10.2.2: Specialized Cells - Biology

10.2.2: Specialized Cells - Biology

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Specialized Cells From the Protoderm

Guard Cells

Stomata are pores (holes) in the epidermis of plants. Guard cells are the pairs of cells, shaped a bit like parentheses or two sides of a donut, that flank the stoma. The guard cells regulate when the stoma is open or closed, which in turn regulates gas exchange with the environment and the rate of transpiration.

Figure (PageIndex{1}): This image is of an epidermal peel. The epidermal cells look like transparent, interlocking puzzle pieces. Interspersed between these cells, there are stomata with guard cells. Each stoma looks a bit like a mouth, with one guard cell acting as the upper lip and another guard cell acting as the lower. Chloroplasts are visible within the guard cells. Photo by Melissa Ha, CC BY-NC .

Figure (PageIndex{2}): There are two images above. The one on the left shows an open stoma, while the one on the right shows a closed stoma. Photo by Melissa Ha, CC BY-NC.


Trichomes are hairs composed of cells on the epidermis of a plant.

Figure (PageIndex{2}): Trichomes on the underside (abaxial) of a leaf (left) and on a leaf margin (right). The trichomes in the photo on the left are long, fuzzy, and intertwining. The trichomes in the photo on the right are straight, separate from each other, and rigid-looking. The different appearance and texture of trichomes is one of the characteristics used to identify plants. Photos by Maria Morrow, CC BY-NC.

Figure (PageIndex{3}): These nettle trichomes are structured much like a hypodermic needle. The long silica tip breaks off to inject a cocktail of stinging chemicals into any animal that brushes by it. Photo by Maria Morrow, CC BY-NC.

Figure (PageIndex{4}): The ends of sundew (Drosera sp.) leaves are covered with glandular trichomes. Each trichome has a blob of sticky fluid at the tip. Insects are attracted to the fluid, becoming trapped in it, and the plant can slowly digest their bodies for the mineral nutrients that are lacking in their environment. Photo by Maria Morrow, CC BY-NC.

Figure (PageIndex{5}): The underside (abaxial surface) of this sage leaf has two types of trichomes: glandular trichomes and peltate trichomes. The glandular trichomes are in tufts on raised bumps of the epidermis. Scattered amongst them are small white pearl-like structures (one is indicated by a black arrow). These are the peltate trichomes, which are disc-like instead of hair-like. Photo by Maria Morrow, CC BY-NC.

Specialized Cells From the Ground Meristem

Two types of specialized sclerenchyma cells that can be produced by the ground meristem are sclereids and fibers.


Figure (PageIndex{6}): Sclereids are a type of sclerenchyma cell. These sclereids, called stone cells, are produced in the fruits of pears. In the image on the left, two stone cells are surrounded by many large parenchyma cells. There are plasmodesmata (not distinguishable) connecting the sclereids to these parenchyma cells. In the image on the right, there are 6 or 7 stone cells. They have a thick secondary wall with channels running through it (pits). These pits are the former locations of the plasmodesmata. Photos by Maria Morrow, CC BY-NC.


Figure (PageIndex{7}): Clusters of fibers in the secondary phloem of a Tilia stem. Phloem cells are parenchyma, so they are not as rigid as xylem cells. A way for plants to strengthen and protect phloem tissue is to include bundles of fibers. One of these bundles has been circled in the image. Photo by Maria Morrow, CC BY-NC.

Specialized Cells From the Procambium

Tracheids and Vessel Elements

Tracheids and vessel elements are cells in the xylem that transport water. They have secondary walls with lignin and are dead at functional maturity.

Figure (PageIndex{8}): All of these cells are dead and have secondary walls with lignin. The cell labeled A is a vessel element, with larger perforation plates on its ends and strange wall thickenings. B is a tracheid, thinner than a vessel element with rows of holes lining its sides (no perforation plates). C is a fiber, long and thin. Photo by Maria Morrow, CC BY-NC.

