I. General Features of Cells
Cells are the structural and functional units of life (and of disease processes) in all tissues, organs, and organ systems. Each cell's capabilities and limitations are implicit in its structure.
B. Prokaryotes and Eukaryotes
There are two basic cell types. Prokaryotes are small, single-celled organisms (e.g., bacteria) that lack a nuclear envelope, histones, and membranous organelles. Eukaryotic cells exist primarily as components of multicellular organisms. This chapter covers the basic structural and functional features of eukaryotic cells. Specific human cell types are described in later chapters.
Eukaryotic cells have three major components:
Cell membranes (II) separate a cell from its environment and form distinct functional compartments (nucleus, organelles) in the cell. The outer cell membrane is called the plasma membrane, or plasmalemma.
The cytoplasm (III) surrounds the nucleus and is enclosed by the plasma membrane. It contains the structures and substances that decode the instructions of DNA and carry on the cell's activities.
The membrane-limited nucleus (Chapter 3) contains the DNA, which carries the genetic code for protein synthesis and thus for all cell activities. It also has components that determine which parts of the genetic code are used and that deliver coded information to the cytoplasm.
Three activities basic to living organisms are nourishment, growth, and reproduction. Functions directed toward these activities are described in this chapter and in Chapter 3. More specialized cell functions receive detailed treatment in subsequent chapters.
A. Biochemical Components (Fig. 2–1)
Lipids in cell membranes include phospholipids (Fig. 2–1, E), sphingolipids, and cholesterol (Fig. 2–1, A). Phospholipids (e.g., lecithin) are the most abundant form. Each phospholipid molecule has a polar (hydrophilic), phosphate-containing head group (Fig. 2–1, G) and a nonpolar (hydrophobic) pair of fatty acid tails (Fig. 2–1, F). Membrane phospholipids are arranged in a bilayer, with their tails directed toward one another at the center of the membrane. In electron micrographs (EMs) of osmium-stained tissue, a single membrane, or unit membrane, has two dark outer lines with a lighter layer between them. This trilaminar appearance reflects the deposition of reduced osmium on the hydrophilic head groups.
Proteins may contribute more than 50% of membrane weight. Most membrane proteins are globular and belong to one of two groups:
Integral membrane proteins (Fig. 2–1, C and D) are tightly lodged in the lipid bilayer; detergents are required to extract them. They are folded, with hydrophilic amino acids in contact with the membrane phospholipids’ phosphate groups and hydrophobic amino acids in contact with the fatty acid tails. Some protrude from only one membrane surface (Fig. 2–1, D). Others, called transmembrane proteins (Fig. 2–1, C), penetrate the entire membrane and protrude from both sides. Some transmembrane proteins, such as protein-3-tetramer, are hydrophilic channels for the passage of water and water-soluble materials through hydrophobic regions. Some transmembrane proteins pass multiple times through the bilayer to form channels and receptors. Cryofracture preparations often split plasma membranes through the hydrophobic region, between the ends of the phospholipids’ fatty acid tails (Fig. 2–1). Most integral proteins exposed in this way remain in the side closest to the cytoplasm, termed the P (protoplasmic) face. The membrane half nearest to the environment, the E (ectoplasmic) face, usually appears smoother.
Peripheral membrane proteins (Fig. 2–1, H) are ionically associated with the inner or outer membrane surface and are released in high-salt solutions; some are globular, some filamentous. In erythrocytes, examples on the cytoplasmic surface include adapter proteins like spectrin, which helps maintain membrane integrity, and ankyrin, which links spectrin to protein-3-tetramer.
Carbohydrates occur on plasma membranes mainly as oligosaccharide moieties of glycoproteins (Fig. 2–1, B) and glycolipids. Membrane oligosaccharides have a characteristic branching structure and project from the cell's outer surface, forming a surface coat called the glycocalyx that participates in cell adhesion and recognition.
The fluid mosaic model describes biologic membranes as “protein icebergs in a lipid sea”. Integral proteins exhibit lateral mobility and may rearrange through their association with peripheral proteins, cytoskeletal filaments within the cell (III.I), membrane components of adjacent cells, and extracellular matrix components. Integral proteins may diffuse to and accumulate in one membrane region. Membrane asymmetry refers to differences in chemical composition between the bilayer's inner and outer halves. Oligosaccharides occur only on the plasma membrane's outer surface. Phospholipid asymmetries also occur. The outer half has more phosphatidyl choline and sphingomyelin and the inner half has more phosphatidyl serine and phosphatidyl ethanolamine.
Selective permeability. Cell membranes separate the internal and external environments of a cell or organelle, preventing the intrusion of harmful substances, the dispersion of macromolecules, and the dilution of enzymes and substrates. This selective permeability is essential for maintaining the functional steady state, or homeostasis, required for cell survival. Homeostatic mechanisms attributable to cell membranes maintain optimal intracellular concentrations of ions, water, enzymes, and substrates. Three mechanisms allow selected molecules to cross membranes.
Passive diffusion. Some substances (e.g., water and lipids) can cross the membrane in either direction following a concentration gradient, without the cell expending energy.
Facilitated diffusion. Some molecules (e.g., glucose) are helped across the membrane by a membrane component. This facilitated diffusion is often unidirectional, but it follows a concentration gradient and requires no energy.
Active transport. Some molecules enter or exit a cell against a gradient. This requires energy, usually as adenosine triphosphate (ATP). One active transport mechanism is the sodium pump (Na+/K+-ATPase), which expels sodium ions from a cell even when the sodium concentration is higher outside than inside.
Signal transduction. Integral membrane receptor proteins with strong binding affinities for signal molecules (e.g., neurotransmitters, peptide hormones, and growth factors) are found on cell surfaces. The signal molecule to which a receptor specifically binds is its ligand. The receptor transduces the signal across the membrane without the ligand entering the cell. These receptors are critical to intercellular communication (IV.B). Signal transmission depends on the receptor class involved. There are three major receptor classes.
Ligand-gated (e.g., transmitter-gated) ion channels are long proteins that pass multiple times through the plasma membrane (Fig. 2–2A). The binding of a neurotransmitter (as ligand) to its receptor domain at the cell surface induces a conformational change in the rest of the receptor that opens (or closes) a transmembrane ion channel. Ligand binding and the resulting opening are brief, permitting regulation. A signal of adequate strength (threshold) allows enough ions to cross the membrane to open additional channels (voltage-gated ion channels) in response to the change in potential. Reaching threshold allows a self-perpetuating membrane depolarization to propagate as a wave along the nerve cell surface (9.VII.B.2).
