What are three ways growth may occur

List the different levels of the taxonomic classification system.

Biology 2e

Define histology.

Fundamentals of Anatomy & Physiology (11th Edition)

Define histology.

Fundamentals of Anatomy & Physiology Plus Mastering A&P with eText - Access Card Package (10th Edition) (New A&P Titles by Ric Martini and Judi Nath)

A fertilized mouse egg and a fertilized human egg are similar in size, yet they produce animals of very different sizes. What factors in the control of cell behavior in humans and mice are responsible for these size differences? The same fundamental question can be asked for each organ and tissue in an animal's body. What factors in the control of cell behavior explain the length of an elephant's trunk or the size of its brain or its liver? These questions are largely unanswered, at least in part because they have received relatively little attention compared with other questions in cell and developmental biology. It is nevertheless possible to say what the ingredients of an answer must be.

The size of an organ or organism depends mainly on its total cell mass, which depends on both the total number of cells and the size of the cells. Cell number, in turn, depends on the amounts of cell division and cell death. Organ and body size are therefore determined by three fundamental processes: cell growth, cell division, and cell death. Each is independently regulated—both by intracellular programs and by extracellular signal molecules that control these programs.

The extracellular signal molecules that regulate cell size and cell number are generally either soluble secreted proteins, proteins bound to the surface of cells, or components of the extracellular matrix. The factors that promote organ or organism growth can be operationally divided into three major classes:

Mitogens, which stimulate cell division, primarily by relieving intracellular negative controls that otherwise block progress through the cell cycle.

Growth factors, which stimulate cell growth (an increase in cell mass) by promoting the synthesis of proteins and other macromolecules and by inhibiting their degradation.

Survival factors, which promote cell survival by suppressing apoptosis.

Some extracellular signal molecules promote all of these processes, while others promote one or two of them. Indeed, the term growth factor is often used inappropriately to describe a factor that has any of these activities. Even worse, the term cell growth is often used to mean an increase in cell number, or cell proliferation.

In this section, we first discuss how these extracellular signals stimulate cell division, cell growth, and cell survival, thereby promoting the growth of an animal and its organs. We then consider how other extracellular signals can act in the opposite way, to inhibit cell growth or cell division or to stimulate apoptosis, thereby inhibiting organ growth.

Unicellular organisms tend to grow and divide as fast as they can, and their rate of proliferation depends largely on the availability of nutrients in the environment. The cells of a multicellular organism, however, divide only when more cells are needed by the organism. Thus, for an animal cell to proliferate, nutrients are not enough. It must also receive stimulatory extracellular signals, in the form of mitogens, from other cells, usually its neighbors. Mitogens act to overcome intracellular braking mechanisms that block progress through the cell cycle.

One of the first mitogens to be identified was platelet-derived growth factor (PDGF), and it is typical of many others discovered since. The path to its isolation began with the observation that fibroblasts in a culture dish proliferate when provided with serum but not when provided with plasma. Plasma is prepared by removing the cells from blood without allowing clotting to occur; serum is prepared by allowing blood to clot and taking the cell-free liquid that remains. When blood clots, platelets incorporated in the clot are triggered to release the contents of their secretory vesicles (Figure 17-40). The superior ability of serum to support cell proliferation suggested that platelets contain one or more mitogens. This hypothesis was confirmed by showing that extracts of platelets could serve instead of serum to stimulate fibroblast proliferation. The crucial factor in the extracts was shown to be a protein, which was subsequently purified and named PDGF. In the body, PDGF liberated from blood clots probably has a major role in stimulating cell division during wound healing.

PDGF is only one of over 50 proteins that are known to act as mitogens. Most of these proteins are broad-specificity factors, like PDGF and epidermal growth factor (EGF), that can stimulate many types of cells to divide. Thus, PDGF acts on a range of cell types, including fibroblasts, smooth muscle cells, and neuroglial cells. Similarly, EGF acts not only on epidermal cells but also on many other cell types, including both epithelial and nonepithelial cells. At the opposite extreme lie narrow-specificity factors such as erythropoietin, which induces the proliferation of red blood cell precursors only.

In addition to mitogens that stimulate cell division, there are factors, such as some members of the transforming growth factor-β (TGF-β) family, that act on some cells to stimulate cell proliferation and others to inhibit it, or that stimulate at one concentration and inhibit at another. Indeed, like PDGF, many mitogens have other actions beside the stimulation of cell division: they can stimulate cell growth, survival, differentiation, or migration, depending on the circumstances and the cell type.