Figure (PageIndex{9}): Conducting cells from the xylem. The first photo shows a vessel element with large openings at the ends. The second photo shows a tracheid, with tapered ends and sets of bordered pits along the length of it. Photos by Maria Morrow, CC BY-NC.

Figure (PageIndex{10}): A longitudinal section through pine wood. "Gymnosperms like Pinus rely entirely on tracheids for water flow and mechanical support. True xylem vessels and wood parenchyma are lacking. While fibers are generally absent older stems may contain some fiber tracheids. The uniform wood of Pinus consists almost entirely of longitudinally oriented bands of narrow, pale staining tracheids interrupted by short bands of horizontally oriented xylem rays (indicated by box). Xylem rays, usually one cell thick and a few cells high, are composed of clear, thin walled parenchyma cells. The lumen of many tracheids is crossed by branched trabeculae. Tracheid side walls contain large circular bordered pits (indicated by arrow) that function to move water from tracheid to tracheid while preventing air flow from embolized tracheids. The membrane of each bordered pit is marked by a pale, thin, porous outer margo and a thicker, darker, inner torus. Large horizontally oriented resin ducts are lined with living secretory parenchyma that produce resins and many toxic terpenes including turpentines." Image and caption text from Berkshire Community College Bioscience Image Library, CC0, via Wikimedia Commons. Labels added by Maria Morrow.

Figure (PageIndex{11}): A cross section through a vascular bundle of Helianthus, 400x. "The highly lignified cells walls of xylem (black arrows indicate vessel elements) and mature sclerenchyma (white arrow indicates fibers) are stained red orange. These cells are dead at maturity and can also be distinguished by a heavy cell wall and absence of cytoplasm." Image and caption text from Berkshire Community College Bioscience Image Library, CC0, via Wikimedia Commons. Labels added by Maria Morrow.

Sieve cells, Sieve Tube Elements, and Companion Cells

Figure (PageIndex{12}): Longitudinal section through phloem. Most of the cells are sieve tube elements. The cells have sieve plates at the ends (marked by arrows). In the event of an injury, P-protein rushes to form a slime plug (B) and close the wound. Sieve tubes are alive but contain very little cell contents, they do not even have a nucleus. Instead, they are controlled by smaller companion cells (A). Photo by Berkshire Community College Bioscience Image Library, CC0, via Wikimedia Commons. Labels added by Maria Morrow.

Figure (PageIndex{13}): A cross section through a vascular bundle with the phloem circled (white). Within the phloem, there are two types of specialized parenchyma cells: the larger sieve tube elements (black arrow with white border) and the much smaller companion cells (white arrow with black border). Photo by Berkshire Community College Bioscience Image Library, CC0, via Wikimedia Commons. Labels added by Maria Morrow.


Content by Maria Morrow, CC BY-NC

The salamander that eats its siblings’ arms could one day help you grow a new one

Imagine you’re a smiley-faced, feathery-gilled Mexican salamander called an axolotl . You’ve just been born, along with hundreds of brothers and sisters. But salamanders like you live in the wild only in one lake near Mexico City , and that habitat isn’t big enough for all of you. There’s not enough food. Only the strongest can survive. What do you do?

If you’re an axolotl, you have two choices—eat your siblings’ arms, or have your arms eaten.

But even if you are the unfortunate victim of this sibling violence, not all hope is lost. In a few months, you’ll grow a whole new arm—bones, muscle, skin, nerves and all.

“It’s pretty gruesome, but cannibalism is a possible reason why they grow their arms back,” says associate biology professor James Monaghan . His lab studies regeneration in axolotls, a peculiar species that can grow back limbs and other organs to various degrees.

“When an injury occurs, some cues are released in that animal that tells cells near the injury to go from a resting state into a regenerative state,” Monaghan says.

His lab is trying to figure out what those cues are, and how we might induce that response in humans, who have very limited regenerative abilities.