Enzyme-linked receptors (Fig. 2–2B) comprise a heterogeneous group of transmembrane (typically single-pass) proteins associated with an enzyme (typically a protein kinase) or possessing kinase activity of their own (e.g., tyrosine kinase). Protein kinases activate additional proteins by phosphorylating them (III.B.1.b). In this way, a ligand binding to its receptor initiates a cascade of enzyme activations. Receptor tyrosine kinases (e.g., insulin and growth factor receptors) often act by activating an adaptor protein, which subsequently activates the guanosine triphosphate (GTP)-binding protein (G protein) Ras by causing it to exchange the guanosine diphosphate (GDP) it binds in its inactive state for GTP. Activated Ras then activates a cascade of cytoplasmic protein kinases, ending with a gene-regulatory protein that changes the pattern of gene expression. Ras differs from the G proteins described in the next section in that it is monomeric rather than trimeric. It resembles the α subunit in Figure 2–3 and acts without a β or γ subunit.
G protein–coupled receptors (GPCRs) comprise a family of proteins that make seven passes through the membrane (Fig. 2–3). Each receptor binds a different ligand, and the effect of signal binding on the ultimate target protein (either an ion channel [Fig. 2–2A] or an enzyme [Fig. 2–2B]) is indirect and is mediated by a trimeric G protein (Fig. 2–3A). On binding its ligand (Fig. 2–3B), the receptor interacts with and activates the trimeric G protein (Fig. 2–3C). The inactive G protein carries a GDP molecule, which it trades for GTP on activation (Fig. 2–3D and E). The activated G protein complex dissociates from the receptor, allowing the GTP-binding portion to interact with and activate its target (Fig. 2–3F;), thus inactivating the G protein (i.e., GTP → GDP; Fig. 2–3F). Ligand binding to GPCRs results in downstream activation of multiple signaling pathways involving the generation of intracellular second messengers. Two key examples with different second messengers are described below.
Cyclic adenosine monophosphate (cAMP). Some adrenalin receptors are key examples of GPCRs that lead to cAMP production. Adrenalin binding activates membrane-bound adenylyl cyclase through a G-protein mediated mechanism, causing the production of cAMP as the second messenger. Among cAMP's primary downstream targets is protein kinase A (PKA), which in turn activates a number of other target molecules including the transcription factors CREB, CREM, and ATF-1.
Diacyl glycerol (DAG) and phosphatidylinositol triphosphate (IP3). Some serotonin, calcitonin, and histamine receptors are GPCRs that lead to DAG and IP3 production. Ligand binding activates membrane bound phospholipase C (PLC) through a G-protein mediated mechanism. PLC hydrolyzes the membrane lipid, inositol bisphosphonate (IP2), generating DAG and IP3.
DAG remains in the plasma membrane, facilitating the translocation of protein kinase C (PKC) from the cytosol to the plasma membrane and serving as an activator of that enzyme. PKC then activates other downstream target molecules including MARCK.
IP3 diffuses into the cytosol to bind to IP3-receptors in the smooth endoplasmic reticulum (SER) membranes. IP3-receptors are ligand-gated calcium channels that open upon IP3 binding to allow the release of calcium in the SER into the cytosol. The resulting increase in cytosolic calcium concentration induces the activation of many calcium-dependent processes such as muscle contraction, membrane polarization, and enzyme activity.
Steroid hormone receptor family. Most peptide hormones and growth factors signal the cell interior through cell-surface receptors and associated second messengers. Steroid hormones (e.g., hydrocortisone and estrogen), retinoids (vitamin A–related compounds), and vitamin D pass more readily through the membrane and bind to receptors in the nucleus. The thyroid hormone receptor also belongs to this group (Because thyroid hormone consists mainly of the aromatic amino, tyrosine, it passes readily through the plasma membrane). This family of nuclear receptors is characterized by a specific ligand-binding site and a DNA-binding site. They typically form dimers and activate or repress gene transcription through their direct association with response elements in the DNA flanking the gene-coding regions
Endocytosis. Cells engulf extracellular substances and bring them into the cytoplasm in membrane-limited vesicles by mechanisms described collectively as endocytosis.
In phagocytosis (cell eating), the cell engulfs insoluble substances, such as large macromolecules or entire bacteria. The vesicles formed are termed phagosomes.
In pinocytosis (cell drinking), the cell engulfs small amounts of fluid, which may contain a variety of solutes. Pinocytotic vesicles are smaller than phagosomes.
In receptor-mediated endocytosis (Fig. 2–4), a cell engulfs ligands along with their surface receptors. The binding of ligand to receptor causes the ligand–receptor complexes to collect in a coated pit, a shallow membrane depression whose cytoplasmic surface is covered with the coat protein, clathrin. After further invagination, the protein, dynamin, coils around the neck of the budding vesicle and pinches it off to create a coated vesicle, which carries the ligand–receptor complexes into the cell. The clathrin coat is released from the vesicle, now termed an early endosome, and the ligands dissociate from the receptors. The late endosome, or compartment of uncoupling of receptor and ligand (CURL), becomes more tubular and divides into two portions, segregating the receptors from the ligands. The portion with the receptors pinches off and returns to fuse with the plasma membrane. The portion with the ligands fuses with a lysosome.
Exocytosis ejects substances from the cell. Cells use exocytosis both for secretion and for excretion of undigested material. A membrane-limited vesicle or secretory granule fuses with the plasma membrane and releases its contents into the extracellular space, without disrupting the plasma membrane.
Compartmentalization. Membranes selectively block the passage of most water-soluble substances. The cytoplasm has many membrane-limited compartments (organelles), each with different internal solute concentrations. This compartmentalization prevents the dilution of substrates, metabolic intermediates and cofactors in multistep biochemical reactions, and protects sensitive reactions from the intrusion of extraneous substances.
Spatial–temporal organization of metabolic processes. Some cell membranes (e.g., inner mitochondrial membrane and Golgi complex) contain enzymes arranged in series so that intermediates in multistep metabolic processes are passed from enzyme to enzyme. This arrangement maintains the chronologic order of such processes and sets rate limits by maintaining local concentrations of intermediates.