In the absence of a mitogenic signal to proliferate, Cdk inhibition in G1 is maintained, and the cell cycle arrests. In some cases, cells partly disassemble their cell-cycle control system and exit from the cycle to a specialized, nondividing state called G0.

Most cells in our body are in G0, but the molecular basis and reversibility of this state vary in different cell types. Neurons and skeletal muscle cells, for example, are in a terminally differentiated G0 state, in which their cell-cycle control system is completely dismantled: the expression of the genes encoding various Cdks and cyclins are permanently turned off, and cell division never occurs. Other cell types withdraw from the cell cycle only transiently and retain the ability to reassemble the cell-cycle control system quickly and reenter the cycle. Most liver cells, for example, are in G0, but they can be stimulated to divide if the liver is damaged. Still other types of cells, including some lymphocytes, withdraw from and re-enter the cell cycle repeatedly throughout their lifetime.

Almost all the variation in cell-cycle length in the adult body occurs during the time the cell spends in G1 or G0. By contrast, the time taken for a cell to progress from the beginning of S phase through mitosis is usually brief (typically 12–24 hours in mammals) and relatively constant, regardless of the interval from one division to the next.

For the vast majority of animal cells, mitogens control the rate of cell division by acting in the G1 phase of the cell cycle. As discussed earlier, multiple mechanisms act during G1 to suppress Cdk activity and thereby hinder entry into S phase. Mitogens act to release the brakes on Cdk activity, thereby allowing S phase to begin. They do so by binding to cell-surface receptors to initiate a complex array of intracellular signals that penetrate deep into the cytoplasm and nucleus (discussed in Chapter 15). The ultimate result is the activation of G1-Cdk and G1/S-Cdk complexes, which overcome the inhibitory barriers that normally block progression into S phase.

As we discuss in Chapter 15, an early step in mitogen signaling is often the activation of the small GTPase Ras, which leads to the activation of a MAP kinase cascade. By uncertain mechanisms, this leads to increased levels of the gene regulatory protein Myc. Myc promotes cell-cycle entry by several overlapping mechanisms (Figure 17-41). It increases the transcription of genes that encode G1 cyclins (D cyclins), thereby increasing G1-Cdk (cyclin D-Cdk4) activity. In addition, Myc increases the transcription of a gene that encodes a component of the SCF ubiquitin ligase. This mechanism promotes the degradation of the CKI protein p27, leading to increased G1/S-Cdk (cyclin E-Cdk2) activity. As discussed earlier, increased G1-Cdk and G1/S-Cdk activities stimulate phosphorylation of the inhibitory protein Rb, which then leads to activation of the gene regulatory protein E2F. Myc may also stimulate the transcription of the gene encoding E2F, further promoting E2F activity in the cell. The end result is the increased transcription of genes required for entry into S phase (see Figure 17-30). As we discuss later, Myc also has a major role in stimulating the transcription of genes that increase cell growth.

As we discuss in Chapter 23, many of the components of intracellular signaling pathways are encoded by genes that were originally identified as cancer-promoting genes, or oncogenes, because mutations in them contribute to the development of cancer. The mutation of a single amino acid in Ras, for example, causes the protein to become permanently overactive, leading to constant stimulation of Ras-dependent signaling pathways, even in the absence of mitogenic stimulation. Similarly, mutations that cause an overexpression of Myc promote excessive cell growth and proliferation and thereby promote the development of cancer.

Surprisingly, however, when Ras or Myc is experimentally hyperactivated in most normal cells, the result is not excessive proliferation but the opposite: the activation of checkpoint mechanisms causes the cells to undergo either cell-cycle arrest or apoptosis. The normal cell seems able to detect abnormal mitogenic stimulation, and it responds by preventing further division. Such checkpoint responses help prevent the survival and proliferation of cells with various cancer-promoting mutations.

Although it is not known how a cell detects excessive mitogenic stimulation, such stimulation often leads to the production of a cell-cycle inhibitor protein called p19 ARF, which binds and inhibits Mdm2. As discussed earlier, Mdm2 normally promotes p53 degradation. Activation of p19ARF therefore causes p53 levels to increase, thereby inducing either cell-cycle arrest or apoptosis (Figure 17-42).

How do cancer cells ever arise if these mechanisms block the division or survival of mutant cells with overactive proliferation signals? The answer is that the protective system is often inactivated in cancer cells by mutations in the genes that encode essential components of the checkpoint responses, such as p19ARF or p53.