“Humans are notoriously bad at regenerating,” Monaghan says. After we’re done growing, the genes that tell our cells to grow new organs are turned off.

“That’s a good thing because otherwise it’d be chaos,” he says. No one wants to spontaneously grow an extra finger.

“Axolotls can turn back on those genes that we turn off permanently,” Monaghan says.

This researcher's risky idea could mean big things for regenerative medicine

Understanding the specific mechanisms that induce regenerative responses in axolotls is no small task since axolotls have the largest genome ever sequenced .

So far, the lab has identified one molecule, neuregulin-1, which is essential for regeneration of limbs , lungs , and possibly hearts.

“When we removed it, regeneration stopped. And when we added it back in, it induced the regenerative response,” Monaghan says. “I’m not saying it’s a golden bullet for inducing regeneration in humans, too, but it could be part of the puzzle.”

A lot of researchers study limb regeneration in axolotls. But Monaghan’s lab is interested in extending this research to other organs, as well.

“When you think of the human condition, most of our issues with disease are with internal organs,” Monaghan says.

Take retina regeneration, for example. Monaghan says we can either learn the process axolotls undergo that allows their specialized cells to return back to developmental cells, and then mimic that process in human eyes. Or, we can learn which elements of the axolotl enable their cells to behave this way, and then add those elements to human stem cell therapy.

To test the latter, Monaghan has teamed up with a Northeastern associate professor of chemical engineering, Rebecca Carrier , and her lab to figure out the best way to transplant mammalian retinal cells using molecules found in the axolotl.

In the experiment, Monaghan and Carrier used pig eyes, which are similar to human eyes. When they transplanted stem cells from the retina of one pig into the retina of another, 99 percent of the transplanted cells died. “Something’s missing,” Monaghan says. “The cells don’t have the right cues.”

But when Carrier and Monaghan injected those same pig stem cells into the axolotl eye, fewer cells died. “They were much happier,” Monaghan says. “There’s something in the axolotl retina that the mammalian cells like.”

This salamander can regenerate limbs like Deadpool. Can it teach us to do the same?

One reason axolotls are so good at receiving transplants is because, unlike humans, they don’t have a learned immune system, meaning they can’t distinguish between themselves and foreign entities.

“It’s really easy to do grafts between animals because the axolotls can’t tell that the new tissue isn’t theirs,” he says. “They don’t reject it like we might.”

An obvious example of this can be seen in axolotls that are genetically modified with a green fluorescent protein found in jellyfish. These naturally white axolotls glow neon green in certain lighting.

“With this we can ask really basic questions, like do cells change their fate when they participate in regeneration?” Monaghan says.

For example, if Monaghan grafts muscle tissue from a green fluorescent animal onto a white axolotl and then that axolotl regenerates, does the axolotl grow green muscle? Do its bones glow green, too? What about its skin?

Researchers have found, however, that cells don’t actually change. Green muscle yields green muscle only.

The axolotl isn’t the only animal that can regrow organs. Starfish, worms, frogs, and other species of salamanders can also regenerate. But axolotls are special because, unlike other animals, they can regrow organs that are just as robust as the originals, no matter how old they get.

For example, tadpoles can regenerate limbs. But once they undergo metamorphosis and become frogs, “they can only regrow a spike,” Monaghan says. “They lose the ability to grow back their digits.”

The axolotl’s ability to fully regrow organs, even as it ages, could be partially due to its perpetual juvenile state. Axolotls, unlike most other amphibians, don’t undergo metamorphosis naturally, which means they never technically reach adulthood, even though they can reproduce. This condition is called neoteny.

“Axolotls come from a species that used to walk on land,” Monaghan says. They do have legs, after all. “But some mutation occurred that keeps them in the lake and from reaching adulthood.”

To test whether their neotenic state is responsible for their ability to regenerate, Monaghan took a group of axolotl siblings and induced metamorphosis in one half by exposing them to thyroid hormones, a chemical that flips on the maturity switch in these amphibians. The other half was kept in the juvenile state.