Storage, transport, and secretion. Substances in vesicles may be kept for later use (storage), shuttled from one compartment to another for further processing (transport, II.D), or expelled from the cell (secretion, II.C.4).
D. Membrane Flow (Traffic)
Membrane moves from one organelle to another. This general feature of organelle function allows homeostatic regulation of membrane area. Membranes bud as vesicles from one organelle and fuse with another, allowing the amount of membrane in a particular organelle to change without membrane synthesis or breakdown. Vesicle budding is energetically facilitated by coat proteins (e.g., β-COP, clathrin) that vesicles acquire on their cytoplasmic surfaces during budding. These coats help shape the bud and ensure its inclusion of molecules that direct vesicle transport toward the proper destination. After a coated vesicle is formed, the coat is released. The resulting transfer vesicle may diffuse to nearby targets (e.g., ER to Golgi) or attach to microtubules for transport over longer distances (e.g., neuron cell body to axon terminal). Transmembrane proteins in budding vesicle membranes, called v-SNAREs, remain after the coat is shed. They are critical in allowing transfer vesicles to recognize, bind, fuse with, and unload their contents, in a GTP-dependent process, at the correct target organelle. Receptors in the target organelle membranes, called t-SNAREs, selectively bind the v-SNAREs and recruit other cytoplasmic proteins required for fusion with the target membrane.
Schematic diagram of the biochemical components of plasma membranes (II.A). Labeled components include cholesterol (A), the oligosaccharide moiety (B) of a glycoprotein on the extracellular surface, integral proteins (C and D), phospholipid molecules (E) with their fatty acid tails (F) and polar head groups (G), and peripheral protein (H).
Schematic diagrams of signal-transducing membrane receptors. A. Ion channel–linked receptor (transmitter-gated ion channel). Ligand binding causes a conformational change in the receptor, allowing ions to pass through the membrane. B. Enzyme-linked receptor. Ligand binding causes a conformational change in the receptor that activates an enzyme (e.g., kinase) that is part of the receptor (shown) or another protein associated with the cytoplasmic domain of the receptor (not shown).
Schematic diagram of G protein–coupled receptor activation. A. System components. B. Ligand-receptor binding causes a conformational change in the receptor allowing the α subunit of the trimeric G protein complex to interact with it. C. As the α subunit interacts with the receptor, it undergoes a conformational change, allowing release of its GDP. D. GDP is traded for GTP. E. This causes a further conformational change of the α subunit. F. This change causes the subunit to disengage from the receptor, dissociate from the γ and β subunits, and interact with a G-protein–activated kinase. In this interaction, one high-energy bond in the GTP is used to activate the kinase, resulting in the transformation of GTP to GDP.
Schematic diagram of receptor-mediated endocytosis (II.C.3.c).
Cytoplasmic structures comprise three groups. Organelles are membrane-bound, enzyme-containing subcellular compartments (e.g., mitochondria). Each organelle has a distinctive structure and unique functions. Cytoplasmic inclusions are structures, membrane-limited or not, that are generally transient cargo and passive in cell metabolism (e.g., lipid droplets). Organelles and inclusions are discussed in sections III.A–H. The cytoskeleton (III.I) is a proteinaceous supporting network within the cytoplasm, components of which also form discrete cytoplasmic structures.
These large organelles generate chemical energy (ATP) from nutrients for use by the cell.
Structure. Mitochondria are comparable in size to bacteria (typically 2–6 μm long and 0.2 μm wide). They have various shapes (e.g., spherical, ovoid, filamentous). Each is bounded by two unit membranes.
The outer mitochondrial membrane (Fig. 2–5, A) has a smooth contour and forms a continuous but porous covering. It is permeable to small molecules (<5 k Da) owing to large channel-forming proteins called porins.
The inner mitochondrial membrane (Fig. 2–5, B) is less permeable and has many infoldings, or cristae (Fig. 2–5, C). The cristae of most mitochondria are shelflike, but in steroid-secreting cells are tubular. Its inner surface contains inner membrane subunits (Fig. 2–5, H), also called F1 subunits (or lollipops, because of their shape in certain preparations); these are sites of mitochondrial ATP synthase activity. Intercalated in the inner membrane are components of the electron transport system, including enzymes and cofactors that have important roles in mitochondrial function (e.g., cytochromes, proton pumps, dehydrogenases, flavoproteins). Mitochondrial ribosomes also associate with the inner surface.
Membrane-limited spaces. The mitochondrial membranes create two membrane-limited spaces. The intermembrane space (Fig. 2–5, F) is located between the inner and outer membranes and is continuous with the intracristal space (Fig. 2–5, E), which extends into the cristae. The intercristal space, or matrix space (Fig. 2–5, D), is enclosed by the inner membrane and contains the mitochondrial matrix.
Mitochondrial matrix. This matrix contains water, solutes, and large matrix granules (Figure 2–5, G), which play a role in mitochondrial calcium ion concentration. It also contains circular DNA and mitochondrial ribosomes similar to those of bacteria. The matrix contains numerous soluble enzymes involved in such specialized mitochondrial functions as the citric acid (Krebs, tricarboxylic acid) cycle, lipid oxidation, and mitochondrial protein and DNA synthesis.
Function. Mitochondria provide energy for chemical and mechanical work by storing energy from cellular metabolites in the high-energy bonds of ATP. Energy is generated by oxidative phosphorylation, which involves four main steps: (1) the citric acid cycle uses acetyl CoA generated from pyruvate (from carbohydrates) or fatty acids (from lipids) to generate high-energy electrons, which it donates to NADH and FADH2; (2) these intermediates release the high-energy electrons to the electron transport chain in the inner mitochondrial membrane, which uses their energy to drive pumps that eject protons from the matrix; (3) the proton imbalance establishes an electrochemical gradient across the inner mitochondrial membrane—a form of potential energy; (4) the flow of protons back into the matrix, down the gradient established by the proton pumps, occurs through a channel in ATP synthase. ATP synthase captures that energy by converting adenosine diphosphate (ADP) plus inorganic phosphate to ATP. ATP exits the mitochondrion and releases its stored energy at various intracellular sites. Mitochondria synthesize their own DNA and some proteins. They grow and reproduce by fission or budding and can undergo rapid movement and shape changes.
