CELLULAR BIOLOGY

1. Movement. Muscle cells can generate forces that produce motion. Muscles that are attached to bones produce limb movements, whereas those that enclose hollow tubes or cavities move or empty contents when they contract. For example, the contraction of smooth muscle cells surrounding blood vessels changes the diameter of the vessels; the contraction of muscles in walls of the urinary bladder expels urine.

2. Conductivity. Conduction as a response to a stimulus is manifested by a wave of excitation, an electrical potential, that passes along the surface of the cell to reach its other parts. Conductivity is the chief function of nerve cells.

3. Metabolic absorption. All cells take in and use nutrients and other substances from their surroundings. Cells of the intestine and the kidney are specialized to carry out absorption. Cells of the kidney tubules reabsorb fluids and synthesize proteins. Intestinal epithelial cells reabsorb fluids and synthesize protein enzymes.

4. Secretion. Certain cells, such as mucous gland cells, can synthesize new substances from substances they absorb and then secrete the new substances to serve as needed elsewhere. Cells of the adrenal gland, testis, and ovary can secrete hormonal steroids.

5. Excretion. All cells can rid themselves of waste products resulting from the metabolic breakdown of nutrients. Membrane-bound sacs (lysosomes) within cells contain enzymes that break down, or digest, large molecules, turning them into waste products that are released from the cell.

6. Respiration. Cells absorb oxygen, which is used to transform nutrients into energy in the form of adenosine triphosphate (ATP). Cellular respiration, or oxidation, occurs in organelles called mitochondria.

7. Reproduction. Tissue growth occurs as cells enlarge and reproduce themselves. Even without growth, tissue maintenance requires that new cells be produced to replace cells that are lost normally through cellular death. Not all cells are capable of continuous division (see Chapter 2).

8. Communication. Communication is critical for all the other functions above that enable the survival of the society of cells. Pancreatic cells, for instance, secrete and release insulin to tell muscle cells to take up sugar from the blood for energy. Constant communication allows the maintenance of a dynamic steady state.

Nucleus

The nucleus, which is surrounded by the cytoplasm and generally is located in the center of the cell, is the largest membrane-bound organelle. Two membranes comprise the nuclear envelope (Figure 1-2, A). The outer membrane is continuous with membranes of the endoplasmic reticulum. The nucleus contains the nucleolus, a small dense structure composed largely of RNA; most of the cellular DNA; and the DNA-binding proteins, the histones, that regulate its activity. The DNA chain in eukaryotic cells is so extensive that the risk of breakage is high. Therefore, the histones that bind to DNA cause the folding of DNA into chromosomes (Figure 1-2, C). The wrapping of DNA into tight packages of chromosomes is essential for cell division in eukaryotes.

The primary functions of the nucleus are cell division and control of genetic information. Other functions include the replication and repair of DNA and the transcription of the information stored in DNA. Genetic information is transcribed into RNA, which can be processed into messenger, transport, and ribosomal RNA and introduced into the cytoplasm, where it directs cellular activities. Most of the processing of RNA occurs in the nucleolus. (The role of DNA and RNA in protein synthesis is discussed in Chapter 4.)

Cytoplasmic Organelles

Cytoplasm is an aqueous solution (cytosol) that fills the cytoplasmic matrix—the space between the nuclear envelope and the plasma membrane. The cytosol represents about half the volume of a eukaryotic cell. It contains thousands of enzymes involved in intermediate metabolism and is crowded with ribosomes making proteins. Newly synthesized proteins remain in the cytosol if they lack a signal for transport to a cell organelle.¹ The organelles suspended in the cytoplasm are enclosed in biologic membranes, which enables them simultaneously to carry out functions that require different biochemical environments. These functions, many of which are directed by coded messages carried from the nucleus by RNA, include synthesis of proteins and hormones and their transport out of the cell, isolation and elimination of waste products from the cell, metabolic processes, breakdown and disposal of cellular debris and foreign proteins (antigens), and maintenance of cellular structure and motility. Also the cytosol functions as a storage unit for fat, carbohydrate, and secretory vesicles.

