14.2: Introduction to the Reproductive System - Biology

14.2: Introduction to the Reproductive System - Biology

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It’s All About Sex

A tiny sperm breaks through the surface of a huge egg. Voilà! In nine months, a new baby will be born. Like most other multicellular organisms, human beings reproduce sexually. In human sexual reproduction, individuals with testes produce sperm, and individuals with ovaries produce eggs, and a new offspring forms when a sperm unites with an egg. How do sperm and eggs form? And how do they arrive together at the right place and time so they can unite to form a new offspring? These are functions of the reproductive system.

What Is the Reproductive System?

The reproductive system is the human organ system responsible for the production and fertilization of gametes (sperm or eggs) and carrying of a fetus. Both both sexes gonads produce gametes. A gamete is a haploid cell that combines with another haploid gamete during fertilization, forming a single diploid cell called a zygote. Besides producing gametes, the gonads also produce sex hormones. Sex hormones are endocrine hormones that control the development of sex organs before birth, sexual maturation at puberty, and reproduction once sexual maturation has occurred. Other reproductive system organs have various functions, such as maturing gametes, delivering gametes to the site of fertilization, and providing an environment for the development and growth of offspring.

Sex Differences in the Reproductive System

The reproductive system is the only human organ system that is significantly different between males and females. Embryonic structures that will develop into the reproductive system start out the same in males and females, but by birth, the reproductive systems have differentiated. How does this happen?

Sex Differentiation

Starting around the seventh week after conception in genetically male (XY) embryos, a gene called SRY on the Y chromosome (Figure (PageIndex{2})) initiates the production of multiple proteins. These proteins cause undifferentiated gonadal tissue to develop into testes. Testes secrete hormones — including testosterone — that trigger other changes in the developing offspring (now called a fetus), causing it to develop a complete male reproductive system. Without a Y chromosome, an embryo will develop ovaries, that will produce estrogen. Estrogen, in turn, will lead to the formation of the other organs of a female reproductive system.

Homologous Structures

Undifferentiated embryonic tissues develop into different structures in male and female fetuses. Structures that arise from the same tissues in males and females are called homologous structures. The testes and ovaries, for example, are homologous structures that develop from the undifferentiated gonads of the embryo. Likewise, the penis and clitoris are homologous structures that develop from the same embryonic tissues.

Sex Hormones and Maturation

Male and female reproductive systems are different at birth, but they are immature and incapable of producing gametes or sex hormones. Maturation of the reproductive system occurs during puberty when hormones from the hypothalamus and pituitary gland stimulate the testes or ovaries to start producing sex hormones again. The main sex hormones are testosterone and estrogen. Sex hormones, in turn, lead to the growth and maturation of the reproductive organs, rapid body growth, and the development of secondary sex characteristics, such as body and facial hair and breasts.

Role of Sex Hormones in Transgender Treatment

Feminizing or masculinizing hormone therapy is the administration of exogenous endocrine agents to induce changes in physical appearance. Since hormone therapy is inexpensive relative to surgery and highly effective in the development of secondary sex characteristics (e.g., facial and body hair in female-to-male [FTM] individuals or breast tissue in male-to-females [MTFs]), hormone therapy is often the first, and sometimes only, medical gender affirmation intervention accessed by transgender individuals looking to develop masculine or feminine characteristics consistent with their gender identity. In some cases, hormone therapy may be required before surgical interventions can be conducted. Trans-females are prescribed estrogen and anti-testosterone medication, such as cyproterone acetate and spironolactone. Trans-men are prescribed testosterone.

Male Reproductive System

The main structures of the male reproductive system are external to the body and illustrated in Figure (PageIndex{3}). The two testes (singular, testis) hang between the thighs in a sac of skin called the scrotum. The testes produce both sperm and testosterone. Resting atop each testis is a coiled structure called the epididymis (plural, epididymes). The function of the epididymes is to mature and store sperm. The penis is a tubular organ that contains the urethra and has the ability to stiffen during sexual arousal. Sperm passes out of the body through the urethra during a sexual climax (orgasm). This release of sperm is called ejaculation.

In addition to these organs, there are several ducts and glands that are internal to the body. The ducts, which include the vas deferens (also called the ductus deferens), transport sperm from the epididymis to the urethra. The glands, which include the prostate gland and seminal vesicles, produce fluids that become part of semen. Semen is the fluid that carries sperm through the urethra and out of the body. It contains substances that control pH and provide sperm with nutrients for energy.

Female Reproductive System

The main structures of the female reproductive system are internal to the body and shown in Figure (PageIndex{4}). They include the paired ovaries, which are small, oval structures that produce eggs and secrete estrogen. The two Fallopian tubes (aka uterine tubes) start near the ovaries and end at the uterus. Their function is to transport eggs from the ovaries to the uterus. If an egg is fertilized, it usually occurs while it is traveling through a Fallopian tube. The uterus is a pear-shaped muscular organ that functions to carry a fetus until birth. It can expand greatly to accommodate a growing fetus, and its muscular walls can contract forcefully during labor to push the baby into the vagina. The vagina is a tubular tract connecting the uterus to the outside of the body. The vagina is where sperm are usually deposited during sexual intercourse and ejaculation. The vagina is also called the birth canal because a baby travels through the vagina to leave the body during birth.

The external structures of the female reproductive system are referred to collectively as the vulva. They include the clitoris, which is homologous to the male penis. They also include two pairs of labia (singular, labium), which surround and protect the openings of the urethra and vagina.


1. What is the reproductive system?

2. Define gonad.

3. What are sex hormones? What are their general functions?

4. Distinguish between male and female sex hormones.

5. How does the differentiation of the reproductive system occur in males and females?

6. In the context of the human male and female reproductive systems, what are homologous structures?

7. When and how does the human reproductive system mature?

8. List the organs of the male reproductive system.

9. List the organs of the female reproductive system.

10. Female gametes are called _________ and male gametes are called _________ .

11. True or False: The vagina is the homologous structure to the penis.

12. True or False: In the absence of a Y chromosome in humans, ovaries will develop.

13. Which are secondary sex characteristics?

A. Fallopian tubes

B. ovaries

C. breasts

D. all of the above

14. Fertilization usually occurs in the _________________.

A. ovary

B. Fallopian tube

C. uterus

D. vagina

15. Explain the difference between the vulva and the vagina.

Explore More

People's sense of gender identity does not always match their anatomy. Some people do not identify as either male or female, and instead, they identify as non-binary, or genderqueer. Others may identify as a gender that is the opposite of what is typically associated with their chromosomes or reproductive organs. These people are called transgender, and they may choose to transition to the opposite gender, a process that may or may not involve physical modifications. Watch the video below to learn about the use of hormones in gender transitioning.

Sex determination may be more complicated than originally thought. Check out this video to learn more:

14.2: Introduction to the Reproductive System - Biology

The reproductive system involves a number of organs that work together to reproduce new life. There are a number of differences between the two systems within the genders, which will be addressed. Genetic material allows for a greater genetic fitness of the offspring in the reproductive system.

The female reproductive system is made up of the vagina, uterus, fallopian tubes, and ovaries. The fallopian tubes carry the egg from the ovary to the womb. While the ovaries contain eggs, they are also responsible for the creation of sex hormones. These components make up the primary aspects of the female reproductive system.

The menstrual cycle of the female reproductive system is also relevant. The menstrual cycle is controlled by the pituitary gland, which secretes hormones also found in the ovaries. The average menstrual cycle is 28 days (Scanlon 456). The process can result in a fertilized or an unfertilized egg.

The male reproductive system is made up of the penis, testicles, epididymis, the vas deferens, and the prostate glands. Sperm and sex hormones and produced by the testicles. Sperm is collected and stored by the epididymis, which is released by the penis by way of other parts of the male reproductive system.

The sperm contains genetic material and represents the male reproductive cell. It moves towards an egg with its tail. Testosterone promotes the maturation of sperm (Scanlon 243).

There are a number of types of problems that can occur within the female and male reproductive systems. Females may experience endometriosis, fibroids, painful periods, and premenstrual tension. Males may experience impotence or prostrate problems. Both genders can experience infertility and sexually transmitted diseases.

Development of the reproductive organs

The sex of a child is determined at the time of fertilization of the ovum by the spermatozoon. The differences between a male and a female are genetically determined by the chromosomes that each possesses in the nuclei of the cells. Once the genetic sex has been determined, there normally follows a succession of changes that will result, finally, in the development of an adult male or female. There is, however, no external indication of the sex of an embryo during the first eight weeks of its life within the uterus. This is a neutral or indifferent stage during which the sex of an embryo can be ascertained only by examination of the chromosomes in its cells.