Location. Mitochondria occur in nearly all eukaryotic cells, and in most are dispersed throughout the cytoplasm. They accumulate in cell types and intracellular regions with high energy demands. Cardiac muscle cells contain abundant mitochondria. Epithelial cells lining kidney tubules have abundant mitochondria interdigitated between basal plasma membrane infoldings, where active ion and water transport occurs.
These protein-synthesizing organelles are two main types. Mitochondrial (like prokaryotic) ribosomes are smaller (20 nm) than cytoplasmic (eukaryotic) ribosomes (25 nm).
Structure. Each ribosome type has two unequal ribosomal subunits, named for their ultracentrifugal sedimentation values (but often called simply “large” and “small”). Mitochondrial ribosomes (70S overall) have a 50S and a 30S subunit; cytoplasmic ribosomes (80S overall) have a 60S and a 40S subunit. Cytoplasmic ribosomes are composed of ribosomal RNA (rRNA) synthesized in the nucleolus and many proteins synthesized in the cytoplasm. Light microscopy reveals cytoplasmic ribosome accumulations as basophilic patches, formerly termed ergastoplasm in glandular cells and Nissl bodies in neurons. In EMs, ribosomes appear as small, electron-dense cytoplasmic granules.
Location and function. Ribosomal subunits are dispersed in the cytoplasm. Cytoplasmic ribosomes occur in two forms. A free ribosome forms as a large and small subunit assemble on a strand of messenger RNA (mRNA). Polyribosomes, or polysomes, are multiple ribosomes attached to a single mRNA, permitting synthesis of multiple copies of a protein from the same message. Ribosomes translate the mRNA code and thus play a critical role in assembling amino acids into specific proteins. As each ribosome reaches the end of the message, its subunits separate and release the mRNA. Polysomes occur free in the cytoplasm (free polysomes) or attached to RER membranes; free polysomes synthesize intracellular structural proteins and enzymes. RER polysomes synthesize proteins to be secreted, sequestered, or inserted into membranes. Various signal sequences are encoded in the 5′ end of mRNAs, helping to direct the proteins that contain the translated sequence to different organelles. The signal sequences directing secretory proteins to the RER are described in the next section (III.C.1.b). Other signal sequences direct proteins to, and help them enter, the nucleus, mitochondria, and peroxisomes. A common posttranslational modification of cytoplasmic proteins that regulates many cell functions is protein phosphorylation. Phosphate groups, transferred from ATP or GTP to proteins by protein kinases (e.g., tyrosine kinase or serine/threonine kinases), or removed by protein phosphatases, can activate or inactivate a wide variety of proteins after they have been synthesized. Such reactions are often key steps in signal transduction (II.C.2).
C. Endoplasmic Reticulum (ER)
This complex organelle participates in synthesizing, packaging, and processing various molecules. It is a freely anastomosing network (reticulum) of membranes that form vesicles, or cisternae; these may be elongated, flattened, rounded, or tubular. Transfer vesicles (II.D) bud from the ER and cross the intervening cytoplasm, delivering their contents to the Golgi complex (III.D) for further processing or packaging. In mature cells, ER occurs in two forms: rough and smooth.
Rough endoplasmic reticulum (RER)
Structure. RER is studded with ribosomes in polysomal clusters. RER cisternae are typically parallel, flattened, elongated, and especially abundant in cells specialized for protein secretion (e.g., pancreatic acinar cells, plasma cells). Ribosomes render the RER basophilic. RER membranes and individual ribosomes are visible only with the electron microscope. Proteins unique to RER membranes include docking protein, ribophorins, and signal peptidase (III.C.1.b).
Function. The RER synthesizes proteins for sequestration from the cytoplasm, including secretory proteins such as collagen, proteins for insertion into cell membranes (integral proteins; II.A.2.a), and lysosomal enzymes (isolated to prevent autolysis). Ribosomes assemble on and read mRNAs from 5′ to 3′. The 5′ end of mRNAs for secretory, sequestered and integral membrane proteins carries the code for a 20- to 25-amino acid signal sequence. The signal sequence is translated on a free polysome and subsequently binds with a cytoplasmic signal recognition particle (SRP, six polypeptides plus a 7S RNA molecule). SRP halts translation until the SRP–polyribosome complex binds to the RER docking protein; the SRP is then released and translation continues. Ribophorins mediate the attachment of the signal sequence and large ribosomal subunit to the RER membrane and provide a hydrophilic translocation channel for vectorial discharge (unidirectional passage) of nascent protein into the RER lumen. Here the signal sequence is cleaved by signal peptidase and the resulting nascent protein undergoes folding with the aid of protein chaperones (e.g., calnexin, calreticulin) and assembly. Chaperones also assist in quality control, retaining misfolded or unassembled protein complexes in the ER cisterna. If modifications are incomplete or unsuccessful, the new proteins are eventually degraded. Another important posttranslational modification in the ER is core glycosylation, in which preassembled oligosaccharides, often high in mannose, are transferred from a lipid carrier (e.g., dolichol phosphate) to amino acids, especially asparagine. The oligosaccharides help “address” proteins for transport to intracellular destinations.
Location. The RER is suspended in the cytoplasm and continuous with the nuclear envelope's outer membrane. The RER in protein-secreting epithelial cells often lies in the basal cytoplasm, between the plasma membrane and the nucleus.
Smooth endoplasmic reticulum (SER)
Structure. The SER lacks ribosomes and thus appears smooth in electron micrographs. Its cisternae are tubular or vesicular. SER stains poorly, if at all; thus, with the light microscope, it is indistinguishable from the rest of the cytoplasm.
Function. Because it lacks ribosomes, SER cannot synthesize proteins. It has many enzymes that are important in lipid metabolism, steroid hormone synthesis, gluconeogenis (glucose-6-phosphatase), and detoxification. The last occurs by means of enzymatic conjugation, oxidation, and methylation of potentially toxic substances. It plays a key role in regulating cytosolic calcium concentrations by sequestering excess calcium and releasing it when IP3 receptors in its membranes are activated [II.C.2.c.(2)(b)]. Because the steroid hormones it synthesizes diffuse freely through cell membranes, they need not be transferred to the Golgi complex for packaging and release.