In the experiment, the juveniles regenerated normally, but all of their adult siblings regenerated slower than usual, and had deformities in their regrown limbs.

“There is some association with neoteny and the ability to regenerate,” Monaghan says. “But it’s not the main factor.”

That main factor is yet to be discovered. But even though some of this might sound like science fiction, “you already made an arm once,” Monaghan says. “If we could just learn how to turn back on those programs, our bodies might do the rest of the work.”

What is Cell Specialization?

As alluded to earlier, multi-cellular cells are composed of two or more cells that may have different forms, structure, function, and organization.

Also referred to as cell differentiation, cell specialization is the process wherein “general” or “common” cells evolve to form specific cells that have specific functions. This process is very much prevalent and most important during embryological development. During adulthood, cells called stem cells become specialized in order to replace old and worn-out cells.

Roles of DNA and RNA in Cell Differentiation

Dexoyribonucleic Acid, or DNA, controls the way cells function. It also determines what type of specialized cells will be made. Stem cells are cells that have the ability to become any type of specialized cell in the body. After an egg cell and sperm cell unite to begin forming a new organism, all of the DNA in each cell of that organism will be virtually identical. If every part of the DNA in each cell is the same, then how do cells become different types of cells? Let’s look more closely at DNA to find out.

DNA is wound tightly into chromosomes. Different regions of the chromosome code for every different function and cell type. Not all sections of a chromosome are turned on, or expressed, at the same time. Only the regions that are needed to perform a specific function are expressed in each cell. These regions are often depicted as bands or stripes on a drawing of a chromosome. These bands are called genes, and whether or not a gene is expressed determines what type of cell will be created. For example, genes that are expressed (turned on) in a nerve cell are different from the genes that are expressed in a muscle cell. Both cells have the same DNA, but expressing different genes generates different cell types.

This process by which information from a gene is used to make the structures of a cell is called gene expression. Since RNA translates and transcribes the DNA code into proteins (the structures of a cell), it also plays a role in cell differentiation.

Stem cells are defined as precursor cells that have the capacity to self-renew and to generate multiple mature cell types. Only after collecting and culturing tissues is it possible to classify cells according to this operational concept. This difficulty in identifying stem cells in situ, without any manipulation, limits the understanding of their true nature. This review aims at presenting, to health professionals interested in this area, an overview on the biology of embryonic and adult stem cells, and their therapeutic potential.

All the authors declared no competing interests.

TO CITE THIS ARTICLE: Chagastelles PC, Nardi NB. Biology of stem cells: an overview. Kidney inter., Suppl. 2011 1: 63–67.


In recent decades, the skin of sharks has achieved a certain biomimetic status among both science popularizers and in research circles for the notion that the specialized skin surface structure could reduce drag and enhance the efficiency of locomotion. Manufactured body suits have been loosely modeled on shark skin with various ridges and dents, to induce surface roughness, that purportedly enhance swimming performance in humans, and researchers have long suspected that the special surface structure of shark skin contributes to the efficiency of locomotion [shark skin structure has been comprehensively reviewed (e.g. Applegate, 1967 Lang et al., 2008 Reif, 1982 Reif, 1985) also see images in Castro (Castro, 2011)].

A variety of ‘shark-inspired’ engineered materials have also been produced to reduce drag when applied to the surface of submerged bodies. For example, riblets are fine rib-like surface geometries with sharp surface ridges that can be aligned either parallel or perpendicular to the flow direction and might reduce drag. A diversity of riblet shapes and sizes has been investigated experimentally and theoretically (Bechert and Bartenwerfer, 1989 Bechert et al., 2000 Bechert et al., 1997 Büttner and Schulz, 2011 Koeltzsch et al., 2002 Luchini et al., 1991 Luchini and Trombetta, 1995 Neumann and Dinkelacker, 1991), and drag reduction of stiff bodies covered with riblet material has been shown to occur (Bechert et al., 1997 Bechert et al., 1985 Dinkelacker et al., 1987). Experiments with an adjustable surface with longitudinal blade ribs and slits revealed the highest stiff-body drag reduction of 9.9%, with a groove depth of half the size of lateral riblet spacing (Bechert et al., 1997). Scalloped riblets, somewhat similar to the ridges in shark denticles, produce a maximal stiff-body drag reduction of approximately 7% (Bechert et al., 1985).