Plasma Membranes

Whether they surround the cell or enclose an intracellular organelle, membranes are exceedingly important to normal physiologic function because they control the composition of the space, or compartment, they enclose. Membranes can allow or exclude various molecules, and because of selective transport systems, they can move molecules into or out of the space (Figure 1-10). By controlling the movement of substances from one compartment to another, membranes exert a powerful influence on metabolic pathways. In addition to these functions, the plasma membrane has an important role in cell-to-cell recognition. For example, protein receptors for hormones and for other chemical signals are associated with the membrane and act as markers that identify a cell to its neighbors. Other functions of the plasma membrane include cellular mobility and the maintenance of cellular shape (Table 1-1).

Cellular Receptors

Cellular receptors are protein molecules (proteins are discussed on p. 11) on the plasma membrane, in the cytoplasm, or in the nucleus that are capable of recognizing and binding with specific smaller molecules called ligands. Hormones, for example, are ligands. Recognition and binding depend on the chemical configuration of the receptor and its smaller ligand, which must fit together somewhat like pieces of a jigsaw puzzle (see Chapter 20). New data reveal that activation of a receptor also may depend on differences in movement and binding of the extracellular face of the receptor.¹³

Plasma membrane receptors are particularly important for cellular uptake of ligands (Table 1-2). They protrude from or are exposed at the external surface of the membrane and often are attached to integral proteins. Some of these recognition units have all the mobile properties related to membrane fluidity. The ligands that bind with membrane receptors include hormones, neurotransmitters, antigens, complement components, lipoproteins, infectious agents, drugs, and metabolites. The past several years have brought many new discoveries concerning the specific interactions of cellular receptors with their respective ligands. In many instances this information has provided a basis for understanding disease.

Although the chemical nature of both ligands and the receptors to which they bind differs, receptors are classified on the basis of their location and function (see Cellular Communication and Signal Transduction, p. 18). Cellular type determines overall cellular function, but plasma membrane receptors determine which ligands a cell will bind with and how the cell will respond to binding with each. For example, the ability of a hormone or a neurotransmitter to stimulate a cell is regulated by the specificity and number of receptors present on the plasma membrane. Specific processes also control intracellular mechanisms. Hormone binding, for example, depends on special messenger molecules that regulate protein synthesis within the cell (see Chapter 20). Neurotransmitters (discussed in Chapter 14) also operate by causing special messengers to react with specific receptors.

Receptors for different drugs are found on the plasma membrane, in the cytoplasm, and in the nucleus. Membrane receptors have been found for certain anesthetics, opiates, endorphins, enkephalins, antibiotics, cancer chemotherapeutic agents, digitalis, and other drugs. Membrane receptors for endorphins, which are opiate-like peptides isolated from the pituitary gland, are found in large quantities in pain pathways of the nervous system (see Chapters 14 and 15). With binding, the endorphins (or drugs like morphine) change the cell’s permeability to ions, increase the concentration of molecules that regulate intracellular protein synthesis, and initiate molecular events that modulate pain perception.

Receptors for infectious microorganisms, or antigen receptors, bind bacteria, viruses, and parasites. Antigen receptors on white blood cells (lymphocytes, monocytes, macrophages, granulocytes) recognize and bind with antigenic microorganisms and activate the immune and inflammatory responses (see Chapters 6 and 7).

Extracellular Matrix

Cells can be bound together by attachment to one another or via the extracellular matrix (also including the basement membrane), which the cells secrete around themselves. The extracellular matrix is an intricate meshwork of fibrous proteins embedded in a watery, gel-like substance composed of complex carbohydrates (Figure 1-14). The matrix is like glue; however, it does provide a pathway for diffusion of nutrients, wastes, and other water-soluble traffic between the blood and tissue cells. Interwoven within the matrix are three groups of macromolecules: (1) fibrous structural proteins, including collagen and elastin; (2) a diverse group of adhesive glycoproteins, such as fibronectin; and (3) proteoglycans and hyaluronic acid.

Collagen forms cable-like fibers or sheets that provide tensile strength or resistance to longitudinal stress. Collagen breakdown, such as occurs in osteoarthritis, destroys the fibrils that give cartilage its tensile strength.

Elastin is a rubber-like protein fiber most abundant in tissue that must be capable of stretching and recoiling, such as the lungs.

Fibronectin, a large glycoprotein, promotes cell adhesion and cell anchorage. Reduced amounts have been found in certain types of cancerous cells; this allows cancer cells to travel or metastasize to other parts of the body.