Cell division is controlled not only by extracellular mitogens but also by intracellular mechanisms that can limit cell proliferation. Many animal precursor cells, for example, divide a limited number of times before they stop and terminally differentiate into permanently arrested, specialized cells. Although the stopping mechanisms are poorly understood, a progressive increase in CKI proteins probably contributes in some cases. Mice that are deficient in the CKI p27, for example, have more cells than normal in all of their organs because the stopping mechanisms are apparently defective.

The best-understood intracellular mechanism that limits cell proliferation occurs in human fibroblasts. Fibroblasts taken from a normal human tissue go through only about 25–50 population doublings when cultured in a standard mitogenic medium. Toward the end of this time, proliferation slows down and finally halts, and the cells enter a nondividing state from which they never recover. This phenomenon is called replicative cell senescence, although it is unlikely to be responsible for the senescence (aging) of the organism. Organism senescence is thought to depend, in part at least, on progressive oxidative damage to macromolecules, in as much as strategies that reduce metabolism (such as reduced food intake), and thereby reduce the production of reactive oxygen species, can extend the lifespan of experimental animals.

Replicative cell senescence in human fibroblasts seems to be caused by changes in the structure of the telomeres, the repetitive DNA sequences and associated proteins at the ends of chromosomes. As discussed in Chapter 5, when a cell divides, telomeric DNA sequences are not replicated in the same manner as the rest of the genome but instead are synthesized by the enzyme telomerase. By mechanisms that remain unclear, telomerase also promotes the formation of protein cap structures that protect the chromosome ends. Because human fibroblasts, and many other human somatic cells, are deficient in telomerase, their telomeres become shorter with every cell division, and their protective protein caps progressively deteriorate. Eventually, DNA damage occurs at chromosome ends. The damage activates a p53-dependent cell-cycle arrest that resembles the arrest caused by other types of DNA damage (see Figure 17-33).

The lack of telomerase in most somatic cells has been proposed to help protect humans from the potentially damaging effects of runaway cell proliferation, as occurs in cancer. Unfortunately, most cancer cells have regained the ability to produce telomerase and therefore maintain telomere function as they proliferate; as a result, they do not undergo replicative cell senescence (discussed in Chapter 23). The forced expression of telomerase in normal human fibroblasts, using genetic engineering techniques, has the same effect (Figure 17-43).

Normal rodent cells, by contrast, usually maintain telomerase activity and telomere function as they proliferate and therefore do not undergo this type of replicative senescence. When overstimulated to proliferate in culture, however, they frequently activate the p19ARF-dependent checkpoint mechanism described earlier and eventually stop dividing. Mutations that inactivate these checkpoints make it easier for rodent cells to proliferate indefinitely in culture. Such mutant cells are often described as “immortalized”. If cultured in optimal conditions that avoid the activation of checkpoint responses, however, at least some normal rodent cells also seem able to proliferate indefinitely. Nevertheless, rodents age much more rapidly than humans.

The growth of an organism or organ depends on cell growth: cell division alone cannot increase total cell mass without cell growth. In single-celled organisms such as yeasts, cell growth (like cell division) requires only nutrients. In animals, by contrast, cell growth and cell division both depend on signals from other cells.

The extracellular growth factors that stimulate cell growth bind to receptors on the cell surface and activate intracellular signaling pathways. These pathways stimulate the accumulation of proteins and other macromolecules, and they do so by both increasing their rate of synthesis and decreasing their rate of degradation.

One of the most important intracellular signaling pathways activated by growth factor receptors involves the enzyme PI 3-kinase, which adds a phosphate from ATP to the 3 position of inositol phospholipids in the plasma membrane. As discussed in Chapter 15, the activation of PI 3-kinase leads to the activation of several protein kinases, including S6 kinase. The S6 kinase phosphorylates ribosomal protein S6, increasing the ability of ribosomes to translate a subset of mRNAs, most of which encode ribosomal components. Protein synthesis therefore increases. When the gene encoding S6 kinase is inactivated in Drosophila, the mutant flies are small; whereas cell numbers are normal, cell size is abnormally small. Growth factors also activate a translation initiation factor called eIF4E, further increasing protein synthesis and cell growth (Figure 17-44).

Growth factor stimulation also leads to increased production of the gene regulatory protein Myc, which also plays an important part in signaling by mitogens (see Figure 17-41). Myc increases the transcription of a number of genes that encode proteins involved in cell metabolism and macromolecular synthesis. In this way, it stimulates both cell metabolism and cell growth.