A silicone-replica of the skin of the copper shark Carcharhinus brachyurus attached to a rigid flat plate resulted in a drag reduction of 5.2–8.3% compared with that of smooth silicone on a flat plate (Han et al., 2008). A hard plastic shark skin replica achieved a drag reduction of 3% (Bechert et al., 1985). But these cases involved study of a rigid body covered with a biomimetic skin, which is not the situation for a shark in vivo, where body undulations can greatly alter the structure of surface ornamentation and change flow characteristics over the skin. In addition, a variety of tests with the Speedo® FS II swimsuit ‘shark-like’ material resulted in a 7.7% (Benjanuvatra et al., 2002) and 10–15% (Mollendorf et al., 2004) reduction in the stiff-body drag compared with that of normal swimsuits under certain conditions, but other studies or tests showed no significant drag reduction (Benjanuvatra et al., 2002 Toussaint et al., 2002).

Because sharks are self-propelled deforming bodies, thrust and drag forces are hard to decouple (Anderson et al., 2001 Schultz and Webb, 2002 Tytell, 2007 Tytell et al., 2010), which makes it difficult to isolate drag forces alone during normal free-swimming locomotion to assess the effect of surface ornamentation. In order to investigate the possible drag-reducing properties of surface ornamentation such as shark skin denticles or various biomimetic products (or whether surface structures might possibly enhance thrust), it is necessary to use a study system that permits (1) the use of self-propelling bodies possessing different surface ornamentations, where thrust and drag forces are naturally balanced throughout an undulatory cycle, (2) accurate measurement of self-propelled swimming (SPS) speed so that the swimming performance of different surfaces can be compared statistically, (3) the imposition of different motion programs so that the effect of moving the ornamented surfaces in different manners can be assessed, and (4) various experimental manipulations of surface structure to test directly the hypothesis that it is the surface ornamentation alone that causes drag reduction and hence increased swimming speed.

In this study, we use a robotic flapping foil device to test the effect of shark skin surface ornamentation and two biomimetic surfaces on SPS speed. The flapping foil robotic device was developed for the study of fish-like self-propulsion in both rigid and flexible foils and allows accurate measurement of free-swimming speeds, the production of controlled motion programs to move foils under a variety of heave and pitch conditions (Lauder et al., 2007 Lauder et al., 2011a Lauder et al., 2011b) and quantification of flow over the foil surface using digital particle image velocimetry (DPIV). We make foils that are both rigid and flexible out of fresh shark skin and also study the propulsion of two manufactured shark skin mimics. We directly test the hypothesis in each case that surface ornamentation produces an increase in swimming speed by comparing against a control condition with reduced or absent ornamentation.

Interactive resources for schools

Polymerase chain reaction

PCR is a series of temperature-controlled reactions which enable us to amplify a very tiny sample of DNA, producing enough material for it to be analysed or used in DNA profiling.

Genetic engineering

Genetic engineering involves changing the DNA of an organism, usually by deleting, inserting or editing a gene to produce desired characteristics.


The use of biological organisms or enzymes to create, break down or transform a material.

Stem cell

Cells which can divide repeatedly without becoming differentiated and have the capacity to develop into a diverse range of specialised cell types.


A list of often difficult or specialised words with their definitions.

The basic unit from which all living organisms are built up, consisting of a cell membrane surrounding cytoplasm and a nucleus.

What can stem cells do?