The next phase, one of differentiation, begins first in gonads that are to become testes and a week or so later in those destined to be ovaries. Embryos of the two sexes are initially alike in possessing similar duct systems linking the undifferentiated gonads with the exterior and in having similar external genitalia, represented by three simple protuberances. The embryos each have four ducts, the subsequent fate of which is of great significance in the eventual anatomical differences between men and women. Two ducts closely related to the developing urinary system are called mesonephric, or wolffian, ducts. In males each mesonephric duct becomes differentiated into four related structures: a duct of the epididymis, a ductus deferens, an ejaculatory duct, and a seminal vesicle. In females the mesonephric ducts are largely suppressed. The other two ducts, called the paramesonephric or müllerian ducts, persist, in females, to develop into the fallopian tubes, the uterus, and part of the vagina in males they are largely suppressed. Differentiation also occurs in the primitive external genitalia, which in males become the penis and scrotum and in females the vulva (the clitoris, labia, and vestibule of the vagina).

At birth the organs appropriate to each sex have developed and are in their adult positions but are not functioning. Various abnormalities can occur during development of sex organs in embryos, leading to hermaphroditism, pseudohermaphroditism, and other chromosomally induced conditions. During childhood until puberty there is steady growth in all reproductive organs and a gradual development of activity. Puberty marks the onset of increased activity in the sex glands and the steady development of secondary sexual characteristics.

In males at puberty the testes enlarge and become active, the external genitalia enlarge, and the capacity to ejaculate develops. Marked changes in height and weight occur as hormonal secretion from the testes increases. The larynx, or voice box, enlarges, with resultant deepening of the voice. Certain features in the skeleton, as seen in the pelvic bones and skull, become accentuated. The hair in the armpits and the pubic hair becomes abundant and thicker. Facial hair develops, as well as hair on the chest, abdomen, and limbs. Hair at the temples recedes. Skin glands become more active, especially apocrine glands (a type of sweat gland that is found in the armpits and groin and around the anus).

In females at puberty, the external genitalia enlarge and the uterus commences its periodic activity with menstruation. The breasts develop, and there is a deposition of body fat in accordance with the usual contours of the mature female. Growth of axillary (armpit) and pubic hair is more abundant, and the hair becomes thicker.

Study the Male and Female Reproductive Systems

The organs that comprise the male genital system are the testicles, the epididymis, the vas deferens, the seminal vesicles, the ejaculatory duct, the prostate, the bulbourethral glands, the urethra and the penis.

More Bite-Sized Q&As Below

2. Concerning reproduction, what is the function of the testicles?

The testicles are the male gonads that is, the organs where the production of gametes takes place. In human beings, gametes are produced by meiosis that occurs in the testicles.

3. After passing the epididymis, through which structures do sperm cells travel until exteriorization?

After leaving the epididymis in the testicle, sperm cells enter the vas deferens. After that, they receive secretions from seminal vesicles and gather (from the right and left sides) in the ejaculatory duct. They also get secretions from the prostate and the bulbourethral glands and then go through the urethra, inside the penis, to the exterior.

4. What is the function of the secretions of the prostate, seminal vesicle and bulbourethral glands in reproduction?

These secretions, along with sperm cells from the testicles, form semen. These secretions have the function of nourishing the sperm cells and serving as a fluid means of propagation for them. The alkaline pH of seminal fluid also neutralizes the acidic secretions of the vagina, allowing the survival of sperm cells in the vaginal environment after copulation.

5. What endocrine glands regulate sexual activity in males? How does this regulation work and what hormones are involved ?

In males, sexual activity is regulated by the endocrine glands: the hypophysis (the pituitary), the adrenal glands and the gonads (testicles).

FSH (follicle-stimulating hormone) secreted by the adenohypophysis acts on the testicles, stimulating spermatogenesis. LH (luteinizing hormone), another adenohypophyseal hormone, also stimulates the production of testosterone by the testicles. Testosterone, the production of which intensifies after the beginning of puberty, acts on several organs of the body and is responsible for the appearance of secondary male sex characteristics (beard, body hair, deep voice, increase in the muscle and bone mass, maturation of genitalia, etc.). Testosterone also stimulates spermatogenesis.

The Female Reproductive System

6. What organs are a part of the female reproductive system?

The organs that make up the female reproductive system are the ovaries, the Fallopian tubes (or uterine tubes), the uterus, the vagina and the vulva.

7. During which period of life does the formation of gametes begin in women?

The meiosis that forms female gametes begins in the cells of ovarian follicles before birth. After the beginning of puberty, under hormonal stimuli, during each menstrual cycle, one of the cells is released on the surface of the ovary and meiosis resumes. However, the meiotic process is only concluded if fertilization occurs.

8. What organ releases the female gamete under formation? How is this release triggered? What organ collects the released gametes?

The organ that releases the female gamete is the ovary, the female gonad. The releasing of the oocyte is a response to hormonal stimuli. The immature egg cell (still an oocyte) falls into the abdominal cavity and is picked up by the Fallopian tube (uterine tube, or oviduct), a tubular structure that connects the ovary with the uterus.

9. What are the anatomical relationships between the organs of the female reproductive system, from the external vulva to the ovaries?

The external female genitalia is called the vulva. The vulva is the external opening of the vaginal canal, or vagina. The vagina is the copulation organ of females and its posterior extremity communicates with the uterus through the uterine cervix. The uterus is divided into two portions: the cervix and the uterine cavity. The lateral walls of the uterine fundus communicate with the Fallopian tubes. The other extremity of each Fallopian tube ends in fimbria, forming fringes in the abdominal cavity. Between the uterine tube and the ovary is intra-abdominal space.

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The Menstrual Cycle

10. What is the menstrual cycle?

The menstrual cycle is the periodic succession of interactions between the hormones and organs of the female reproductive system that, after the beginning of puberty, regulates the release of female gametes and prepares the uterus for fertilization and pregnancy.

11. What endocrine glands are involved in the menstrual cycle? What hormones are in involved?

The endocrine glands that secrete hormones involved in the menstrual cycle are the hypophysis (the pituitary gland) and the ovaries.

The hormones from the adenohypophysis are FSH (follicle-stimulating hormone) and LH (luteinizing hormone) and the hormones from the ovaries are estrogen and progesterone.

12. What event marks the beginning of the menstrual cycle? What is the blood concentration of FSH, LH, estrogen and progesterone during this phase of the cycle?

By convention, the menstrual cycle begins on the day that menses begins. (Menses is the endometrial hemorrhage excreted through the vaginal canal.) During these days, the hormones FSH, LH, estrogen and progesterone have low concentration.

13. After menses, what hormone influences the maturation of ovarian follicles?

The maturation of ovarian follicles after menses is stimulated by the action of FSH (follicle-stimulating hormone).

14. What hormone is secreted by the growing ovarian follicles? What is the action of that hormone on the uterus?

The follicles growing after menses secrete estrogen. These hormones act on the uterus, stimulating the thickening of the endometrium (the internal mucosa of the uterus).

15. What is the relationship between estrogen level and the LH level in the menstrual cycle? What is the function of LH in the menstrual cycle and when does its blood concentration reach a peak?

The increase in the blood concentration of estrogen with the growing of the ovarian follicle causes the hypophysis to secrete LH. During this phase, LH acts along with FSH to promote the maturation of the follicle, which on the 14th day, ruptures, releasing the female gamete (ovulation). After the release of the ovum, LH stimulates the formation of the corpus luteum, a structure made from the remaining follicular mass. LH concentration is at its maximum on the 14th day of the cycle.

16. What hormones promote the release of the female gamete from the follicle and on which day of the menstrual cycle does this phenomenon happen? What is this event called?

The hormones that promote the release of the ovum from the follicle are FSH and LH, hormones found in maximum blood concentration around the 14th day of the cycle. The release of the female gamete from the ovary is called ovulation. Ovulation happens at (around) on 14th day of the menstrual cycle.

17. How does the female gamete move from the ovary to the uterus?

The female gamete released by the ovary falls into the surrounding abdominal cavity and is collected by the Fallopian tube. The internal epithelium of the uterine tubes has ciliated cells that move the ovum or the fertilized egg cell towards the uterus.

18. How long after ovulation must fertilization occur to be effective?

If fertilization does not occur approximately 24 hours after ovulation, the released ovum often dies.

19. Into what structure is the follicle transformed after ovulation? What is the importance of that structure in the menstrual cycle?