Some extracellular signal proteins, including PDGF, can act as both growth factors and mitogens, stimulating both cell growth and cell-cycle progression. This functional overlap is achieved in part by overlaps in the intracellular signaling pathways that control these two processes. The signaling protein Ras, for example, is activated by both growth factors and mitogens. It can stimulate the PI3-kinase pathway to promote cell growth and the MAP-kinase pathway to trigger cell-cycle progression. Similarly, as described above, Myc stimulates both cell growth and cell-cycle progression. Extracellular factors that act as both growth factors and mitogens help ensure that cells maintain their appropriate size as they proliferate.

Cell growth and division, however, can be controlled by separate extracellular signal proteins in some cell types. Such independent control may be particularly important during embryonic development, when dramatic changes in the size of certain cell types can occur. Even in adult animals, however, growth factors can stimulate cell growth without affecting cell division. The size of a sympathetic neuron, for example, which has permanently withdrawn from the cell cycle, depends on the amount of nerve growth factor (NGF) secreted by the target cells it innervates. The greater the amount of NGF the neuron has access to, the larger it becomes. It remains a mystery, however, how different cell types in the same animal grow to be so different in size (Figure 17-45).

Animal cells need signals from other cells—not only to grow and proliferate, but also to survive. If deprived of such survival factors, cells activate their intracellular death program and die by apoptosis. This arrangement ensures that cells survive only when and where they are needed. Nerve cells, for example, are produced in excess in the developing nervous system and then compete for limited amounts of survival factors that are secreted by the target cells they contact. Nerve cells that receive enough survival factor live, while the others die by apoptosis (Figure 17-46). A similar dependence on survival signals from neighboring cells is thought to control cell numbers in other tissues, both during development and in adulthood.

Survival factors, just like mitogens and growth factors, usually bind to cell-surface receptors. Binding activates signaling pathways that keep the death program suppressed, often by regulating members of the Bcl-2 family of proteins. Some factors, for example, stimulate the increased production of apoptosis-suppressing members of this family. Others act by inhibiting the function of apoptosis-promoting members of the family (Figure 17-47A). In Drosophila, and probably in vertebrates as well, some survival factors also act by stimulating the activity of IAPs, which suppress apoptosis (Figure 17-47B).

When most types of mammalian cells are cultured in a dish in the presence of serum, they adhere to the bottom of the dish, spread out, and divide until a confluent monolayer is formed. Each cell is attached to the dish and contacts its neighbors on all sides. At this point, normal cells, unlike cancer cells, stop proliferating—a phenomenon known as density-dependent inhibition of cell division. This phenomenon was originally described in terms of “contact inhibition” of cell division, but it is unlikely that cell-cell contact interactions are solely responsible. The cell population density at which cell proliferation ceases in the confluent monolayer increases with increasing concentration of serum in the medium. Moreover, passing a stream of fresh culture medium over a confluent layer of fibroblasts reduces the diffusional limitation to the supply of mitogens, and it induces the cells under the stream of medium to divide at densities at which they would normally be inhibited from doing so (Figure 17-48). Thus, density-dependent inhibition of cell proliferation seems to reflect, in part at least, the ability of a cell to deplete the medium locally of extracellular mitogens, thereby depriving its neighbors.

This type of competition could be important for cells in tissues as well as in culture, because it prevents them from proliferating beyond a certain population density, determined by the available amounts of mitogens, growth factors, and survival factors. The amounts of these factors in tissues is usually limited, and increasing their amounts results in an increase in cell number, cell size, or both. Thus, the concentrations of these factors in tissues have important roles in determining cell size and number.

The shape of a cell changes as it spreads and crawls out over a substratum to occupy vacant space, and this can have a major impact on cell growth, cell division, and cell survival. When normal fibroblasts or epithelial cells, for example, are cultured in suspension, unattached to any solid surface and therefore rounded up, they almost never divide—a phenomenon known as anchorage dependence of cell division (Figure 17-49). But when these cells are allowed to settle and adhere to a sticky substrate, they rapidly form focal adhesions at sites of attachment, and then begin to grow and proliferate.

How are the growth and proliferation signals generated by cell attachments? Focal adhesions are places where extracellular matrix molecules, such as laminin or fibronectin, interact with cell-surface matrix receptors called integrins, which are linked to the actin cytoskeleton (discussed in Chapter 19). The binding of extracellular matrix molecules to integrins leads to the local activation of protein kinases, including focal adhesion kinase (FAK), which in turn leads to the activation of intracellular signaling pathways that can promote the survival, growth, and division of cells (Figure 17-50).

