1. Simple epithelia (Figure 1-2), formed by only one layer of cells. They are subdivided into simple squamous, simple cuboidal, and simple columnar, according to the height and width of the cells. The specific name endothelium is used for the simple epithelium lining the blood and lymphatic vessels. Mesothelium is the simple epithelium lining all body cavities (peritoneum, pericardium, and pleura).

2. Stratified epithelia (Figure 1-3) are composed of two or more cell layers. Stratified epithelia are subclassified according to the shapes of the cells at the superficial or outer layer into stratified squamous, stratified cuboidal, and stratified columnar. Stratified squamous is the epithelium most frequently found and can be subdivided into moderately keratinized (also known as nonkeratinized) or highly keratinized types. The cells of the outer layer of a moderately keratinized squamous epithelium can display nuclei (for example, esophagus and vagina). Nuclei are absent in the outer layer of the highly keratinized stratified squamous epithelium (for example, the epidermis of the skin). The basal cells aligned along the basal lamina are mitotically active and replace the differentiating cells of the upper layers.

3. Two special categories are the pseudostratified epithelium and the transitional epithelium (Figure 1-4). The pseudostratified epithelium consists of basal and columnar cells resting on the basal lamina. Only the columnar cells reach the luminal surface, however. Because the nuclei of the basal and columnar cells are seen at different levels, one has the impression of a stratified epithelial organization. Within this category are the pseudostratified columnar ciliated epithelium of the trachea and the pseudostratified columnar epithelium with stereocilia of the epididymis. The transitional epithelium of the urinary passages is also referred to as urothelium. The urothelium also consists of basal and columnar dome-shaped cells. An important feature of this epithelium is that its height varies with distention and contraction of the organ (see Chapter 14, Urinary System).

Figure 1-2 Simple epithelium

Figure 1-3 Stratified epithelium

Figure 1-4 Pseudostratified and transitional epithelia

An important aspect of epithelia is its polarity. Most epithelial cells line surfaces and cavities and have three domains (Figure 1-5):

1. The apical domain is exposed to the lumen or external environment.

2. The lateral domain faces neighboring epithelial cells linked to each other by cell adhesion molecules and junctional complexes.

3. The basal domain is associated with a basal lamina that separates the epithelium from underlying connective tissue. The basal lamina is reinforced by components of the connective tissue. The basal lamina–connective tissue complex is designated the basement membrane.

Figure 1-5 Domains of a polarized epithelial cell

Epithelial cells are attached to each other by junctional complexes and adhesion molecules. Epithelial cells are specialized to fulfill important roles, such as absorption and secretion or to act as a water or gas barrier. Several cell barriers and their functional significance are studied.

EPITHELIAL CELL POLARITY

Epithelial cells have two major domains (Figure 1-5):

1. An apical domain

2. A basolateral domain

Each domain is defined by specific structural and functional characteristics. For example, the apical domain has structures important for the protection of the epithelial surface (such as cilia in the respiratory tract) or for the absorption of substances (such as microvilli in the intestinal epithelium).

Junctional complexes and cell adhesion molecules are present at the basolateral domain to anchor epithelial cells to each other and to the basement membrane.

Apical differentiations

The apical domain of some epithelial cells can display three types of differentiation:

1. Cilia

2. Microvilli

3. Stereocilia

Cilia (singular, cilium; Figure 1-6) are motile cell projections originating from basal bodies anchored by rootlets to the apical portion of the cytoplasm. A basal body contains nine triplet microtubules in a helicoid array without a central microtubular component. By contrast, a cilium consists of an assembly called an axoneme, formed by a central pair of microtubules surrounded by nine concentrically arranged microtubular pairs. This assembly is known as the 9 + 2 microtubular doublet arrangement. The axoneme is also a component of the sperm tail, or flagellum.

Figure 1-6 Apical differentiations of epithelial cells: cilia and primary cilium

The trachea and the oviduct are lined by ciliated epithelial cells. In these epithelia, ciliary activity is important for the local defense of the respiratory system and for the transport of the fertilized egg to the uterine cavity.

Some cells have a primary cilium. The importance of primary cilia emerges from rare recessive human disorders known as ciliopathies caused by structural or functional abnormalities of cilia. The structure and mechanism of assembly of primary cilia are shown in Figure 1-6. The significant aspects of the primary cilium are: (1) they are non-motile; (2) they participate in the early stages of embryonic patterning leading to organogenesis; (3) many components of the hedgehog signaling pathway, essential at least in early development, are present in primary cilia; and (4) the position of the primary cilium, called kinocilium, of the hair cell of the organ of Corti in the inner ear determines the correct polarity of the actin-containing stereocilia (see Chapter 9, Sensory Organs: Vision and Hearing).

Microvilli (singular, microvillus; Figure 1-7) are finger-like cell projections of the apical epithelial cell surface containing a core of cross-linked microfilaments (a polymer of G-actin monomers). At the cytoplasmic end of the microvillus, bundles of actin and other proteins extend into the terminal web, a filamentous network of cytoskeletal proteins running parallel to the apical domain of the epithelial cell.

Figure 1-7 Apical differentiations of epithelial cells: microvilli and stereocilia

The intestinal epithelium and portions of the nephron in the kidney are lined by epithelial cells with microvilli forming a brush border. In general, a brush border indicates the absorptive function of the cell.