The follicle that released the ovum undergoes the action of LH and is transformed into the corpus luteum. The corpus luteum is very important because it secretes estrogen and progesterone.

These hormones prepare the uterine mucosa, also known as endometrium, for nidation (the implantation of the zygote in the uterine wall) and embryonic development, since they stimulate the thickening of the mucous tissue, increase its vascularity and cause the appearance of uterine glycogen-producing glands.

20. What is the importance of uterine glycogen-producing glands?

Uterine glands produce glycogen that can be broken down into glucose to nourish the embryo before the complete development of the placenta.

21. How does negative feedback between the hypophysis and corpus luteum  work? What is the name given to the atrophied corpus luteum after this feedback process?

After ovulation, the estrogen and progesterone secretions from the corpus luteum inhibit hypophyseal FSH and LH secretions (this happens through the inhibition of GnRH, gonadotropin-releasing hormone, a hypothalamic hormone). The blood concentration of these adenohypophyseal hormones falls to basal levels once again. As LH lowers, the corpus luteum (luteum means “yellow”) becomes atrophic and turns into the corpus albicans (“white”). With the regression of the corpus luteum, the production of estrogen and progesterone ceases.

22. In hormonal terms, why does menses occur?

Menses is the monthly਎ndometrial desquamation that occurs as the estrogen and progesterone levels fall after the regression of the corpus luteum. This is because these hormones, mainly progesterone, can no longer support and maintain the thickening of the endometrium.

23. What is the explanation for the bleeding that accompanies menses?

The hemorrhage that accompanies menses occurs because the endometrium is a highly vascularized tissue. The rupture of the blood vessels of the uterine mucosa during menstrual desquamation causes the bleeding.

24. What are the phases of the menstrual cycle?

The menstrual cycle is divided into two main phases: the follicular (or menstrual) phase and the luteal (or secretory) phase.

The menstrual phase begins on the first day of menses and lasts until ovulation (around the 14th day). The luteal phase begins after ovulation and ends when menses begins (around the 28th day).

25. Including the main events and hormonal changes, how can the menstrual cycle be described?

The cycle can be described like an analog clock on which 12 o’clock is the beginning and the end of the menstrual cycle and 6 o’clock corresponds to the 14th day of the cycle.

At 12 o’clock, menses and therefore the menstrual cycle begin and FSH blood levels begins to increase. Around 2 o’clock, the follicles maturing under the effect of FSH are already secreting estrogen and the endometrium is thickening. Around 3 o’clock, estrogen is intensely stimulating the increase of LH blood levels. At 6 o’clock (the 14th day), LH is at its maximum concentration and FSH is also at high levels to promote ovulation. LH then stimulates the formation of the corpus luteum. Around 7 o’clock, the corpus luteum is already secreting a large amount of estrogen and progesterone and the endometrium thickens even more levels of FSH and LH decrease with the increasing of the ovarian hormones. Around 11 o’clock, the reduced LH and FSH levels make the corpus luteum turn into the corpus albicans the production of estrogen and progesterone ceases and the endometrium regresses. At 12 o’clock again (the 28th day), the endometrium desquamates and a new menstrual cycle begins.


26. In general, during what phase of the menstrual cycle can copulation lead to fertilization?

Although this is not a rule, to be effective, fertilization must occur within 24 hours after ovulation (which occurs around the 14th day of the menstrual cycle). Fertilization may occur even if copulation took place up to 3 days before ovulation, since male gametes remain viable for about 72 hours within the female reproductive system.

However, the fertile period of the women is considered to be the period from 7 days before ovulation to 7 days after ovulation.

27. In what part of the female reproductive system does fertilization occur?

Fertilization generally occurs in the Fallopian tubes, but it can also take place in the uterus. There are cases when fertilization may occur even before the ovum enters the uterine tube, which may lead to a severe medical condition known as abdominal pregnancy.

28. How does the sexual arousal mechanism in women facilitate fertilization?

During sexual arousal in women, the vagina secretes substances to neutralize its acidity, thus allowing the survival of sperm cells within it. During the female fertile period, hormones make the mucus that covers the internal surface of the uterus less viscous to help the passage of sperm cells into the uterine tubes. During copulation, the uterine cervix advances inside the vagina to facilitate the entrance of male gametes through the cervical canal.

Nidation and Pregnancy

29. What is nidation? During which phase of the menstrual cycle does nidation occur?

Nidation is the implantation of the embryo into the uterus. Nidation occurs around the 7th day after fecundation, that is, 7 to 8 days after fertilization (obviously, it occurs only if fecundation also occurs). Since it occurs in the luteal phase ,the progesterone level is high and the endometrium is in its best condition to receive the embryo.

30. What is tubal pregnancy?

Often fertilization takes place in the Fallopian tubes. Generally, the newly formed zygote is moved to the uterus, where nidation and embryonic development occur. However, in some cases, the zygote cannot descend into the uterus and the embryo implants itself in the uterine tube tissue, which is the characteristic of tubal pregnancy. Tubal pregnancy is a severe clinical condition since the tube often ruptures during gestation, causing a hemorrhage and even the death of the woman. The most common treatment for tubal pregnancy is surgery.

31. How do hormonal tests to detect pregnancy work?

Laboratory tests to detect pregnancy commonly test for human chorionic gonadotropin (HCG) concentration in blood or urine samples. If the level of this hormone is abnormally high, pregnancy is likely.

32. Does the hypophysis-ovaries endocrine axis work in the same way during pregnancy as in non-pregnant women? If pregnancy does not occur how does another menstrual cycle begin?

The functioning of the hypophysis is altered during pregnancy. Since estrogen and progesterone levels remain elevated during the gestational period, the production of GnRH (gonadotropin-releasing hormone) from the hypothalamus is inhibited. The lack of GnRH therefore inhibits the secretion of FSH and LH by the hypophysis and a new menstrual cycle does not begin.

If pregnancy does not occur, the lowering of estrogen and progesterone levels stimulates the production of GnRH by the hypothalamus. This hormone then hastens the adenohypophyseal secretion of FHS and LH, which in turn stimulate the maturation of follicles and the beginning of a new menstrual cycle.

33. What is the endocrine function of the placenta?

The placenta, in addition to being the organ through which the exchange of substances between the mother and the fetus is carried out, also has the function of secreting estrogen and progesterone to maintain a high level of these hormones during pregnancy. (The placenta still secretes other hormones such as human placental lactogen, which acts in a way similar to that of the hypophyseal hormones that regulate reproduction, and HCG, human chorionic gonadotropin.)

Reproductive Planning Methods

34. How do contraceptive pills generally work?

Contraceptive pills generally contain the hormones estrogen and progesterone. If taken daily from the 4th day after menses, the abnormal elevation of these hormones acts upon the hypophysis-hypothalamus endocrine axis, inhibiting FSH and LH secretions. Since these hormones do not reach their normal high levels during the menstrual cycle, ovulation does not occur.

(Treatment with contraceptive pills must be started under medical supervision.)

35. What are the common contraindications of contraceptive pills?

There are medical reports associating the use of contraceptive pills with vomiting, nausea, vertigo, headaches, hypertension and other pathological conditions. Some research has attempted to relate the medical ingestion of estrogen and progesterone with an increased propensity for cardiovascular diseases (such as heart attacks, strokes and thrombosis) and malignant neoplasms (cancers). Doctors must always be askedꂫout the risks and benefits of the contraceptive pill prior to use.

36. What are the most common methods of male and female surgical sterilization?

Vasectomy is the most common method of surgical sterilization in men. In vasectomy, the vas deferens inside the scrotum are sectioned and closed at a section, forbidding the sperm cells from entering the ejaculatory duct but still allowing the release of seminal fluid during ejaculation.

The surgical sterilization of women is often done by bilateral tubal ligation. With tubal ligation, the ovum does not enter the uterus and, as a result, sperm cells cannot reach it.

37. How does a contraceptive diaphragm work? What are the limitations of this contraceptive method?

A contraceptive diaphragm is a device made of latex or plastic that, when placed on the vaginal fundus, covers the uterine cervix, preventing the passage of sperm cells through the cervical canal. To be more effective, the diaphragm needs to be used with spermicide. However, this method does not prevent sexually transmitted infections (STIs).

38. Why is the use of condoms not just a contraceptive method but also a health protection behavior?

The use of condoms, in addition to being an efficient contraceptive method, also helps the prevention of diseases caused by sexually transmitted agents (STIs), such as syphilis, gonorrhea, HPV (the human papilloma virus, which may lead to genital cancers) infection, HIV infection, etc.