All of these macromolecules occur in intercellular junctions and cell surfaces and may assemble into two different components: interstitial matrix and basement membrane (BM)¹⁴ (see Figure 1-14).

The extracellular matrix is secreted by fibroblasts (“fiber formers”), local cells that are present in the matrix. The matrix and the cells within it are known collectively as connective tissue because they connect cells together to form tissue and organs. Human connective tissues are enormously varied. They can be hard and dense, like bone; flexible, like tendons or the dermis of the skin; resilient and shock-absorbing, like cartilage; or soft and transparent, like the jelly that fills the eye. In all these examples, the majority of the tissue is composed of extracellular matrix, and the cells that produce the matrix are scattered within it like raisins in a pudding¹⁵ (see Figure 1-14).

The matrix is not just a passive scaffolding for cellular attachment; it also helps regulate the functions of the cells within which it interacts. The matrix helps regulate cell growth, movement, and differentiation.

Specialized Cell Junctions

Cells in direct physical contact with neighboring cells are often linked together at specialized regions of their plasma membranes called cell junctions. Cell junctions have two main functions: (1) to hold cells together and (2) to allow small molecules to pass from cell to cell, allowing coordination of the activities of cells that form tissues. The three main types of cell junctions are (1) desmosomes (adhering junctions, or macula adherens), (2) tight junctions (impermeable junctions, or zonula occludens), and (3) gap junctions (adhering [communicating] junctions) (Figure 1-15). Together they form the junctional complex. Desmosomes hold cells together by forming either continuous bands or belts of epithelial sheets or button-like points of contact. Desmosomes also act as a system of braces to maintain structural stability. Tight junctions serve as a barrier to diffusion, prevent the movement of substances through transport proteins in the plasma membrane, and prevent the leakage of small molecules between the plasma membranes of adjacent cells. Gap junctions are clusters of communicating tunnels, connexons, that allow small ions and molecules to pass directly from the inside of one cell to the inside of another. Connexons are joining proteins that extend outward from each of the adjacent plasma membranes. Cells connected by gap junctions are considered ionically (electrically) and metabolically coupled. Gap junctions coordinate the activities of adjacent cells. They are important, for example, in synchronizing contractions of heart muscle cells through ionic coupling and in permitting action potentials to spread rapidly from cell to cell in neural tissues. The reason that gap junctions occur in tissues that are not electrically active is unknown. Although most gap junctions are associated with junctional complexes, they sometimes exist as independent structures.

The junctional complex is a highly permeable part of the plasma membrane. Its permeability is controlled by a process called gating, which depends on concentrations of calcium ions in the cytoplasm. Increased cytoplasmic calcium causes decreased permeability at the junctional complex. Gating is an important cellular defense mechanism because it enables uninjured cells to seal themselves off from injured neighbors. As damaged cells release calcium, it travels through the junctional complex and increases calcium levels in neighboring cells. (The damaging effects of calcium influx are described in Chapter 2.) This decreases the permeability of the junctional complexes of the neighboring cells, which form a relatively impermeable wall around the injured area.

Signal Transduction

Signal transduction involves incoming signals or instructions from extracellular chemical messengers (ligands) that are conveyed to the cell’s interior for execution. Within the outer surface of the plasma membrane, specialized protein receptors bind with the selected chemical messengers. This combination of messenger with receptor triggers a cascade of cellular events important to the maintenance of homeostasis, such as membrane transport, cell division and differentiation, movement, secretion, and metabolism. Some types of altered cell behavior, such as increased cell growth and division, involve changes in gene expression and the synthesis of new proteins and therefore occur slowly. Others, such as changes in cell movement, secretion, or metabolism, do not involve the nuclear machinery and therefore occur more rapidly. If deprived of appropriate signals, most cells undergo a form of cell suicide known as programmed cell death, or apoptosis (see p. 84).

Signaling cascades, or relay chains, of intercellular signaling molecules have several important functions (see Figure 1-18):

Two general responses from binding of the extracellular chemical messenger, or first messenger, to the membrane receptors occur: (1) opening or closing specific channels in the membrane to regulate the movement of ions into or out of the cell, and (2) transferring the signal to an intracellular messenger, or second messenger, which in turn triggers a cascade of biochemical events within the cell.