Like other controls on cell division, anchorage control operates in G1. Cells require anchorage to progress through G1 into S phase, but anchorage is not required for completing the cycle. In fact, cells commonly loosen their attachments and round up as they pass through M phase. This cycle of attachment and detachment presumably allows cells in tissues to rearrange their contacts with other cells and with the extracellular matrix. In this way, tissues can accommodate the daughter cells produced by cell division and then bind them securely into the tissue before they are allowed to begin the next division cycle.

The extracellular signal proteins discussed in this chapter—mitogens, growth factors and survival factors—are positive regulators of cell-cycle progression, cell growth, and cell survival, respectively. They therefore tend to increase the size of organs and organisms. In some tissues, however, cell and tissue size also is influenced by inhibitory extracellular signal proteins that oppose the positive regulators and thereby inhibit organ growth.

The best-understood inhibitory signal proteins are TGF-β and its relatives. TGF-β inhibits the proliferation of several cell types, either by blocking cell-cycle progression in G1 or by stimulating apoptosis. As discussed in Chapter 15, TGF-β binds to cell-surface receptors and initiates an intracellular signaling pathway that leads to changes in the activities of gene regulatory proteins called Smads. This results in complex and poorly understood changes in the transcription of genes encoding regulators of cell division and cell death.

One example of an apoptosis-inducing extracellular signal is bone morphogenetic protein (BMP), a TGF-β family member. BMP helps trigger the apoptosis that removes the tissue between the developing digits in the mouse paw (see Figure 17-35). Like TGF-β, BMP stimulates changes in the transcription of genes that regulate cell death, although the nature of these genes remains unclear.

The overall size of an organ may be limited in some cases by inhibitory signaling proteins. Myostatin, for example, is a TGF-β family member that normally inhibits the proliferation of myoblasts that fuse to form skeletal muscle cells. When the gene that encodes myostatin is deleted in mice, muscles grow to be several times larger than normal (see Figure 22-43). Both the number and the size of muscle cells increase. Remarkably, two breeds of cattle that were bred for large muscles have both turned out to have mutations in the gene encoding myostatin (Figure 17-51).

The life of multicellular organisms begins with a series of division cycles that are controlled according to intricate rules. This is strikingly illustrated by the nematode Caenorhabditis elegans. The fertilized egg of C. elegans divides to produce an adult worm with precisely 959 somatic cell nuclei (in the male), each of which is generated by its own characteristic and absolutely predictable sequence of cell divisions. (The initial cell number is greater than this, but more than 100 cells die by apoptosis during development.) In general, the controls that generate such precise cell numbers do not operate by merely counting cell divisions according to a clocklike schedule. Instead, the organism seems mainly to control total cell mass, which depends not only on cell numbers but also on cell size. Salamanders of different ploidies, for example, are the same size but have different numbers of cells. Individual cells in a pentaploid salamander are about five times the volume of those in a haploid salamander, and in each organ the pentaploids have generated only one-fifth as many cells as their haploid cousins, so that the organs are about the same size in the two animals (Figures 17-52 and 17-53). Evidently, in this case (and in many others) the size of organs and organisms depends on mechanisms that can somehow measure total cell mass.

The development of limbs and organs of specific size and shape depends on complex positional controls, as well as on local concentrations of extracellular signal proteins that stimulate or inhibit cell growth, division, and survival. As we discuss in Chapter 21, some of the genes that help pattern these processes in the embryo are now known. A great deal remains to be learned, however, about how these genes regulate cell growth, division, survival, and differentiation to generate a complex organism (discussed in Chapter 21).

The controls that govern these processes in an adult body are also poorly understood. When a skin wound heals in a vertebrate, for example, about a dozen cell types, ranging from fibroblasts to Schwann cells, must be regenerated in appropriate numbers and in appropriate positions to reconstruct the lost tissue. The mechanisms that control cell proliferation in tissues are likewise central to the understanding of cancer, a disease in which the controls go wrong, as discussed in Chapter 23.

In multicellular animals, cell size, cell division, and cell death are carefully controlled to ensure that the organism and its organs achieve and maintain an appropriate size. Three classes of extracellular signal proteins contribute to this control, although many of them affect two or more of these processes. Mitogens stimulate the rate of cell division by removing intracellular molecular brakes that restrain cell-cycle progression in G1. Growth factors promote an increase in cell mass by stimulating the synthesis and inhibiting the degradation of macromolecules. Survival factors increase cell numbers by inhibiting apoptosis. Extracellular signals that inhibit cell division or cell growth, or induce cells to undergo apoptosis, also contribute to size control.