39. What is the normal duration of the menstrual cycle? How does the calendar contraceptive method work?

The normal duration of the menstrual cycle is 28 days, but it can vary among different women or different cycles in the same woman.

In the calendar contraceptive method, the date n-14 (n minus 14) is taken, considering n the number of days of the normal menstrual cycle of the woman (generally n=28). The safety margin +3 or –3 refers to the days around n-14 during which intercourse should be avoided to prevent pregnancy. (This method is not completely free of failures. A doctor must always be consulted before relying on any contraceptive method.)

40. How is the ovulation date estimated via the measurement of a woman's body temperature?

One method to estimate the exact ovulation date is daily measurement of body temperature always done under same conditions. On the date of ovulation, body temperature often increases about 0.5 degrees centigrade.

41. What is the contraceptive mechanism of an IUD?

An IUD (intrauterine device) is a piece of plastic coated with copper that is inserted into the uterus by a doctor. Copper is then gradually released (an IUD can last from 5 to 10 years) and since it has a spermicidal effect, sperm cells are destroyed before fertilization. in addition to this mechanism, the movement of the IUD inside the uterus causes slight endometrial inflammation, which helps to prevent nidation.

Reproduction in Other Animals

(See zoology subjects for a comprehensive review.)

42. Generally, how does a male animal realize that the female is receptive to copulation?

In most vertebrate species with internal fertilization, females have reproductive cycles with fertile periods. During this period, the female secretes pheromones (odoriferous substances that attract the male of the species) from the skin and mucosae. The presence of the male individual and his pheromones also stimulates the release of pheromones by the female. (Many animals also use pheromones to mark their territories and for signal transmission between individuals about the location of dangers and food.)

43. What is parthenogenesis?

Parthenogenesis is the reproduction or formation of a new individual from the egg cell without fertilization by the male gamete. Depending on the species, individuals born via parthenogenesis may be male or female, or of any sex.

In bees, the drone (the single male bee) is haploid and born via parthenogenesis while the females (queen and workers) are diploid.

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Reproductive System in Humans | Essay | Humans | Reproductive Biology

In this essay we will discuss about the male and female reproductive system in humans.

Essay on the Male Reproductive System :

During the developmental stages, the gonads of the genetic male fetus are induced to differentiate into testes. The testes of the male secrete testosterone, which is responsible for differentiation and development of the urinogenital system characteristic of the male. The testes remain inactive until puberty.

They are activated by the gonadotropins produced by the pituitary gland. Two sets of genes are required for the development of the male phenotype. The first set is located on the Y-chromosome and they encode the testes determining factor. These genes must be expressed for the undifferentiated gonad to form the testis.

In male the Mullerian ducts have to degenerate so that they may not form the female reproductive tract. Somatic cells of the developing testes secrete a Mullerian inhibiting substance, which helps in the regression of the Mullerian duct. Development of the male reproductive tract and secondary sexual characters are dependent upon androgens.

The fetal testes have to synthesize androgens at a critical time during the differentiation of the testis. A second set of genes is required for the complete development of the male. These genes encode the enzymes required for the conversion of cholesterol to testosterone and dihydrotestosterone.

Testes :

Testis performs two important functions. It provides an environment for spermatogenesis and secretes the hormone testosterone, which regulates a number of reproductive activities. Testes are located within the scrotum. Each testis is oval in shape and is about 4 – 6 cm in length and 2 – 3 cm in diameter. A connective tissue sheath, the tunica albuginea, surrounds it. Spermatozoa are produced in convoluted seminiferous tubules.

These tubules converge to form the rete testis, which opens to efferential ductules and epididymis. The epididymis can be differentiated into head, body and tail. The tail continues as vas deferens. The outer layer of seminiferous tubules is made up of connective tissue and smooth muscle, while the inner layer consists of the Sertoli cells within which are embedded the spermatogonia and different stages of mature and immature spermatozoa.

Sertoli cells provide nourishment and other factors necessary for sperm maturation. The fully mature spermatozoa are released into the lumen of the seminiferous tubules and subsequently move slowly to the epididymis where they are stored in the tail region. In-between the seminiferous tubules are scattered the interstitial cells of Leydig, which produce androgens.

The parent substance from which the androgens are synthesized is cholesterol. Pregenolone is synthesized from cholesterol in the Leydig cells. Pregenenolone is converted into 17 – hydroxypregnenolone. This substance is converted to 17 – ketosteroids by side-chain cleavage and these, in turn are converted into testosterone. Testosterone is the principal steroid produced by the Leydig cells.

Hormonal Control of Testicular Function:

The anterior lobe of the pituitary gland secretes gonadotropins namely Follicle Stimulating Hormone (FSH) and Leutenizing Hormone (LH). The anterior pituitary is stimulated to release these hormones through Follicle Stimulating Hormone Releasing Hormone (FSH-RH) and Leutenizing Hormone Releasing Hormone (LH – RH) from the hypothalamus. These hormones are neurohormones.

GnRH and its Effects :

Gn-RH is a peptide containing 10 amino acid residues. It is secreted by the neurons whose cell bodies are located in the arcuate nucleus of the hypothalamus. Gonadotrophic releasing hormone is transported to the anterior pituitary gland through the portal circulation and stimulates the release of the two gonadotrophic hormones, LH and FSH.

GnRH is secreted intermittently a few minutes at a time once every 1 to 3 hours. The intensity in stimulus of this hormone is determined by the frequency of the cycle of secretion and by the GnRH released from each cycle.

Regulation of Spermatogenesis by FSH and Testosterone:

LH and FSH are secreted by the gonadotroph cells in the anterior lobe of the pituitary gland. LH and FSH are glycoproteins. The quantity of the carbohydrate bound to the protein in the hormone molecule varies considerably under different conditions, which may change the activity potential of the hormone.

FSH binds to specific FSH receptors attached to the Sertoli cells in the seminiferous tubules. This causes these cells to grow and stimulate the formation of spermatozoa in the seminiferous tubules of testes after maturity either continuously or seasonally depending upon the species. Simultaneously, testosterone also exerts a strong influence on spermatogenesis.

Hormones that stimulate spermatogenesis are:

a. Leutenizing Hormone:

It is secreted by the luteotrophs present in the anterior pituitary lobe. This hormone stimulates the interstitial cells of Leydig to secrete testosterone. Pituitary gonadotropin secretion is under tonic regulatory control. They undergo wide fluctuation in their circulating concentrations over short periods of time.

In human males LH is released after every 90 minutes. LH is released on exposure of a male to female, but successive presentation of the same female to a male mammal leads to habituation. LH specifically binds to Leydig cells while FSH binds to only Sertoli cells. This clearly establishes the individual roles of pituitary gonadotropins in regulating testicular function.

LH increases the cAMP levels in the interstitial cells of the testes but not the seminiferous tubules. FSH on the other hand, does not increase the cAMP levels in the Leydig cells, but stimulates the production of cAMP in seminiferous tubules in enriched fractions of the Sertoli cells.

Inhibin is a peptide hormone produced by the Sertoli cells. It controls the secretion of pituitary FSH. Inhibin secretion by the Sertoli cells is regulated by influences deriving from maturing sperms. Pituitary FSH secretion normally is regulated by negative feedback by an inhibitory factor of Sertoli cell origin.

Granulosa cells from the ovarian follicle also secrete inhibin that acts directly on pituitary cells from males or females to supress FSH secretion. Administration of anti-inhibin antisera to rats of either sex causes increase in FSH levels. LH levels are not affected by anti inhibin antisera. The role of FSH and inhibin in the control of testicular and ovarian function is shown in Figure 1.

Testicular receptors are maintained by one or more pituitary hormones other than the pituitary gonadotropins. Decrease in pituitary prolactin secretion decreases the testicular LH receptors, whereas prolactin treatment prevents loss of LH receptors in hypophysectomized animals. Thus, prolactin plays a role in the control of testicular Leydig cell LH receptor numbers.

Androgens control the differentiation and development of the male urinogenital system, the accessory sex organs and the external genital organs. Androgens produced by the testis are responsible for the growth and development of all those tissues that characterize the male.

Increased gonadotropins level during puberty increases the circulating levels of androgens. This increase in the level of androgens is responsible for initiating spermatogenesis and for the growth and development of the secondary sexual characteristics.

All androgens are steroid compounds. They can be synthesized from cholesterol or directly from acetyl coenzyme A. After secretion by the testes, about 97% of the hormone becomes either loosely bound to plasma albumin or tightly bound to a beta globulin called sex hormone binding globulin and circulates in blood in these states for 30 minutes to 1 hour.