Extracellular Messengers and Channel Regulation

Membrane channels, or “gates,” can open and close depending on the circumstances of the first messenger. Opening and closing occur because of conformational changes (shaping) of the proteins that form the channels—blocking the channel (closing) or permitting passage through it (opening). Channel opening and closing can be initiated in one of three ways: (1) by binding of a ligand to a specific membrane receptor that is closely associated with the channel (for example, G proteins); (2) by changes in electric current in the plasma membrane, altering flow of Na⁺ and K⁺; and (3) by stretching or other chemical deformation of the channel. Figure 1-19 summarizes ways by which extracellular messengers regulate channel function for the other two methods of controlling channels (see p. 28).

Second Messengers

Many ligands cannot enter their target cells to bring about the desired intracellular response. Instead, the first messengers, or ligands, issue orders by binding with receptors on the surface membrane, triggering a “pass it on” signal. Second messengers are generated in large numbers when the membrane-bound enzyme is activated, and they then rapidly diffuse away from their source, broadcasting the signal throughout the cell (Figure 1-20). Remember, most cell-surface receptor proteins belong to one of three large classes: ion-channel-linked receptors, G-protein-linked receptors, or enzyme-linked receptors.

The two major second messenger pathways are cyclic adenosine monophosphate (cyclic AMP, cAMP) and Ca⁺⁺. In the cAMP pathway, binding of the ligand to its surface receptor eventually activates the enzyme adenylyl cyclase on the inner surface of the membrane. A membrane-bound “middleman,” a G protein, acts as an intermediary between the receptor and adenylyl cyclase. G proteins are named because they are bound to guanine nucleotides—guanosine triphosphate (GTP) or guanosine diphosphate (GDP). An unactivated G protein consists of a complex of alpha (α), beta (β), and gamma (γ) subunits, with a GDP molecule bound to the α subunit. The cAMP pathway with G proteins is summarized in Figure 1-20.

Instead of cAMP, some cells use Ca⁺⁺ as a second messenger. In this pathway, binding of the first messenger to the surface receptor eventually leads, by means of G proteins, to activation of the enzyme phospholipase C, an enzyme protein effector (an ion channel for an enzyme) that is bound to the inner side of the membrane. Figure 1-21 summarizes the Ca⁺⁺ second messenger pathway. The cAMP and Ca⁺⁺ pathways frequently overlap in bringing about a specific cellular response. For example, cAMP and Ca⁺⁺ can influence each other. Calcium-activated calmodulin can regulate adenylyl cyclase and thus influence cAMP; conversely, cAMP-dependent kinase may phosphorylate and thereby change the activity of Ca⁺⁺ channels or carriers. In some instances, both Ca⁺⁺ and cAMP regulate the same intracellular protein. In a few cells, cyclic guanosine monophosphate (cyclic GMP, cGMP) serves as a second messenger similar to the cAMP pathway. For example, cGMP is the signal transduction pathway involved in vision. Some cellular responses mediated by cAMP and phospholipase C are summarized in Table 1-3. Major types of receptors and signal transduction pathways are contained in Table 1-4.

A large number of human disorders involve problematic signaling in cells. Cancer, for example, results from genetic mutations leading to the overactivity of proteins in signal relaying pathways that normally induce the cells to divide. Affected proteins cause cells to behave as if other cells were constantly telling them to reproduce, even when no such orders were sent.¹⁸ Signal blockers are already in use against breast cancer.

Role of Adenosine Triphosphate

For a cell to function it must be able to extract and use the chemical energy contained within the structure of organic molecules. When 1 mole of glucose is metabolically broken down in the presence of oxygen into carbon dioxide (CO₂) and water (H₂O), 686 kilocalories (kcal) of energy are released. In a test tube this energy is released as heat. Because a cell cannot transform heat into work, chemical energy, rather than heat, is created by metabolism. The chemical energy lost by one molecule is transferred to the chemical structure of another molecule by an energy-carrying or transferring molecule, such as ATP. The energy stored in ATP can be used in a variety of energy-requiring reactions and in the process is generally converted to adenosine diphosphate (ADP) and inorganic phosphate (Pi). The energy available as a result of this reaction is about 7 kcal/mol of ATP. In addition to its use in synthesis (anabolism) of organic molecules, ATP is used by the cell for muscle contraction and active transport of molecules across cellular membranes. The function of ATP is not only to store energy but also to transfer it from one molecule to another. Energy is stored by molecules of carbohydrate, lipid, and protein, which, when catabolized, transfer energy to ATP.