Location. The SER is suspended in the cytoplasm of many cells and is especially abundant in cells synthesizing steroid hormones (e.g., in the adrenal cortex and gonads). It is also abundant in liver cells (hepatocytes), where it participates in glucose metabolism and drug detoxification. Specialized SER termed sarcoplasmic reticulum is found in striated muscle cells, where it regulates muscle contraction by sequestering and releasing calcium ions (10.II.B.2).
The Golgi complex (Golgi apparatus) is a focal center of membrane flow (II.D) and vesicle traffic among organelles and has a key role in secretion.
Structure. This membranous organelle comprises three major compartments: (1) a stack of 3–10 slightly curved, flattened cisternae with peripheral dilations; (2) numerous small vesicles peripheral to the stack; and (3) a few large condensing vacuoles at the concave surface of the stack. The cis face (convex face, forming face) of the stack is usually close to adjacent dilated ER cisternae and surrounded by transfer vesicles. Its cisternae stain with osmium. The trans face (concave face, maturing face) often harbors several condensing vacuoles and faces away from the nucleus. It connects to a system of tubules and vesicles called the trans Golgi network (TGN), from which secretory vesicles and lysosomes exit.
Polysaccharide synthesis. The Golgi complex contains glycosyltransferases that initiate, lengthen, or shorten polysaccharide or oligosaccharide chains one sugar at a time.
Modifying secretory products. The cis Golgi contains enzymes that glycosylate proteins and lipids and sulfate glycosaminoglycans (GAGs). It is thus important in synthesizing secretory glycoproteins, proteoglycans, glycolipids, and sulfated GAGs. It also participates in tagging lysosomal enzymes with mannose-6-phosphate for segregation by the TGN.
Sorting secretory products. Products synthesized by the RER and modified in the cis Golgi are sorted in the TGN. For example, lysosomal enzymes marked with mannose-6-phosphate and secretory proteins destined for constitutive versus regulated exocytosis are segregated into vesicles with different v-SNAREs in their membranes allowing them to directed to different target sites (II.D).
Packaging secretory products. The TGN packages the segregated products into vesicles. These secretory vesicles, or secretory granules, are transported to the plasma membrane for exocytosis (II.C.5).
Concentrating and storing secretory products. The Golgi complexes of some cells concentrate and store secretory products prior to secretion. Concentration (e.g., by removing water from the vesicle lumen) is a major function of the TGN's condensing vacuoles, which also often serve as precursors to secretory granules.
Location. The Golgi complex typically lies near the nucleus (juxtanuclear) and is often found near centrioles (which organize the Golgi membranes and direct vesicle traffic). Its membranes typically associate with microtubules by binding the molecular motor, dynein. Because dynein generally moves vesicles toward the microtubule's minus end, Golgi membranes collect near the microtubule organizing center (III.H.1.b) adjacent to the nucleus. Golgi complexes are well developed in neurons and secretory (gland) cells.
Flow through the Golgi complex. Secretory materials were long thought to follow a one-way route (cis to trans) through the Golgi complex. This is an oversimplification. Golgi-associated vesicles differ in their source, destination, function, contents, and surface composition. Certain nonclathrin, vesicle-coating proteins (e.g., β-COP, COPI, COPII) and SNAREs are associated with specific Golgi regions, indicating that various vesicle types fuse with, and bud from, the cis, trans, or intermediate Golgi membranes.
These membrane-limited vesicles of variable size contain material destined for lysosomal digestion. Heterophagosomes contain extracellular material. Autophagosomes contain intracellular material, such as worn or damaged organelles). Digestion begins when a phagosome fuses with one or more primary lysosomes to form a secondary lysosome, as described below. (Note: Some authors use the term “heterophagosome” to refer to secondary lysosomes [III.F.2].)
These spherical, membrane-limited vesicles function as the cell's digestive system and may contain more than 50 enzymes each. Their enzyme activities distinguish them from other cellular granules. Acid phosphatase occurs almost exclusively in lysosomes and is often used to identify them. Other common lysosomal enzymes include ribonucleases, deoxyribonucleases, cathepsins, sulfatases, beta-glucuronidase, phospholipases, various proteases, glucosidases, and lipases. An inherited lysosomal enzyme deficiency can cause life-threatening lysosomal storage diseases (substrate accumulations in the cytoplasm). Lysosomal enzymes usually occur as glycoproteins and are most active at acidic pH. Lysosomes vary in sizes and electron density, depending on their activity.
Primary lysosomes are small (5–8 nm diameter) and electron-dense, appearing as solid black circles in EMs. Enzymes in these storage lysosomes (released directly from the TGN) are inactive. Lysosomal enzymes synthesized and core-glycosylated in the RER are transferred to the Golgi complex for further glycosylation and packaging in vesicles (III.D.2). Primary lysosomes disperse through the cytoplasm and are abundant in phagocytic cells (e.g., macrophages, neutrophils).
Secondary lysosomes are larger, less electron-dense, and have a mottled appearance in EMs. They form by the fusion of one or more primary lysosomes with a phagosome. Fusion activates a proton pump in their membrane, which acidifies the contents and activates the enzymes. Their primary function is digesting products of heterophagy and autophagy. Lysosomal digestion yields metabolites for cell maintenance and growth (small molecules diffuse into the surrounding cytoplasm) and aids in organelle turnover. Lysosomal enzymes also digest some cell synthesis products, thus regulating the quality and quantity of secretory material. Secondary lysosomes occur throughout the cytoplasm in many cells, in numbers that reflect the cell's lysosomal and phagocytic activity.
Residual bodies are membrane-limited inclusions of various sizes and electron densities associated with the terminal phase of lysosome function. They contain indigestible materials, such as pigments, crystals, and certain lipids. Some cells (e.g., macrophages) expel residual bodies as waste, but long-lived cells (e.g., nerve, muscle) accumulate them. In the latter, waste-containing residual bodies reflect cellular aging and are termed lipofuscin granules. These granules appear yellow–brown in light microscopy and as electron-dense particles in EMs.
Peroxisomes are membrane-limited, enzyme-containing vesicles slightly larger than primary lysosomes. In rats, they differ from lysosomes because of their electron-dense, granular urate oxidase nucleoid. Peroxisomes function in hydrogen peroxide metabolism. They contain urate oxidase, hydroxyacid oxidase, and d-amino acid oxidase, which produce hydrogen peroxide capable of killing bacteria; they also contain catalase, which oxidizes various substrates and uses the hydrogen removed in the process to convert toxic hydrogen peroxide to water. Peroxisomes also participate in gluconeogenesis by assisting in fatty acid oxidation. They occur dispersed in the cytoplasm or in association with SER.