By that time much of the testosterone becomes fixed to the tissues and is converted to a more active hormone known as dihydrotestosterone especially in some target organs like the prostate gland in adult and in external genitalia of the male fetus.

Intracellular Action of Testosterone :

Testosterone increases the rate of formation of proteins in target cells. In prostate gland testosterone enters the cells within a few minutes after secretion and is converted into dihydrotestosterone by an intracellular enzyme 5, α-reductase, Dihydrotestosterone binds to an intracelular receptor protein and this complex migrates to the nuclei and binds to a nuclear protein to induce the transcription process within 30 minutes.

RNA polymerase is activated and ultimately, the concentration of the cellular protein progressively increases. Thus, testosterone stimulates the production of protein and specifically those proteins in target organs responsible for the development of secondary sexual characteristics.

Some important target tissues do not have the reductase enzyme in their cells to convert testosterone into dihydrotestosterone. Therefore, in these tissues testosterone functions directly but only with its half potency. This direct action of testosterone is essential in male fetal tissues for the development of epididymis, vas deferens and seminal vesicles.

During sexual maturation, FSH and testosterone initiate spermatogenesis. FSH increases the size of the testis but does not increase the appearance of mature spermatozoa and secretory activity of the Leydig cells. For completion of spermatogenesis, testosterone is required. FSH initiates tie process of spermatogenesis while testosterone maintains it.

FSH interacts with the receptors located in the plasmalemma of the Sertoli ceils resulting in increased production of cAMP and the synthesis of an androgen binding protein (ABP). The ABP is subsequently secreted into the lumen of the seminiferous tubules. Leydig cells contain specific receptors for LH. In response to LH, testosterone released by the Leydig cells enters the seminiferous tubules through blood circulation.

Testosterone is actively taken up by the Sertoli cells. Within the Sertoli cells testosterone is bound to the androgen binding protein. This binding brings testosterone in close contact to spermatozoa on which maturation of the spermatozoa is dependent. Thus, the hormonal effects of testosterone on spermatogenesis are mediated through the Sertoli cells.

Androgen binding protein provides a mechanism for the accumulation of androgens within the Sertoli cells and its release into the lumen of the seminiferous tubules. From the lumen, ABP transports testosterone to the epididymis, where they mature and develop the potential for fertilization and motility.

Physiological Functions of Estrogens:

Estrogens and androgens are necessary for normal reproductive function in male. Information on estrogen functions in the male is obtained by the use of gene knockout technique involving a mutant mouse line without a functional estrogen receptor.

In mice without functional estrogen receptor, the testes atrophy progressively with decrease in the number of spermatozoa in epididymis, their viability and motility. Estrogen regulates the reabsorption of luminal fluid in the head of epididymis. Disruption of estrogen function causes spermatozoa to enter the epididymis in diluted rather than concentrated, resulting in infertility.

Essay on the Female Reproductive System:

As in the male, in female the ovary also performs the functions of gametogenesis and as an endocrine gland. Ova released from the ovary enter the oviducts and Fallopian tube. Fertilization of the ovum usually occurs in the fallopian tubes by the spermatozoa released by the male during copulation. The fertilized ovum descends into the uterus and is embedded in its wall for further development and embryogenesis.

The ovary consists of both epithelial and mesenchymal elements. The mesenchyme differentiates into interstitial tissue, which produces estrogen. The epithelial tissue is closely associated with the germinal elements of the ovary. It provides nutrition to the oocytes and is an important source of hormones required for different stages of the ovarian cycle.

The ovary is a solid structure covered by a visceral peritoneum of flattened cells. Inside this is a layer of cuboidal cells, the germinal epithelium. The stroma is divided into outer ovarian cortex and inner ovarian medulla. At birth, each oocyte is surrounded by a single layer of flattened granulosa cells. The combined structure is known as primordial follicle.

The primordial follicles are located near the periphery or cortex of the ovary, and are separated from each other by the stromal connective tissue and interstitial tissue. The primordial follicles remain inactive until puberty. The follicular epithelial cells are transformed into a single layer of cuboidal cells surrounding the oocyte.

The entire structure is known as primary follicle. During puberty, under the influence of hormones 6 to 12 primary follicles develop into secondary follicles. The granulosa cells secrete a mucoid material that forms the zona pellucida around the oocyte.

The granulosa cells develop protoplasmic processes that penetrate the zona pellucida and touch the plasmalemma of the oocyte. Out of the 6 to 12 primary follicles, during each menstrual cycle only one develops into a mature follicle while others become atretic and disappear. The granulosa cells continue to increase in number.

The interstitial tissue adjacent to the follicle becomes arranged concentrically around it to form the theca. Thecal cells adjacent to the follicle, the theca interna, are surrounded by an outer layer of interstitial cells forming the theca externa. Granulosa cells continue to proliferate and surrounding interstitial cells are incorporated into the theca. These changes are accompanied by accumulation of a fluid in the spaces between the granulosa cells.

A large vesicle or antrum is formed due to the enlargement of the follicle. Graulosa cells adhering to the surface of the oocyte form the coronal granulosa cells while the remaining cells in contact with the surrounding theca form the membrana granulosa. A streak of granulosa cells connects the cells around the ovum with the membrana granulosa.

The layer of granulosa cells around the ovum form the corona radiata. The oocyte acquires two membranes, the inner thin vitelline membrane and outer thick zona pellucida. The zona pellucida is surrounded by the corona radiata composed of enlarged follicular cells with fine canals between them. The fully formed mature follicle is known as Graffian follicle.


The solid follicle develops in it a small cavity called as the antrum. The antrum gradually becomes wider. Release of the oocyte is known as ovulation. A single ovum is expelled from a ovarian follicle into the abdominal cavity in the middle of each monthly menstrual cycle.

Ovulation is caused due to increased turgidity and contraction of smooth muscle fibers around the follicles. The ovum passes through one of the Fallopian tubes into the uterus, if it is fertilized by the sperm.

In rabbit ovulation occurs after copulation. Mammalian ovulation is a unique biological process as it involves the physical disruption of healthy tissue at the surface of the ovary. Ovulation requires a surge of pituitary gonadotropins. The capillaries in the follicular wall dilate after about 4-6 hours of the starting of the ovulatory process.

As the time of rupture approaches, the apex of a mature follicle protrudes more and more above the surface of the ovary and the follicle wall gradually becomes thinner. The apical most part of the follicle becomes translucent and rapidly protrudes above the normal wall of the follicle wall to form a stigma.

The follicle ruptures within several minutes after the formation of the stigma. The final rupture of the follicle is dependent upon the degradation of the collagenous connective tissue in the thecal layer of the follicle wall. After the rupture of the wall of the follicle, the oocyte and surrounding cells are extruded within a couple of minutes.

The purposes of the ovarian follicle are to:

(i) Preserve the resident oocyte,

(ii) Mature the oocyte at the optimal time,

(iii) Produce hormones to develop a proliferating endometrium,

(iv) Release of the oocyte at the proper time,

(v) Formation of a high quality corpus luteum for implantation, and

(vi) Secrete hormones required for gestation.

Corpus Luteum:

After rupture and release of the ovum from the Graffian follicle, the granulosa and thecal cells increase in number and the blood clot is absorbed. The granulosa cells start to accumulate large quantity of cholesterol and this process of leutenization forms the corpus luteum. Cells derived from the theca interna migrate into the luteal tissue to give rise to small luteal cells, theca lutein cells and fibrobasts.

Hormonal Influence on the Ovary:

The female hormonal system consists of three hormones:

(i) A hypothalamic releasing hormone, the gonadotropin releasing hormone (GnRH),

(ii) The anterior pituitary hormones, follicle stimulating hormone (FSH) and leutenizing hormone (LH) both of which are secreted on stimulation by the GnRH, and

(iii) The ovarian hormones, estrogen and progesterone secreted by the ovaries in response to the two pituitary hormones. These different hormones are not secreted in constant amounts and continuously throughout the menstrual cycle but are secreted at different rates during different parts of the cycle.

Estrogen Biosynthesis:

The two cell theory of estrogen secretion states that the thecal cells produce carbon 19 androgens and these are delivered to the granulose cells where they are converted into aromatic compounds, the estrogens (Fig. 3). This theory is supported by the observation that granulose cells from several species secrete estrogens if provided with an androgen substrate.

This theory is supported by the observation that granulosa cells of several species secrete estrogens if given an androgen substrate. Thecal cells also produce large amount of androgens. The CYP 17 gene that codes for 17-hydroxylase and C17-20 lyase is actively regulated by LH. Granulosa cells have an aromatase system.