Stem cells exist naturally in the body to replace damaged cells and tissues. Research into stem cells could allow:

  • the development of a treatment that allows people paralysed because of spinal injuries to walk again
  • the possibility of growing replacement organs that are matched to the patient
  • genetic disorders to be studied and new drugs to treat them tested.

None of these are readily available yet - but trials are underway to investigate whether they are possible.


Click here to download the Stem Cell Research poster shown below.

To order a free set of ABPI schools posters on the following topics: biotechnology, cloning, genetic engineering, unravelling the genome, polymerase chain reaction and stem cells, please fill in our order form.

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There are a number of interactive features in this e-source:

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Follow us on twitter. We use cookies on our website.By using our website you consent to our use of cookies in accordance with our Cookie & Privacy Policy. The Association of the British Pharmaceutical Industry ABPI Resources for Schools, ABPI, 7th floor, Southside, 105 Victoria Street, London, SW1E 6QT Analytical balances have finer readability, are much more sensitive to changes, and can detect smaller variations in mass than toploading balances. Semi-micro and microbalances are part of this category with much smaller capacities and higher resolutions. Moisture balances measure the amount of liquid in a substance and are often used in food testing. They weigh the existing item or product, apply heat to evaporate any moisture, and re-weigh to provide the data used to calculate the moisture content.


A cell is a collection of biological matter enclosed by a membrane. Cells are the basic unit of all forms of life.

Cell Theory

Cell Theory is a widely accepted theory which describes properties of cells. It is based on several key points:

  • All living things are composed of at least one cell
  • Cells are the most basic unit of life
  • All cells are produced from pre-existing cells

Prokaryotic Cells

Prokaryotes are cells whose genetic material is not contained within a nucleus and lack membrane-bound organelles. Despite lacking membrane-bound organelles, some prokaryotes contain protein-based microcompartments, which are believed to serve as primordial organelles. Prokaryotes can be split into two domains: bacteria and archaea. Prokaryotes reproduce asexually by means of binary fission.


Bacteria are one of two domains of single-celled prokaryotes. Being some of the first known life forms on Earth, bacteria constitute a large chunk of Earth's biomass, and are found in an extremely wide range of environments. Bacteria reproduce asexually by means of binary fission. Some bacteria are autotrophic and obtain their energy by chemosynthesis or photosynthesis, while others are heterotrophic and break down organic matter as a source of energy. While some bacteria are pathogenic and cause disease in organisms, many are mutualistic and carry out extremely important biological processes, such as nitrogen fixation.


Archaea are one of two domains of single-celled prokaryotes. Archaea share some characteristics with bacteria and eukaryotes, but also have unique characteristics of their own, such as their cell wall structure. Archaea utilize a variety of different energy sources, some undergoing forms of photosynthesis, while others being chemoautotrophic. No archaea have been distinctly identified as directly causing disease, and many are known to be commensalistic or mutualistic. Although many archaea have been identified in seemingly inhospitable environments, such as volcanic hot springs, many also inhabit much more favorable environments, such as oceans, marshes, and even the human body.

Eukaryotic Cells

Eukaryotic cells are cells whose genetic material is contained within a nucleus. In addition, eukaryotic cells also almost always contain membrane-bound organelles, such as mitochondria. Organisms comprised of eukaryotic cells are called eukaryotes. All multicellular organisms are eukaryotes, and some unicellular organisms are eukaryotes. Eukaryotic cells divide either by mitosis or meiosis. Likewise, eukaryotic cells are usually a lot bigger than prokaryotic cells.


Cytoplasm refers the portion of a cell outside the nucleus that's enclosed within the cell membrane. Since prokaryotes don't have a nucleus, all of their contents are considered part of the cytoplasm. The materials composing the cytoplasm can be split into three main categories organelles, cytosol, and small, insoluble particles called inclusions. Cytosol refers to the area of the cytoplasm not confined within an organelle, and is made of water, organic molecules, and salts. The cytosol also includes the cytoskeleton and other small structures (e.g., ribosomes, etc.). In cell division, the cytoplasm is split between the two daughter cells during cytokinesis.