H. Other Cytoplasmic Inclusions
Prominent among storage inclusions are spherical lipid droplets, which differ in appearance depending on the histologic preparation. Glycogen granules are PAS-positive in light microscopy and appear in EMs as rosettes of electron-dense particles. Both lipid droplets and glycogen granules lack a limiting membrane. Melanin is a brown pigment widely distributed in vertebrates and often found in electron-dense, membrane-limited granules termed melanosomes. It is particularly abundant in epidermal cells, the retina's pigment epithelium, and the brain's substantia nigra.
The cytoskeleton, a mesh of filamentous elements called microtubules, microfilaments, and intermediate filaments, provides structural stability for maintaining cell shape. It is also important in cell movement and rearranging cytoplasmic components.
Structure. Microtubules (Fig. 2–6) are the thickest (24-nm) cytoskeletal components. In EMs these fine tubules vary in length and have dense walls (5-nm thick) and a clear internal space (14-nm across). The walls consist of subunits called tubulin heterodimers, each of which comprises one α-tubulin and one β-tubulin protein molecule. The tubulin heterodimers are arranged in threadlike chains called protofilaments, 13 of which align parallel to one another to form the wall of each microtubule. Each microtubule is polarized, with a plus (+) and minus (−) end. They exist in a state of dynamic instability, undergoing abrupt changes in length through changes in the balance between polymerization and depolymerization.
Function. Microtubules form a network of roadways in the cell, deploy cytoplasmic organelles (including the ER and Golgi complex), shuttle vesicles from one part of the cell to another, and move chromosomes during mitosis. Their instability is critical to their function. The drug Taxol causes permanent stabilization and leads to cell death. Most microtubules anchor by their minus ends in the γ-tubulin rings of satellite bodies that act as nucleation sites in the microtubule organizing center (MTOC). The MTOC is a juxtanuclear region containing the centrioles and pericentriolar (satellite) bodies. Microtubules extend from their nucleation sites by adding GTP-containing tubulin heterodimers to their plus end. The GTP is hydrolyzed by tubulin to GDP shortly after heterodimer incorporation into a growing microtubule, causing destabilization, depolymerization, and retraction as the GDP-containing heterodimers are released. Rapidly growing tubules develop a GTP cap on their plus end, briefly protecting them from retraction, but will depolymerize, if they fail to attach to an organelle or a stabilizing microtubule-associated protein (MAP) in time. Stabilized microtubules acquire other MAPs; large, ATPase-containing molecular motors (e.g., kinesin and dynein) capable of binding cellular structures and “walking” them along the microtubules, provide an intracellular transport system. Kinesin carries its cargoes mainly toward the plus end, and dynein toward the minus end, of stabilized microtubules. Different motors appear to exist for each organelle or vesicle type. Thus microtubule struts that spread the ER through the cytoplasm with the aid of kinesin, when exposed to colchicine (a drug causing net microtubule depolymerization), collapse, allowing the ER to collapse around the nucleus. Conversely, dynein-assisted aggregation of the Golgi complex toward the nucleus fails, causing Golgi dispersion, in the presence of the same drug.
Location. Microtubules originate repeatedly from the MTOC, growing outward through the cytoplasm and retracting if they fail to connect. Those that do connect stabilize and provide a latticework supporting organelle deployment and vesicle traffic. Stabilized microtubule arrays occur in cilia (III.J), flagella (III.K), basal bodies (III.L), centrioles (III.M), and the mitotic spindle apparatus (III.N).
Structure. Microfilaments are the thinnest cytoskeletal elements (5–7 nm) and are more flexible than microtubules. They are filamentous polymers of one of several types of globular actin protein monomers. In striated muscle cells, actin filaments form a stable paracrystalline array in association with myosin filaments. Actin filaments in other cells are less stable and repeatedly dissociate and reassemble. These changes are regulated in part by calcium ions, cAMP, and by a host of actin-binding proteins in the cytoplasm and attached to the plasma membrane's cytoplasmic surface. In addition to regulating polymerization and depolymerization, actin-binding proteins arrange microfilaments into the networks and bundles that carry out their many important functions.
Function. Microfilaments are contractile, but to contract they must interact with myosin, the only actin-associated motor-protein family. In muscle cells, myosin forms thick filaments. In nonmuscle cells, it exists in soluble form and binds to microfilaments by its globular head, leaving its tail (free end) to attach to the plasma membrane or other cellular components to move them. Each actin monomer harbors an ATP molecule that promotes binding during polymerization but hydrolyzes to ADP shortly after binding to destabilize the microfilament, unless stabilizing actin–binding proteins are present. Cytochalasins disrupt microfilament organization and interfere with the following functions: endocytosis; exocytosis; extension and contraction of microvilli; cell movement; cytoplasmic streaming; maintenance of cell shape; and equatorial constriction of dividing cells. Phalloidin binds to intact microfilaments, stabilizes them, and interferes with these same functions. Because it fluoresces, phalloidin may be used to localize actin filaments.
Location. In nonmuscle cells, microfilaments form an irregular cytoplasmic mesh. Local accumulations occur as a thin sheath under the plasma membrane called the terminal web; as parallel strands in cores of microvilli; in the cytoplasm at the leading edge of pseudopods; in association with the plasma membrane, organelles, or other cytoplasmic components; or as a belt (“purse string”) around the equator of dividing cells.
Structure. Intermediate filaments are ropelike and composed of shorter threadlike protein subunits twisted around one another to form filaments with a diameter (10–12 nm) intermediate between microtubules and microfilaments. Their protein subunits are globular at their amino and carboxy terminals, with an elongated, linear central domain. The individual proteins belong to the same family as nuclear lamins (3.II.B) and differ depending on the cell type. Examples: cytokeratins in epithelial cells, vimentin in mesenchyme derived cells (e.g., fibroblasts, chondrocytes), desmin in muscle cells, glial fibrillary acidic protein in glial cells, and neurofilaments (intermediate filament bundles) in neurons. The stability and longevity of these proteins, together with their cell-type specificity, make them particularly useful in immunohistochemical determination of the origin of neoplastic cells.