Food and Production of Cellular Energy

The process of catabolism of the proteins, lipids, and polysaccharides found in food can be divided into three phases (Figure 1-22). In phase 1, large molecules are broken down into their smaller subunits—proteins into amino acids, polysaccharides into simple sugars, and fats into fatty acids and glycerol. These processes are called digestion and occur outside the cell by the action of secreted enzymes.

In phase 2 the small molecules enter cells and are further broken down in the cytoplasm. Most of the sugars are converted into pyruvate. Pyruvate then enters mitochondria and is converted to the acetyl groups of acetyl coenzyme A (acetyl CoA). Acetyl CoA, like ATP, releases energy when it is hydrolyzed. The most important part of phase 2 is the lysis (splitting) of glucose, known as glycolysis (Figure 1-23). Glycolysis produces a net of two molecules of ATP per glucose molecule through the process of oxidation, or the removal and transfer of a pair of electrons. This process, often called oxidative cellular metabolism, involves 10 biochemical reactions. In reactions 1 through 5, glucose is converted to two, three-carbon aldehyde (glyceraldehyde-3-phosphate [G3P]), which requires energy in the form of ATP. The next five reactions convert G3P molecules into pyruvate molecules and generate four molecules of ATP for each two molecules of G3P. In addition, two molecules of nicotinamide adenine dinucleotide (NAD) are further oxidized to produce four more molecules of ATP. After subtracting two molecules of ATP to drive the reactions, the net yield is six ATP molecules for each molecule of glucose.

Phase 3 occurs when the acetyl group of acetyl CoA is completely degraded to CO₂ and H₂O. It is in this final phase that most of the ATP is generated. Phase 3 begins with the citric acid cycle (also called the Krebs cycle or the tricarboxylic acid cycle) and ends with oxidative phosphorylation. The citric acid cycle accounts for approximately two thirds of the total oxidation of carbon compounds in most cells. Its major end products are CO₂ and two dinucleotides, reduced NADH, and the reduced form of flavin adenine dinucleotide (FADH₂), which transfer their electrons into the electron-transport chain.

Oxidative Phosphorylation

Oxidative phosphorylation occurs in the mitochondria and is the mechanism by which the energy produced from carbohydrates, fats, and proteins is transferred to ATP. During the breakdown (catabolism) of foods, many of the reactions involve the removal of electrons from various intermediates. These reactions generally require a coenzyme (a nonprotein carrier molecule), such as nicotinamide adenine dinucleotide (NAD), to transfer the electrons and thus are called transfer reactions.

In oxidative phosphorylation, molecules of NAD and flavin adenine dinucleotide (FAD) transfer electrons they have gained from the oxidation of substrates to molecular oxygen, O₂. The electrons from reduced NAD and FAD, NADH and FADH₂, are transferred to a series of carrier molecules (the electron-transport chain) on the inner surfaces of the mitochondria with the release of hydrogen ions. Some of the carrier molecules are a group of brightly colored iron-containing proteins known as cytochromes that accept a pair of electrons. After passing through a sequence of different cytochromes, these electrons are eventually combined with molecular oxygen. If oxygen is not available to the electron-transport chain, ATP will not be formed by the mitochondria. Instead, an anaerobic (without oxygen) metabolic pathway synthesizes ATP. This process, called substrate phosphorylation, or anaerobic glycolysis, does not take place in the mitochondria and is linked to the breakdown (glycolysis) of carbohydrate (Figure 1-24).

Because glycolysis occurs in the cytoplasm of the cell, it provides energy for cells that lack mitochondria. However, as noted, glycolysis also provides energy to the cell when oxygen delivery is insufficient or delayed (e.g., with strenuous exercise). The reactions in anaerobic glycolysis involve the conversion of glucose to pyruvic acid (pyruvate) with the simultaneous production of ATP. With the glycolysis of one molecule of glucose, two ATP molecules and two molecules of pyruvate are liberated. If oxygen is present, the two molecules of pyruvate move into the mitochondria, where they enter the citric acid cycle. If oxygen is absent, pyruvate is converted to lactic acid, which is released into the extracellular fluid (see Figure 1-24). The conversion of pyruvic acid to lactic acid is reversible; therefore, once oxygen is restored, lactic acid is quickly converted back to either pyruvic acid or glucose. The anaerobic generation of ATP from glucose, through the reactions of glycolysis, is not as efficient as the aerobic generation of ATP. The addition of an oxygen-requiring stage to the catabolic process (stage 3) provides cells with a much more powerful method for extracting energy from food molecules.