Organelles are specialized structures within a cell. While most eukaryotic cells contain a multitude of organelles, prokaryotes only sometimes contain protein-based microcompartments.

  • Cell Wall: The cell wall is a tough layer that surrounds plant, fungi, and prokaryotic cells. It is composed of a different material depending on the organism - in plants it is made of cellulose, while in fungi it is made of chitin and in bacteria it is composed of peptidoglycan. It functions to provide protection and support, as well as preventing the cell from getting too large.
  • Cell Membrane: The cell membrane is composed of two layers of phospholipids with proteins embedded in it. It is selectively permeable, protecting the cell from its surroundings and only allowing certain molecules into the cell.
  • Nucleus: The nucleus is a compartment surrounded by two membranes known as the nuclear envelope. These membranes regulate transport in and out of the nucleus, as it contains the genetic material of the cell (DNA). RNA transcription is also performed within the nucleus, as well as the control of other activities within the cell.
  • Nucleolus: The nucleolus is a body located within the nucleus where ribosome synthesis takes place. It is not bound by a membrane.
  • Endoplasmic Reticulum (ER): The ER takes two forms: smooth and rough. In the rough ER, ribosomes are found in the outside that perform protein synthesis. In the smooth ER, molecules such as hormones and lipids are synthesized. Ultimately, the ER serves to produce molecules and transport chemicals between cells as a part of the endomembrane system.
  • Ribosome: The ribosome is made of a special type of RNA known as rRNA. It serves to synthesize proteins for use in the body.
  • Golgi Apparatus: The Golgi apparatus is made of sacs of unit membrane known as cisternae. Its function is to modify proteins and package them within vesicles to be transported elsewhere in the body.
  • Lysosome: The lysosome is an organelle containing enzymes that use water to break chemical bonds. These enzymes are known as hydrolytic enzymes, and are used in order to break down or recycle certain parts of the cell.
  • Mitochondrion: The mitochondria are composed of double membranes, where the inner membrane is folded up to form a structure known as cristae. Cellular respiration occurs in the mitochondria.
  • Chloroplast: Chloroplasts are composed of a double layer of membrane, where the inner membrane forms layers known as grana. There is a high concentration of chlorophyll in the grana, used to perform photosynthesis.

Movement Through Cell Boundaries

Cell Division

Cell division is a part of the cell cycle in which a parent cell's growth stops and it separates into two or more daughter cells. In prokaryotes, it occurs through the process of binary fission, in which the cell nearly doubles in size, replicates its DNA, and divides in half. Although binary fission does not involve the exchange or recombination of genetic information, many bacteria exchange genetic information through a process called conjugation. In eukaryotes, cell division occurs through both mitosis and meiosis, described in more detail below.


Mitosis is a form of cell division in which after a parent cell replicates its genetic information, the parent cell splits to form two identical daughter cells. Because the two daughters are genetically identical to the parent cell, mitosis is essential for growth, development, and the replacement of cells in multicellular organisms. Additionally, asexually reproducing eukaryotes reproduce through mitosis (e.g., if an organism reproduces through asexual budding, the cells that comprise the mass that will become the new organism reproduce through mitosis). The process of mitosis can be defined in several distinct phases:

  • Prophase-Chromatin condenses into chromosomes and the spindle apparatus is synthesized. In animal cells, centrosomes (a pair of centrioles surrounded by proteins) organize the spindle apparatus, while in plant cells the nuclear envelope serves as the primary organizer of the spindle apparatus.
  • Prometaphase-Nuclear envelope disintegrates and spindle apparatus attaches to the chromosomes.
    • Note that prometaphase is not always recognized, sometimes being included as part of prophase. It is also sometimes referred to by a different term, such as late prophase.

    In the final stage of cell division, known as cytokinesis, the cytoplasm of the parent cell is divided between the daughter cells as it splits to become the two new daughter cells. Once this separation occurs, the process of cell division is complete.