As follicular maturation progresses, the ability of the granulosa cells to aromatize androgens increases. Estrogen production increases within the follicles during the preovulatory phase and is highest at the time of the LH and FSH surge.

Before exposure to a high level of LH, androgen and estrogen levels predominate after the LH surge and during the luteal phase of the cycle, progesterone is the major steroid produced. A complex number of interactions involving LH, FSH, androgens, progesterone and estrogens are involved in the shift from estrogens to progesterone synthesis.

Modified version of the two cell theory states that LH stimulates androgen production within the thecal cells. Androgens are then aromatized within the thecal cells but are also made available to the granulosa cells for aromatization to estrogens.

The estrogens produced by the thecal cells are the major source of circulating levels of the steroid, whereas estrogens synthesized by the granulosa cells are regulated through FSH stimulation of cAMP production and later activation of aromatase activity.

Estradiol is oxidized to estrone in the liver (Fig. 4). Estrone is hydrated to form estriol. During pregnancy, the placenta is an additional source of androgens. Aromatization of androstenedione and testosterone is a major source of estrogens in the male and old females.

Biosynthesis of Progesterone:

The growth of the ovarian follicles before ovulation is dependent on FSH and LH, both acting together FSH promotes the growth of the follicle by acting through receptors on the granulosa cells and inducing the aromatase enzyme required for the conversion of androgens to estrogens.

The action of FSH is enhanced by androgens. Androgens arise from the theca under the control of LH. FSH receptors occur only on granulosa cells, while LH receptors present initially only in the theca. appear in the granulosa cells and are coupled to the cAMP as FSH receptors.

During the preovulatory surge, LH acting on the granulosa cells initiates luteinization, resulting in a reduction of aromatase activity and the enhancement of progesterone synthesis and secretion as the luteinized granulosa cells transform to become the corpus luteum. Synthesis of progesterone is an early step in the biosynthesis of androgens and estrogens within the thecal cell. Carpus luteum is the major source of circulating progesterone.

Effect of Gonadotrophic Hormones:

The ovarian changes during the sexual cycle depend completely on the gonadotrophic hormones, FSH and LH secreted by the anterior pituitary gland. Ovaries remain inactive until stimulated by these hormones. At the age of 11 to 16 years pituitary begins to secrete more FSH and LH, which initiates the menstrual cycle.

During the life cycle of a normal female, the reproductive phase is characterized by monthly rhythmical changes in the rate of secretion of female hormones and corresponding changes in the ovaries and sex organs. This rhythmical pattern is called as the female sexual cycle or menstrual cycle. The duration of the cycle usually is 28 days.

The menstrual cycle can be divided into three phases:

(i) Proliferation of the endometrium or proliferative phase,

(ii) Development of secretory changes in the endometrium or secretory phase, and

(iii) Degeneration of the endometrium of the uterus or degenerative phase. The menstrual cycle includes a period of bleeding called menstruation and the days are counted from the first day of bleeding in the menstrual period.

In the beginning of the menstrual cycle, the endometrium of the uterine wall is in a state of degeneration. After menstruation, only a thin layer of endometrial stroma and epithelial cells are left and these are located in the deeper portion of glands and crypts of endometrium.

Therefore, to prepare the uterine wall for implantation after ovulation, following changes occur:

(1) The adenohypophysis secretes FSH under the influence of FSH-RH from the hypothalamus. The hypothalamus secretes FSH-RH when decrease in the level of estrogen and progesterone secreted in the proceeding menstrual cycle remove its inhibitory effect.

(2) FSH stimulates the growth of selected few primary follicles and maturation of primary follicles.

(3) FSH stimulates the follicular cells to secrete estrogen, which inhibits the secretion of FSH and stimulates hypothalamic secretion of LH-RH. This in turn stimulates the secretion of leutenizing hormone by the pituitary.

(4) LH induces the mature Graffian follicle to burst and release the ovum. This process occurs after 14 days and is known as ovulation. The ovum at this stage is in secondary oocyte stage with the second meiotic division in progress.

(5) Estrogen prepares the wall of the uterus for implantation by proliferating the endometrial epithelial cells. Blood vessels in the uterine wall become elongated and coiled, and vascularization increases. Uterine glands secrete a nutritive fluid into the cavity. Glycogen and fat accumulate.

(6) Under the influence of estradiol, the lining of Fallopian tube is thickened and the ciliary movements are increased. These changes help in conveying the ovum to the uterus.

Secretory Phase (Progesterone Phase):

The stage lasting for about ten days can be further differentiated into the following steps:

(1) LH and prolactin are at their peak of secretion from the anterior lobe of the pituitary. They stimulate the follicular cells of the empty Graffian follicles after the release of the ovum to grow rapidly and become filled with a yellow substance called luteum.

The cavity of follicle becomes filled with blood and broken down thecal cells. The follicle is known as corpus luteum. The yellow corpus luteum secretes the ovarian hormones, progesterone and .small quantities of estradiol.

(2) Estrogen increases cellular proliferation in the uterine endometrium while progesterone causes swelling and secretory activity of the endometrium. This hormone stimulates the endometrial glands to secrete a nutrient fluid for the fetus.

Cytoplasm of the stromal cells, lipid and glycogen deposits and blood supply to the endometrium increase tremendously. At the peak of the secretory phase, usually about one week after ovulation, the endometrium becomes 5-6 mm thick. Progesterone is also required for proper implantation of the fetus.

(3) Progesterone inhibits the release of FSH to prevent the development of additional follicles and ova.

Menstrual or Bleeding Phase:

This phase lasting for 4-5 days is characterized by:

(1) In case of failure of fertilization, the secondary oocyte undergoes degeneration and autolysis. High levels of progesterone in blood inhibit the release of pituitary LH.

(2) Absence of LH brings about autolysis of the corpus luteum and subsequent decrease in the level of progesterone. Regression of the corpus luteum begins about a week after ovulation and after ten days it is replaced by a small, non-functional whitish structure called as the corpus albicans.

(3) The uterine wall degenerates due to the deficiency of progesterone and is sloughed off. Blood vessels rupture causing bleeding. The lining of the Fallopian tubes also degenerates and breaks down. The cast off uterine and Fallopian tissue, and blood from the ruptured blood vessels passes out through the vaginal opening.

This process is called as menstruation or menstrual flow. This occurs after about 25 days and continues for 3-5 days. During normal menstruation, about 50 to 100 ml of blood and 30 ml of serous fluid are lost. The menstrual flow does not clot because fibrinolysin is released along with endometrial breakage.

In case if excessive bleeding occurs, fibrinolysin cannot prevent clotting. The presence of clot during menstrual flow is a serious problem that needs clinical treatment. The basal part of the endometrium remains in tact during menstruation and is the source of new lining wall to the uterus during the next cycle.

(4) Degeneration of the carpus luteum decreases the levels of progesterone and estradiol, which in turn removes their inhibitory effect on hypothalamus. Therefore, the hypothalamus starts to secrete FSH-RH and this stimulates the pituitary to produce FSH.

Functions of Ovarian Hormones:

The ovarian follicles are sources of three types of steroid hormones progestins, androgen, and estrogens. Their levels vary in the menstrual cycle and drastically change in the pregnancy. In the follicular stage of the menstrual cycle, estradiol is dominant, whereas during the luteal phase and during pregnancy, progesterone prevails over others.


Estrogens are mainly secreted by the ovary but minute quantities are also produced by adrenal cortex. During pregnancy, the placenta also contributes to the secretion of estrogens. Estrogens produced during the puberty in female are responsible for growth and development of the vagina, uterus, and oviducts, organs essential for the transportation of ovum, maturation of the zygote and implantation of the embryo.

Estrogens exert their effect on deposition of fat, growth and development of the mammary glands. In human females, three estrogens occur in significant quantities in the plasma. These are estradiol, estrone and estriol. The principal estrogen is estradiol. Small amount of estrone is secreted by the ovaries but most of it is formed from androgens secreted by the adrenal cortex.

Estriol a weak estrogen, is an oxidative product derived from both estradiol and estrone in the liver. The estrogenic potency of estradiol is 12 times higher than that of estrone and 80 times greater than that of estriol.

(i) Estrogens promote proliferation and growth of specific cells in the body and are mainly responsible for development of secondary sexual characteristics of the female.