Movement of Water and Solutes

Cellular membranes are semipermeable and generally allow passage of water and small particles of dissolved substances called solutes. The movement of solute molecules through membranes is related to their size, solubility, electrical properties, and concentration on either side of the membrane. Small lipid-soluble particles, such as oxygen, carbon dioxide, and urea, can readily pass through the lipid bilayers of the plasma membrane. Larger, water-soluble particles may pass through pores in the membranes. Although large protein molecules, such as albumin and globulin, pass through membranes by endocytosis, they influence the movement of water by exerting an osmotic effect (see p. 27).

Body fluids are composed of two types of solutes: electrolytes, which are electrically charged and dissociate into constituent ions when placed in solution; and nonelectrolytes, such as glucose, urea, and creatinine, which do not dissociate. Electrolytes account for approximately 95% of the solute molecules in body water. Electrolytes exhibit polarity by orienting themselves toward the positive or negative pole. Ions with a positive charge are known as cations and migrate toward the negative pole, or cathode, if an electrical current is passed through the electrolyte solution. Anions carry a negative charge and migrate toward the positive pole, or anode, in the presence of electrical current. Anions and cations are located in both the intracellular fluid (ICF) and extracellular fluid (ECF) compartments, although concentration of particular ions varies depending on their location. (Fluid and electrolyte balance between body compartments is discussed in Chapter 3.) For example, Na⁺ is the predominant extracellular cation, and K⁺ is the principal intracellular cation. The difference in ICF and ECF concentrations of these ions is important to the transmission of electrical impulses across the plasma membranes of nerve and muscle cells.

Electrolytes are measured in milliequivalents per liter (mEq/L) or milligrams per deciliter (mg/dl). Milliequivalents per liter indicate the number of electrical charges per unit volume of fluid. The term milliequivalent thus indicates the chemical-combining activity of an ion, which depends on the electrical charge, or valence, of its ions. In abbreviations, valence is indicated by the number of plus or minus signs. Monovalent ions, or ions with one charge, include sodium (Na⁺), chloride (Cl⁻), and potassium (K⁺). Divalent ions, which have two charges, include calcium (Ca⁺⁺) and magnesium (Mg⁺⁺). One milliequivalent of any cation can combine chemically with 1 mEq of any anion: one monovalent anion will combine with one monovalent cation. Divalent ions combine more strongly than monovalent ions. To maintain electrochemical balance, one divalent ion will combine with two monovalent ions (e.g., Ca⁺⁺ + 2 Cl⁻ = CaCl₂).

Transport by Vesicle Formation

Movement of Electrical Impulses: Membrane Potentials

All body cells are electrically polarized, with the inside of the cell more negatively charged than the outside. The difference in electrical charge, or voltage, is known as the resting membrane potential and is about −70 to −85 millivolts. The difference in voltage across the plasma membrane is a result of the differences in ionic composition of ICF and ECF. Sodium ions have a greater concentration in the ECF, and potassium ions have a greater concentration in the ICF. The concentration difference is maintained by the active transport of Na⁺ and K⁺ (the sodium-potassium pump), which transports sodium outward and potassium inward (Figure 1-32). Because the resting plasma membrane is more permeable to K⁺ than to Na⁺, K⁺ can diffuse easily from its area of higher concentration in the ICF to its area of lower concentration in the ECF. Because Na⁺ and K⁺ are both cations, the net result is an excess of anions inside the cell, resulting in the resting membrane potential.

Nerve and muscle cells are excitable and can change their resting membrane potential in response to electrochemical stimuli. Changes in resting membrane potential convey messages from cell to cell. When a nerve or muscle cell receives a stimulus that exceeds the membrane threshold value, there is a rapid change in the resting membrane potential known as the action potential. The action potential carries signals along the nerve or muscle cell and conveys information from one cell to another. (Nerve impulses are described in Chapter 14.) When a resting cell is stimulated through voltage-regulated channels, the cell membranes become more permeable to sodium. There is a net movement of sodium into the cell, and the membrane potential decreases, or “moves forward,” from a negative value (in millivolts) to zero. This decrease is known as depolarization. The depolarized cell is more positively charged, and its polarity is neutralized.