    Meiosis is a form of cell division in which after a parent replicates its genetic information, it splits to form two daughter cells, and those daughter cells split once more without replicating its genetic information, resulting in four cells with only half the number of chromosomes as the original parent cell. Those cells produced through meiosis are referred to as haploid cells, as they contain a single set of chromosomes, as opposed to cells which contain two pairs of chromosomes, which are known as diploid cells. For example, most human cells are diploid and contain 23 pairs of chromosomes, or 46 total, while sperm and egg cells contain only one pair of 23 chromosomes and are therefore haploid cells.

    The process by which the original parent cell and its daughter cells divide in meiosis are the same as in mitosis, going through prophase, prometaphase, metaphase, anaphase, and telophase in respective order. Note that during meiosis when the original parent cell divides, each phase is referred to as (name of stage) I, such as prophase I, prometaphase I, and so forth, and when the daughter cells of the original parent cell split into 4 haploid cells, each phase is referred to as (name of stage) II, such as prophase II, prometaphase II, and so forth.

    Why It Matters

    Stem cells are being used in many laboratories today for research into the causes of and treatments for ALS. Most commonly, researchers use iPSCs to make a unique source of motor neurons from individual ALS patients to try to understand why and how motor neurons die in ALS. Two types of motor neurons are affected in ALS are upper coriticospinal motor neurons, that when damaged, cause muscle spasticity (uncontrolled movement), and lower motor neurons, that when damaged, cause muscle weakness. Both types can be made from iPSCs to cover the range of pathology and symptoms found in ALS. Astrocytes, a type of support cell, called glia, of the central nervous system (CNS), are also being generated from iPSCs. It is well established that glia play a role in disease process and contribute to motor neuron death.

    Motor neurons created from iPSCs have many uses. The availability of large numbers of identical neurons, made possible by iPSCs, has dramatically expanded the ability to search for new treatments. For example, they can also be used to screen for drugs that can alter the disease process. Motor neurons derived from iPSCs can be genetically modified to produce colored fluorescent markers that allow clear visualization under a microscope. The health of individual motor neurons can be tracked over time to understand if a test compound has a positive or negative effect.

    Because iPSCs can be made from skin samples or blood of any person, researchers have begun to make cell lines derived from dozens of individuals with ALS. One advantage of iPSCs are that they capture a person’s exact genetic material and provide an unlimited supply of cells that can be studied in a dish, which is like person’s own avatar. Comparing the motor neurons derived from these cells lines allows them to ask what is common, and what is unique, about each case of ALS, leading to further understanding of the disease process. They are also used to correlate patients’ clinical parameters, such as site of onset and severity with any changes in the same patient’s motor neurons.

    Stem cells may also have a role to play in treating the disease. The most likely application may be to use stem cells or cells derived from them to deliver growth factors or protective molecules to motor neurons in the spinal cord. Clinical trials of such stem cell transplants are in the early stages, but appear to be safe. In addition, transplantation of healthy astrocytes have the potential to be beneficial in supporting motor neurons in the brain and spinal cord.

    While the idea of replacing dying motor neurons with new ones derived from stem cells is appealing, using stem cells as a delivery tool to provide trophic factors to motor neurons is a more realistic and feasible approach. The significant challenge to replacing dying motor neurons is making the appropriate connections between muscles and surrounding neurons.

    Isolation of IPSCs from people with ALS in clinical trials is extremely valuable for the identification of unique signatures in the presence or absence of a specific treatment approach and as a read out to test whether a drug or test compound has an impact on the health of motor neurons and/or astrocytes. A positive result gives researchers confidence to move forward to more advanced clinical trials. For example, The ALS Association is currently funding a clinical trial to test the effects of retigabine on motor neurons, which use the enrolled patients’ individual iPSCs lines derived from collected skin samples and testing whether there is a change in the excitability of motor neurons in people with ALS. (see above).

    Watch the video: Specialised Cells BBC19LS02 (June 2022).


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