(ii) During puberty secretion of the estrogens increases by 20 times or more. The female sex organs increase in size due to their influence. Ovaries, fallopian tubes, uterus and vagina are all enlarged. Fat deposits in the external genitalia, pubis, labia majora and labia minora. The vaginal epithelium becomes converted from cuboidal into a stratifed type which is more resistant to infection and trauma.

(iii) Estrogens cause glandular tissue to proliferate and number of ciliated epithelial cells increases. The activity of cilia is also increased to promote the transport of fertilized ovum towards uterus.

(iv) Estrogens promote the development of the stromal tissue of breasts, growth of duct system and deposition of fat in breasts.


The most important progestin is progesterone. Small quantities of 17-hydroxy progesterone is also secreted and it has the same effect. Progesterone is the ovarian hormone of pregnancy and is responsible for preparing the reproductive tract for implantation of the zygote and subsequent and maintenance of the pregnant state.

The most important function of progesterone is to promote secretory changes in the uterine endometrium during the later half of monthly menstrual cycle thus preparing the uterus for implantation of fertilized ovum. Progesterone decreases the frequency and intensity of uterine contraction and thus helps in preventing expulsion of the implanted zygote.

(b) Effect on Fallopian Tubes:

Progesterone also promotes secretory changes in the mucosal lining of the Fallopian tubes. These secretions are required for the nutrition of the fertilized ovum.

(c) Effects on Breasts:

Progesterone promotes development of the lobules and alveoli of the breasts. This causes enlargement of the breasts. It cannot initiate the secretion of milk because milk secretion requires the hormone prolactin. It is speculated that preovulatory plasma levels of progesterone may trigger sexual behaviours in some species.

In rodents, progesterone is necessary for induction of sexual receptivity. Progesterone also plays a role in the nest- building activity and brooding behaviours in some birds. The following table summarizes the physiological actions of progesterone and estradiol.

Mechanisms of Action of Ovarian Steroid Hormones:

Estradiol and progesterone interact with cytoplasmic and nuclear protein receptors resulting in the release of the two-receptor subunits with attached steroid hormones from association with a heat shock protein. Identical subunits either singly or together interact directly with the DNA hormone responsive element to activate the transcriptional events leading to the translation of a cell specific protein.

Ovulation and Role of Luteinizing Hormone :

Ovulation is the process bf release of ovum from the ovarian Graffian follicles. This usually occurs in healthy normal women on fourteenth day after the last menstruation. Shortly before ovulation, the protruding outer wall of the follicles swells rapidly and a small area in the capsule called the stigma protrudes out.

In the next thirty minutes, fluid begins to ooze from the follicle through the stigma. Two minutes later, as a follicle becomes smaller due to loss of fluid, stigma ruptures and the ovum comes out surrounded by several thousand granulosa cells called the corona radiata.

Initiation of Ovulation:

Large quantity of luteinizing hormone is secreted by the pituitary gland. The LH in turn causes rapid secretion of follicular steroid hormone, progesterone.

Within a few hours two events occur:

(i) The theca externa (outer capsule of follicle) begins to release proteolytic enzymes from lysosomes that causes swelling of the follicle and degeneration of the stigma.

(ii) Simultaneously, there is rapid growth of new blood vessels into the wall of the follicles and at the same time prostaglandins are secreted in the follicular tissues. Finally, the combined follicle swelling and simultaneous degeneration of the stigma cause follicle rupture to discharge the ovum.

Ovulatory Surge of Leutenizing Hormone:

LH is essential for the growth and development of the follicle, and ovulation. About two days before the LH secretion increases markedly. FSH secretion also increases and the two hormones act together to produce rapid growth of the follicle during last few days before ovulation. LH secretion also has an effect on the theca cells to secrete progesterone.

The changes that occur can be summarized into:

(i) Rapid growth of the follicle,

(ii) Diminishing level of estrogen secretion, starting of the secretion of progesterone and ovulation occurs.

At the age of 40 to 50 years, sexual cycle in the female becomes irregular and ovulation fails to occur during many of the cycles. When secretion of female sex hormones and the process of ovulation stop completely, it is called menopause.

Throughout a womens reproductive life, about 400 follicles grow and ovulate. But at the age of about 45 years, only a few primordial follicles remain to be stimulated by FSH and LH, and production of estrogen decreases to zero when primordial follicles number becomes almost nil. Therefore, estrogen cannot inhibit the production of FSH and LH which are produced in large quantity in menopause.

The loss of estrogen causes many physiological changes in the function of the body such as:

(i) Extreme flushing of the skin,

(v) Decreased strength and calcification of bones throughout the body.

These symptoms are sometimes cured by regular administration of estrogen.

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Reproductive Health

Reproductive health refers to the condition of male and female reproductive systems during all life stages. These systems are made of organs and hormone-producing glands, including the pituitary gland in the brain. Ovaries in females and testicles in males are reproductive organs, or gonads, that maintain health of their respective systems. They also function as glands because they produce and release hormones.

Reproductive disorders affect millions of Americans each year.

The following clinical trial is currently recruiting

  • Early or delayed puberty.
  • Endometriosis, a condition where the tissue that normally lines the inside of the womb, known as the endometrium, grows outside of it.
  • Inadequate breastmilk supply.
  • Infertility or reduced fertility (difficulty getting pregnant).
  • Menstrual problems including heavy or irregular bleeding.
  • Polycystic ovary syndrome, ovaries produce more male hormones than normal.
  • Problems during pregnancy.
  • Uterine fibroids, noncancerous growths in a woman&rsquos uterus or womb.

Scientists believe environmental factors likely play a role in some reproductive disorders. Research shows exposure to environmental factors could affect reproductive health in the following ways:

  • Exposure to lead is linked to reduced fertility in both men and women. 1 exposure has been linked to issues of the nervous system like memory, attention, and fine motor skills. 2
  • Exposure to diethylstilbestrol (DES), a drug once prescribed to women during pregnancy, can lead to increased risks in their daughters of cancer, infertility, and pregnancy complications. 3
  • Exposure to endocrine-disrupting compounds, chemicals that interfere with the body&rsquos hormones, may contribute to problems with puberty, fertility, and pregnancy. 4

What is NIEHS Doing?

NIEHS conducts and funds research to understand environmental factors that may affect human reproductive health.

Heavy lifting or shift work and decreased fertility &ndash Two occupational factors for women &ndash lifting heavy loads or working non-daytime schedules &ndash are associated with fewer eggs in their ovaries, which could indicate decreased fertility. 5

Chemical exposure and assisted reproductive technology &ndash Exposure to high levels of flame retardants 6 and plasticizers 7 may have a negative impact on the outcomes of in vitro fertilization (IVF), a technology used to help people get pregnant. The researchers found that women with higher levels of these chemicals in their urine had lower levels of ovary cells necessary for reproduction, and fewer successful pregnancies and live births.

Chemical exposure and fetal growth &ndash Exposure during pregnancy to phthalates 8 and phenols 9 , chemicals commonly found in plastics, as well as arsenic, a naturally occurring chemical found in food, air, soil, and water, could lead to low birth weight 10 , early onset of puberty, and obesity. 11

BPA and breastfeeding &ndash Women who had more contact with bisphenol A (BPA), an endocrine-disrupting chemical found in many plastics, were more likely to report that they stopped breastfeeding because they believed they were not producing enough breastmilk. 12

Phthalates, parabens, and phenols associated with early puberty &ndash The daughters of pregnant women whose bodies had high levels of these chemicals (common in personal care products) started puberty earlier than normal. 13

Soy formula and menstrual pain &ndash Girls who were fed soy formula as infants are more likely to develop heavy menstrual bleeding 14 , severe menstrual pain 15 , endometriosis 16 , and larger fibroids 17 later in life.