To generate an action potential and the resulting depolarization, a critical value known as the threshold potential must be reached. Generally this occurs when the cell has depolarized by 15 to 20 millivolts. When the threshold is reached, the cell will continue to depolarize with no further stimulation. The sodium gates open, and sodium rushes into the cell, causing the membrane potential to reduce to zero and then become positive (depolarization). The rapid reversal in polarity results in the action potential.

During repolarization the negative polarity of the resting membrane potential is reestablished. As the voltage-gated sodium channels begin to close, voltage-gated potassium channels open. Membrane permeability to sodium decreases, and potassium permeability increases, with an outward movement of potassium ions. The sodium gates close, and with the outward movement of potassium, the membrane potential becomes more negative. The Na⁺-K⁺ pump then returns the membrane to the resting potential by pumping potassium back into the cell and sodium out of the cell.

During most of the action potential, the plasma membrane cannot respond to an additional stimulus. This time is known as the absolute refractory period and is related to changes in permeability to sodium. During the latter phase of the action potential, when permeability to potassium increases, a stronger-than-normal stimulus can evoke an action potential known as the relative refractory period.

When the membrane potential is more negative than normal, the cell is in a hyperpolarized (less excitable) state. A larger-than-normal stimulus is then required to reach the threshold potential and generate an action potential. When the membrane potential is more positive than normal, the cell is in a hypopolarized (more excitable than normal) state, and a smaller-than-normal stimulus is required to reach the threshold potential. Changes in the intracellular and extracellular concentration of ions or a change in membrane permeability can cause these alterations in membrane excitability.

Phases of Mitosis and Cytokinesis

Interphase (the G₁, S, and G₂ phases) is the longest phase of the cell cycle. During interphase the chromatin consists of very long, slender rods that are jumbled together in the nucleus. Late in interphase, strands of chromatin (the substance that gives the nucleus its granular appearance) begin to coil, causing them to shorten and thicken.

The M phase of the cell cycle, mitosis and cytokinesis, begins with prophase, the first appearance of chromosomes. As the phase proceeds, each chromosome is seen as two identical halves called chromatids, which lie together and are attached at some point by a spindle attachment site called a centromere. (The two chromatids of each chromosome, which are genetically identical, are sometimes called sister chromatids.) The nuclear membrane, which surrounds the nucleus, disappears. Spindle fibers are microtubules formed in the cytoplasm. Spindle fibers radiate from two centrioles located at opposite poles of the cell. The role of the spindle fibers is to pull the chromosomes to opposite sides of the cell.

During metaphase, the next phase of mitosis and cytokinesis, the spindle fibers begin to pull the centromeres of the chromosomes. The centromeres become aligned in the middle of the spindle, which is called the equatorial plate (or metaphase plate) of the cell. In this stage chromosomes are easiest to observe microscopically because they are highly condensed and arranged in a relatively organized fashion in the two-dimensional equatorial plate.

Anaphase begins when the centromeres split and the sister chromatids are pulled apart. The spindle fibers shorten, causing the sister chromatids to be pulled, centromere first, toward opposite sides of the cell. When the sister chromatids are separated, each is considered to be a chromosome. Thus the cell has 92 chromosomes during this stage. By the end of anaphase, 46 chromosomes are lying at each side of the cell. Barring mitotic errors, each of the two groups of 46 chromosomes is identical to the original 46 chromosomes present at the start of the cell cycle.

During telophase, the final stage, a new nuclear membrane is formed around each group of 46 chromosomes, the spindle fibers disappear, and the chromosomes begin to uncoil. Cytokinesis causes the cytoplasm to divide into roughly equal parts during this phase. At the end of telophase, two identical diploid cells, called daughter cells, have been formed from the original cell.

Rates of Cellular Division

Although the complete cell cycle lasts 12 to 24 hours, about 1 hour is generally required for the four stages of mitosis and cytokinesis. All types of cells undergo mitosis during formation of the embryo, but many adult cells, such as nerve cells, lens cells of the eye, and muscle cells, lose their ability to replicate and divide. The cells of other tissues, particularly epithelial cells (e.g., of the intestine, lung, skin), divide continuously and rapidly, completing the entire cell cycle in less than 10 hours.