Function. Intermediate filaments are notable for their tensile strength and durability. Their abundance in cells subjected to mechanical stress (e.g., cells of skin, connective tissue, and muscle) indicates a role in stabilizing cell structure and in the many functions that depend on maintaining cell shape.
Location. In most cells, intermediate filaments form a network surrounding the nucleus and extend throughout the cytoplasm. Their ordered arrangement in certain cells (e.g., neurons and keratinocytes of the skin) reflects their special role in maintaining cell shape. Cytokeratin-containing tonofilaments of desmosomes (4.IV.B.3) are examples.
A ciliated cell typically has hundreds of cilia, which are motile, 5- to 10-μm long, 0.25-μm wide, plasma-membrane covered, cell-surface extensions. Each contains a core, or axoneme, composed of nine peripheral microtubule doublets surrounding a pair of discrete microtubules (the “9+2” arrangement). The peripheral doublets consist of a full A microtubule and a partial B microtubule (Fig. 2–6, B). Attached to each A microtubule are two dynein arms, which interact with the B microtubule in the adjacent doublet and drive the movement of the doublets past one another. Other proteins that crosslink the doublets prevent simple sliding and convert the motion into bending. Ciliary movement occurs in two phases: a forward power stroke, in which the distal part of the cilium remains straight and rigid, and a return or recovery stroke, in which the cilium is more flexible and bent.
Most mammalian cells exhibit a single non-motile cilium extending like an antenna from the cell surface. They have a microtubule core resembling that of motile cilia, but lacking the two central microtubules (i.e., “9+0”). These cilia have sensory functions, receiving mechanical and chemical signals from other cells and the environment. Primary cilia extending from receptor cells in the olfactory epithelium are specialized to detect odors. The stalk containing the inner and outer segments of visual receptors in the retina contain a primary cilium-like structure. Inherited defects in the form and function of these structures on the surface of cells lining the renal tubules result in polycystic kidney disease.
A flagellum is similar to a motile cilium but it is longer and typically only one or two are present per cell. In mammals, flagella, typically are 50- to 55-μm long and 0.2- to 0.5-μm thick, occur in the tails of spermatozoa. The flagellar axoneme is identical to that of a motile cilium but is separated from the surrounding plasma membrane by large protein masses. Flagellar movement resembles a turning corkscrew more than the whiplike action of the cilia.
These structures serve as the anchoring points and microtubule organizers for all cilia or flagella. Centrioles (III.N) migrate to the plasma membrane and give rise to basal bodies as in centriole self-duplication. Basal bodies resemble centrioles (Fig. 2–6, C), with nine microtubule triplets.
Structure. A centriole is a cylinder of microtubules, 150 nm in overall diameter and 350- to 500-nm long, containing nine microtubule triplets in a pinwheel array (Fig. 2–6, C). Each microtubule in a triplet shares a portion of its neighbor's wall. An interphase (nondividing) cell has a perpendicular pair of adjacent centrioles, each surrounded by several electron-dense satellites, or pericentriolar bodies containing γ-tubulin. Cytoplasmic microtubules radiate from the pericentriolar bodies into the cytoplasm.
Function. Centrioles are the cell's structural organizers. Centriole duplication is required for cell division. During mitosis, the centrioles organize the mitotic spindle (III.O). Even in vitro, isolated centrioles control microtubule polymerization; centrioles transmit physical organizing forces by means of the microtubules radiating from the pericentriolar bodies. Through their effects on microtubules, centrioles control organelle, vesicle, and granule traffic within the cell. Centrioles also give rise to basal bodies (III.M). However, centrioles are not nucleation sites for cytoplasmic microtubules; the γ-tubulin rings in the pericentriolar bodies serve this function.
Location. Between cell divisions, centrioles lie near the nucleus, often surrounded by Golgi complexes. The centrioles and associated Golgi complexes constitute the centrosome (cytocenter), which appears as a clear juxtanuclear zone. During the S phase of interphase (Fig. 3–2), each centriole duplicates, forming a procentriole perpendicular to the original. During mitosis, new centriole pairs migrate to opposite cell poles to organize the spindle.
O. Mitotic Spindle Apparatus
In preparation for mitosis, cytoplasmic microtubules depolymerize and repolymerize as the mitotic spindle apparatus. This spindle-shaped microtubule array assembles between two centriole pairs at opposite poles of dividing cells (3.VI.A.2.a). Some spindle microtubules (continuous fibers) extend from centriole to centriole. Others (chromosomal fibers) extend from one centriole to the centromere of a chromosome at the equatorial plate. The spindle apparatus is crucial for chromosome separation during mitosis.
Schematic diagram of a mitochondrion (III.A). Labeled components include the outer mitochondrial membrane (A), inner mitochondrial membrane (B), cristae (C), mitochondrial matrix in the intercristal space (D), intracristal extension (E) of the intermembrane space (F), matrix granules (G), and inner membrane subunits (F1 subunits) (H).
Schematic diagrams of microtubules and their contributions to cilia and centrioles. A. Microtubules as seen by the electron microscope. Cross-sections of tubules show a ring of 13 subunits of dimers arranged in a spiral. Changes in microtubule length are caused by the addition or loss of individual tubulin subunits. B. A cross-section through a cilium reveals a microtubule complex, or axoneme, at its core. An axoneme consists of two central microtubules surrounded by nine microtubule doublets (9 + 2). In the doublets, the A microtubule is complete and consists of 13 subunits, whereas the B microtubule shares two or three heterodimers with the A. When activated by ATP, the dynein arms (which harbor ATPase) link adjacent tubules and provide for the sliding of doublets past each other. C. Centrioles consist of nine microtubule triplets in a pinwheel array. In the triplets, the A microtubule is complete and consists of 13 subunits, whereas the B and C microtubules share tubulin subunits. These organelles typically occur in pairs disposed at right angles to each other. (Reproduced, with permission, from Junqueira LC, Carneiro J, Basic Histology: Text & Atlas, 11th ed. McGraw-Hill, Inc., New York, 2005.)