Vitamin D and uterine fibroids &ndash Women with adequate levels of vitamin D are less likely to develop uterine fibroids than those with inadequate levels. 18

Endocrinology Edit

Human reproductive biology is primarily controlled through hormones, which send signals to the human reproductive structures to influence growth and maturation. These hormones are secreted by endocrine glands, and spread to different tissues in the human body. In humans, the pituitary gland synthesizes hormones used to control the activity of endocrine glands. [3]

Reproductive systems Edit

Internal and external organs are included in the reproductive system. There are two reproductive systems including the male and female, which contain different organs from one another. These systems work together in order to produce offspring. [4]

Female reproductive system Edit

The female reproductive system includes the structures involved in ovulation, fertilization, development of an embryo, and birth. [3]

Estrogen is one of the sexual reproductive hormones that aid in the sexual reproductive system of the female. [2]

Male reproductive system Edit

The male reproductive system includes testes, rete testis, efferent ductules, epididymis, sex accessory glands, sex accessory ducts and external genitalia. [3]

Testosterone, an androgen, although present in both males and females, is relatively more abundant in males. Testosterone serves as one of the major sexual reproductive hormones in the male reproductive system However, the enzyme aromatase is present in testes and capable of synthesizing estrogens from androgens. [2] Estrogens are present in high concentrations in luminal fluids of the male reproductive tract. [5] Androgen and estrogen receptors are abundant in epithelial cells of the male reproductive tract. [6]

Animal reproduction occurs by two modes of action, including both sexual and asexual reproduction. [1] In asexual reproduction the generation of new organisms does not require the fusion sperm with an egg. [1] However, in sexual reproduction new organisms are formed by the fusion of haploid sperm and eggs resulting in what is known as the zygote. [1] Although animals exhibit both sexual and asexual reproduction the vast majority of animals reproduce by sexual reproduction. [1]

Gametogenesis is the formation of gametes, or reproductive cells.

Spermatogenesis Edit

Spermatogenesis is the production of sperm cells in the testis. In mature testes primordial germ cells divide mitotically to form the spermatogonia, which in turn generate spermatocytes by mitosis. [7] Then each spermatocyte gives rise to four spermatids through meiosis. [7] Spermatids are now haploid and undergo differentiation into sperm cells. [7] Later in reproduction the sperm will fuse with a female oocyte to form the zygote.

Oogenesis Edit

Oogenesis is the formation of a cell who will produce one ovum and three polar bodies. [7] Oogenesis begins in the female embryo with the production of oogonia from primordial germ cells. Like spermatogenesis, the primordial germ cell undergo mitotic division to form the cells that will later undergo meiosis, but will be halted at the prophase I stage. [7] This is known as the primary oocyte. Human females are born with all the primary oocytes they will ever have. [7] Starting at puberty the process of meiosis can complete resulting in the secondary oocyte and the first polar body. [7] The secondary oocyte can later be fertilized with the male sperm.

Methods of Contraception (With Diagram) | Reproductive Health | Biology

The contraceptive methods may be broadly grouped into two classes – the spacing or temporary methods and the terminal or permanent methods.

Contraceptive methods are preventive methods to help women avoid unwanted pregnancies. The term contraception and fertility control are not synonymous. Fertility control includes fertility inhibition or contraception and fertility stimulation. Contraception includes temporary and permanent measures to prevent pregnancy.

The method of contraception by men and women helps families space births, prevents unwanted pregnancy, and in the case of consistent condom use, prevents transmission of sexually transmitted infections (STIs), including HIV.

An ideal contraceptive should be safe, effective, acceptable, inexpensive, reversible, simple to administer, long lasting and should require minimal medical supervision. But there can never be an ideal contraceptive. This is because what is suitable to one group may be unsuitable to another due to different cultural patterns, religious beliefs and socio-economic milieu.

So the search for an ideal contraceptive has been given up. The present approach in family planning is to provide the user a variety of choice from which he may choose according to his needs and wishes and to promote family planning as a way of life. When properly provided and used, currently available contraceptives are safe and effective for the vast majority of users.

i. Spacing or Temporary Methods:

The spacing methods are commonly used to postpone or space births. These methods are commonly used by couple who do not desire to have children.

The temporary methods are of the following types:

The aim of the barrier methods is to prevent the live sperm from meeting the ovum. Barrier methods suitable for both men and women are available. They have both contraceptive and non- contraceptive advantages.

The main contraceptive advantage is the absence of side effects and the non-contraceptive advantage is the protection from sexually transmitted diseases or STD. These methods prevent sperm deposition in the vagina or prevent sperm penetration through the cervical canal. This is achieved by mechanical devices or by chemical or combined means.

i. Physical or Mechanical Methods:

The mechanical methods include the condoms for males and diaphragm for females. Condoms are the most widely used barrier device by males and popularly known as ‘Nirodh’ which in Sanskrit means prevention. The condom prevents the semen from being deposited in the vagina and protects both men and women from STD. The diaphragm cervical caps and vaults block the entry of sperm. But they are not popular methods.

Various spermicidal agents are available in the market either in the form of spermicidal cream, jelly or tablets. These agents produce sperm immobilisation and kill the sperms. But these are not popular and the failure rate is quite high. Also, concern about possible teratogenic effects on fetuses has been suggested. There may be occasional local allergic reactions in the vagina.

b. Natural Contraception:

The rhythm method of natural contraception is a method based on identification of the fertile period (between the 13th and 18th days of the menstrual cycle) of a cycle and to abstain from sexual intercourse during that period. Withdrawal or coitus interrupts is another natural method in which the make withdraws his penis from the vagina just before ejaculation so as to avoid insemination.

Lactational amenorrhea (absence of menstruation) method is based on the fact that ovulation does not occur during the period of intense lactation after parturition. But this method is effective only for six months. During this method, no contraceptive devices or chemicals are required. But the failure rate is high.

c. Intra-Uterine Contraceptive Devices or IUCD:

The IUCD is a small device usually made of polyethylene or other polymers. It is inserted into the uterus by a doctor. There are two types of IUCD – the medicated and the non-medicated. The medicated IUD release either metal ions (copper) or hormones (progesterone) gradually into the female’s body. Copper T 200 containing a copper wire, progestasert containing progesterone are some examples of IUCD. The mode of action is not clear, probably it produces non-specific biochemical and histological changes in the endometrium and the ionised copper has spermolytic and gametotoxic effects.

These changes impair the viability of the gamete and reduce the chances of fertilisation rather than its implantation. The copper ions may affect sperm motility, capacitation and survival. The hormone releasing devices increase the viscosity of the cervical mucous and thereby prevent the sperm from entering the cervix. The high levels of progesterone and relatively low levels of oestrogen make it unfavourable for implantation. The IUCD should be changed every three years.

The advantages of IUCD are – simplicity in insertion insertion takes only a few minutes inexpensive virtually free of metabolic side effects associated with hormonal pills and reversibility to fertility is immediate soon after removal.

The disadvantages are heavy and irregular menstruation and pelvic inflammatory diseases. Pelvic inflammatory disease (PID) is a collective term that includes acute and chronic conditions of the ovaries, ducts, uterus and it occurs as a result of infection. Women with PID suffer from vagina discharge, pelvic pain, tenderness, abnormal bleeding, chills and fever. Some women suffer uterine perforations leading to inflammatory response in the abdomen.

d. Hormonal Contraceptives:

Hormonal contraceptives are the most effective spacing methods of contraception. The gonadal steroids, oestrogen and progesterone are effective in contraception. The synthetic oestrogens used in contraception are ethinyl oestradiol and mestranol. Synthetic progesterones include three groups – pregnanes, oestranes and gonanes. The hormonal contraceptives currently in use are summarised in Table 3.

ii. Terminal or Permanent Methods of Contraception:

Permanent surgical contraception is a surgical method where by the reproductive function of an individual male or female is permanently destroyed. Surgical method is also known as sterilisation. The surgery done on males is vasectomy and on females is tubectomy.

Sterilisation in males is done by vasectomy (Fig. 1). It is a permanent sterilisation done in males where a segment of vas deferens of both the sides is cut and the cut ends are ligated. The technique is simple and can be performed even in primary health centres by trained doctors under local anesthesia. The failure rate is minimal and it is a very reliable method of contraception but it is an irreversible operation. So it is very important to get the consent of the person undergoing vasectomy.

b. Female Sterilisation:

Occlusion of the fallopian tubes is the underlying principle to achieve female sterilisation. This is known as tubectomy (Fig. 2). Female sterilisation can be done by two procedures — laproscopy and minilaparotomy. The technique of female sterilisation through abdominal approach with a special instrument called laproscope is known as laproscopy.

The minilaparotomy is a modification of abdominal tubectomy and requires a small incision in the abdomen and is conducted under local anaesthesia. It is a suitable procedure at the primary health centre and very safe, effective and easy also.

Hormones of the Testes


Testicular hormones are essential regulators of the endocrine and reproductive systems . The hormones produced by the testis are classified into two major groups, the steroid hormones, in particular testosterone, and the peptides hormones, those of activin/inhibin family. Both groups play divergent roles in the human physiology and provoke their actions within the testis, intratesticular, and outside of the organ, extragonadal. This chapter presents a current state of knowledge of the variety and roles of testicular hormones with a particular focus on their synthesis, secretion, metabolism, mechanism of action, and physiologic functions.

Watch the video: Embryology. Development of Reproductive System (August 2022).