The difference between cells that divide slowly and cells that divide rapidly is the length of time spent in the G₁ phase of the cell cycle. Some cells that divide very slowly remain in the G₁ phase for days or even years. Once the S phase begins, however, progression through mitosis takes a relatively constant amount of time. Once a cell has progressed out of the G₁ phase, there is no turning back; it is committed to completing the S, G₂, and M phases. Times associated with the four successive phases differ.

The mechanisms that control cell division depend on “social control genes” and protein growth factors. Individual cells are members of a complex cellular society in which survival of the entire organism is key and not survival or proliferation of just the individual cells. To grow and divide, a cell must receive specific positive signals from other cells. Many of these signals are protein growth factors that act by overriding intracellular negative controls that block progress of the cell cycle.¹

When a need arises for new cells, as in repair of injured cells, previously nondividing cells must be rapidly triggered to reenter the cell cycle. With continual wear and tear, the cell birth rate and the cell death rate must be kept in balance. Therefore, cell-division controls must govern this balance. Protein growth factors governing the proliferation of different cell types and genes involved in the social control of cell division are currently being identified.¹

The best model for understanding disruption of cell division and study of these so-called social control genes is tumor biology. Current emphasis in locating and identifying these genes is to study tumor cells that have presumably originated because of mutations to these genes, or proto-oncogenes. Proto-oncogenes are thought to encode key components of the normal system of social controls of cell division¹; that is, the mechanisms by which signals from a cell’s neighbors can impel it to divide, differentiate, or die. Some proto-oncogenes code for growth factors, some for growth factor receptors, some for intracellular regulatory proteins that are involved in cell adhesion, and some for proteins that help relay signals for cell division to the cell nucleus.¹ Although more than 50 proto-oncogenes have been identified, many more are yet to be discovered (see Chapter 11).

Growth Factors

Growth factors, also called cytokines, are peptides that transmit signals within and between cells. They have a major role in the regulation of tissue growth and development (Table 1-6). Having nutrients is not enough for a cell to proliferate; it must also receive stimulatory chemical signals (growth factors) from other cells, usually its neighbors. These signals act to overcome intracellular braking mechanisms that tend to restrain cell growth and block progress through the cell cycle.

Different types of cells require different factors; for example, platelet-derived growth factor (PDGF) stimulates the production of connective tissue cells. Table 1-6 summarizes the most significant growth factors. Cells that respond to a particular growth factor have specific receptors for the growth factor in their plasma membrane. Recent evidence shows that some growth factors are also regulators of other cell processes, such as cellular differentiation. In addition to growth factors that stimulate cellular processes, there are factors that inhibit functions; these factors are not well understood. Cells that are starved of growth factors come to a halt after mitosis and enter the arrested, or G₀, state of the cell cycle¹ (see p. 33 for cell cycle).

Tissue Formation

To form tissues cells must exhibit intercellular recognition and adhesion. Specialized cells are thought to form a tissue in one of two ways. First and simplest is mitosis of one or more founder cells (the most basic precursor cell). Founder cells are prevented from “wandering away” by macromolecules in the extracellular matrix and by adherence to one another at specialized junctions on their plasma membranes. Mitosis of founder cells forms, for example, epithelial cell sheets (Figure 1-34).

The second way in which specialized cells form tissues involves their migration to and subsequent assembly at the site of tissue formation. During embryonic development, for example, cells from the neural crest migrate to several different regions, where they differentiate and assemble into a variety of tissues, including those of the peripheral nervous system. Migrant cells are thought to arrive at the site of tissue formation through chemotaxis or contact guidance. Chemotaxis is movement along a chemical gradient caused by chemical attraction (see Chapter 6). Cells at the migrant cells’ destination secrete a chemical, called chemotactic factor, that attracts specific migrant cells. Contact guidance is movement along a pathway, or “pavement,” in the extracellular matrix.¹

Tissues are not randomly arranged into organs. No matter how tissue is formed, staying together in groups means that cells must recognize each other and remain distinct from the cells of surrounding tissues. Little is known about the mechanisms involved in these processes.

Types of Tissues

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