Cells are both empowered and constrained by their available resources. The amounts and types of energy and raw materials at their disposal, the information encoded in their genes, and intrinsic and extrinsic (epigenetic) factors controlling access to that information are major determinants of cell function. Cells in tissues undergoing growth or repair use more resources preparing for, and carrying out, cell division. Fully differentiated cells concentrate on more specialized functions (e.g., secretion, contraction). Maintaining a constant internal environment (homeostasis), even in quiescent cells, requires the expenditure of significant energy and other resources.
A. Cellular Differentiation
Refinements in cell structure and function accompany embryonic and fetal development, maturation, aging, and death. This cellular differentiation generally results in dividing less often and having fewer, but more efficient, capabilities than embryonic cells. The functions of a differentiated cell can be gauged by the organelles it contains. For example, cells specialized for protein secretion contain abundant RER and a well-developed Golgi complex. Although differentiation can result in dramatic changes, it does not occur suddenly. It occurs in steps, often separated by one or more passes through the cell cycle, and involves interactions among the cell's environment, metabolic machinery, and DNA. Among the more obvious changes taking place during differentiation are the changes in the genes being expressed. Cancer often involves a process called dedifferentiation in which a cell returns to a more embryonic character. Differentiation-inducing agents (e.g., retinoic acid) may be employed to reverse carcinogenesis.
B. Intercellular Communication
Tissues, organs, and organ systems are collections of cells and cell products acting in concert to carry out complex functions. The embryonic cells that ultimately form a tissue develop communication strategies early in embryogenesis. Many types of intercellular communication occur—direct and indirect.
Direct communication. During cell-cell recognition or contact inhibition of cell division, signal transduction may require temporary physical contact. In some tissues, especially in epithelia, cells have direct contact with their neighbors over a large surface area. These areas of contact are often marked by specialized junctional complexes (4.IV.B). Some components of junctional complexes are specialized for attachment (physical communication), and others (gap junctions) provide cytoplasmic channels that transmit electric and chemical signals.
Indirect communication. Signals are also transmitted from cell to cell even when they are not in contact. In paracine communication, the signal traverses a short distance (e.g., hormones, growth factors, or other signal molecules produced by one cell type have effects on another in the same tissue). In endocrine and neural communication, the signal travels farther (e.g., when hormone-producing cells in one tissue elicit responses from targets in others).
Many cell functions, especially those involving cell shape and tissue integrity, depend on cell adhesion. Cell-cell adhesion (e.g., in epithelia), requires physical linkages between the cytoskeletons of neighboring cells. Intercellular binding is mediated by transmembrane proteins called cadherins. Their intracellular domains are linked to the cytoskeleton by adaptor proteins that may form plaques on the cytoplasmic surfaces of opposing membranes. Cell–substrate adhesion involves linkages between the cytoskeleton and collagen fibers of the extracellular matrix. Transmembrane proteins in these attachments are called integrins. In these junctions, intracellular adaptor proteins attach integrins to the cytoskeleton and extracellular adaptor proteins (e.g., laminin and fibronectin) attach integrins to collagen. Specific examples of cell adhesion are presented in subsequent chapters (4.IV.B,C,E; 5.II.A.1.b and D.2).
Apoptosis is the term applied to the processes leading to programmed cell death. Because the process involves activating cellular mechanisms whereby the cell participates in its own destruction, many refer to it as cellular suicide. Whether a cell does or does not follow an apoptotic pathway depends on the balance between protective and destructive signals.
Structural changes. During apoptosis, the cell shrinks in size. Its plasma membrane undergoes irregular blebbing and becomes leaky; this exposes phosphatidylserine, normally buried in the membrane's inner leaflet, to the surface. Internally the cell's organelles, especially the mitochondria, swell and break apart releasing proteins that are normally sequestered. Nuclear chromatin condenses, fragments, and is degraded. Ultimately the entire cell breaks into irregular, membrane-limited apoptotic bodies, displaying phosphatidyl serine on their surface, which attracts and activates phagocytic cells to remove them.
Functions of Apoptosis. In adults, apoptosis is primarily a response to cell or tissue damage beyond the cell's ability to restore its normal structure and function (e.g., sublethal mechanical or chemical trauma, viral infection, cancer). During development, it is a specialized form of differentiation in response to signals dictating the normal removal of otherwise healthy cells (e.g., removal of interdigital tissue leading to digit separation during embryonic development).
Mechanisms of Apoptosis. The two primary mechanisms leading to apoptosis are referred to as the intrinsic and extrinsic pathways. Although the initiation factors differ, both pathways converge on the same execution mechanism carried out by cysteine-aspartic-acid proteases (caspases), resulting in apoptotic body formation.
Intrinsic (Mitochondrial) Pathway. Mitochondria express a protein, Bcl-2, on their surface that is associated with protection against apoptosis by preventing the release of mitochondrial cytochrome C. Mitochondrial damage, as occurs for example in the presence of reactive oxygen species and resulting lipid peroxidation, allows other mitochondrial proteins, (e.g., Bax, Bad, or Bak) to reach the mitochondrial surface and inactivate Bcl-2's anti-apoptotic effect. These Bcl-2 inactivators thus initiate perforation of the mitochondrial membrane, releasing cytochrome C into the cytoplasm, where it forms a complex with apoptotic protease-activating factor-1 (APAF-1) and ATP. These complexes aggregate to form apoptosomes, which bind and activate the protease caspase-9 (an initiator caspase). By then activating effector caspases (e.g., caspase 3 and caspase 7), caspase 9 initiates a cascade of protease activity that leads to destruction of cytoplasmic proteins, degradation of nuclear DNA, cell fragmentation, and phagocytosis of apoptotic bodies. Another intrinsic pathway involves the tumor suppressor protein p53, which halts the cell cycle when sufficient DNA breaks or mutations occur (e.g., after exposure to excess radiation; 3.VI.B.5.a). Sufficient p53 enhances apoptosis through interactions with multiple pro-apoptotic mitochondrial factors (e.g., Bax and Bid).
Extrinsic Pathway. Several external signals affect apoptosis. Growth factors (e.g., FGF) and cytokines (e.g., IL-2) can promote cell survival and thus protect against apoptosis. Other signals (e.g., FAS ligand and tumor necrosis factor (TNF) bind to cell surface receptors that then trigger apoptosis by activating the initiator caspase, caspase-8, which, like caspase-9, activates a protease cascade leading to protein and DNA degradation, cell fragmentation, and death.