Stereocilia (singular, stereocilium; see Figure 1-7) are long and branching finger-like projections of the apical epithelial cell surface. Similar to microvilli, stereocilia contain a core of cross-linked actin with other proteins. Stereocilia do not have axonemes. Stereocilia are typical of the epithelial lining of the epididymis and contribute to the process of sperm maturation occurring in this organ.

CELL ADHESION MOLECULES

A sheet of epithelial cells results from the tight attachment of similar cells to each other and to the basal lamina, a component of the extracellular matrix. Cell adhesion molecules enable interepithelial cell contact, and this contact is stabilized by specialized cell junctions. A consequence of this arrangement is the apical and basolateral domain polarity of an epithelial sheet.

Although cell adhesion molecules and cell junctions are considered here within the framework of epithelia, nonepithelial cells also can use cell adhesion molecules and junctions to establish contact with each other, enabling cell-cell communication. A typical example of nonepithelial cells connected by specialized junctions is the cardiac muscle (see Chapter 7, Muscle Tissue).

There are two major classes of cell adhesion molecules (see Box 1-B):

1. Ca²⁺-dependent molecules, including cadherins and selectins

2. Ca²⁺-independent molecules, which compose the immunoglobulin super-family and integrins

Box 1-B Cell adhesion molecules

• Cell adhesion molecules can be classified as Ca²⁺-dependent and Ca²⁺-independent.

• Ca²⁺-dependent adhesion molecules include cadherins and selectins.

• Ca²⁺-independent adhesion molecules include cell adhesion molecules of the immunoglobulin superfamily (CAMs) and integrins.

• Cadherins and CAMs display trans-homophilic interaction across the intercellular space.

• Integrins are the only cell adhesion molecules consisting of two subunits: a and β.

• Cadherins and integrins interact with F-actin through adapters (catenins for cadherins, and vinculin, talin, and α-actinin for integrins).

Many cells can use different cell adhesion molecules to mediate cell-cell attachment. Integrins are mainly involved in cell–extracellular matrix interactions. Cadherins and integrins establish a link between the internal cytoskeleton of a cell and the exterior of another cell (cadherins) or the extracellular matrix (integrins).

Cadherins (Figure 1-8) are a family of Ca²⁺-dependent molecules with a major role in cell adhesion and morphogenesis. A loss of cadherins is associated with the acquisition of invasive behavior by tumor cells (metastasis) (see Chapter 4, Connective Tissue).

Figure 1-8 Cadherins

There are more than 40 different cadherins. E-cadherin is an epithelial cadherin found along the lateral cell surfaces and is responsible for the maintenance of most epithelial layers. The removal of calcium or the use of a blocking antibody to E-cadherin in epithelial cell cultures breaks down cell-cell attachment, and the formation of stabilizing junctions is disrupted. E-cadherin molecules form cis-homophilic dimers (“like-to-like”), which bind to dimers of the same or different class of cadherins in the opposite cell membrane (trans-homophilic or heterophilic [“like-to-unlike”] interaction). These forms of binding require the presence of calcium and result in a specialized zipper-like cell-cell adhesion pattern.

N-cadherin is found in the central nervous system, the lens of the eye, and in skeletal and cardiac muscle. P-cadherin is observed in placenta (trophoblast).

The cytoplasmic domain of cadherins is linked to actin through intermediate proteins known collectively as the catenin (Latin catena, chain) complex. The complex includes catenins (α, β, and γ) and actin-binding proteins, such as α-actinin, vinculin, and formin-1, among others.

The catenin complex has at least three distinct roles in the function of cadherins: (1) catenins mediate a direct link to filamentous actin; (2) they interact with regulatory molecules of the actin cytoskeleton; and (3) they control the adhesive state of the extracellular domain of cadherins. The association of actin to the cadherin-catenin complex is essential for cell morphogenesis, changes in cell shape, and the establishment of cell polarity.

Members of the cadherin family also are present between cytoplasmic plaques of the zonula and the macula adherens. β-catenin plays a significant role in colorectal carcinogenesis (see Chapter 16, Lower Digestive Segment).

Selectins (Figure 1-9), similar to cadherins, are Ca²⁺-dependent cell adhesion molecules. In contrast to cadherins, selectins bind to carbohydrates and belong to the group of lectins (Latin lectum, to select). Each selectin has a carbohydrate-recognition domain (CRD) with binding affinity to a specific oligosaccharide attached to a protein (glycoprotein) or a lipid (glycolipid). The molecular configuration of the CRD is controlled by calcium.

Figure 1-9 Selectins

Selectins participate in the movement of leukocytes (Greek leukos, white, kytos, cell) circulating in blood (neutrophils, monocytes, B and T cells) toward tissues by extravasation. Extravasation is the essence of homing, a mechanism that enables leukocytes to escape from blood circulation and reach the sites of inflammation (see Figure 1-12). Homing also permits thymus-derived T cells to home in on peripheral lymph nodes (see Chapter 10, Immune-Lymphatic System).

The three major classes of cell surface selectins are as follows:

1. P-selectin, found in platelets and activated endothelial cells lining blood vessels

2. E-selectin, found on activated endothelial cells

3. L-selectin, found on leukocytes

P-selectin is stored in cytoplasmic vesicles in endothelial cells. When endothelial cells are activated by inflammatory signaling, P-selectin appears on the cell surface. On their surface, leukocytes contain sialyl Lewis-x antigen, a specific oligosaccharide ligand for P-selectin. P-selectin binding to the antigen slows down streaming leukocytes in blood, and they begin to roll along the endothelial cell surfaces. P-selectins get additional help from members of the immunoglobulin (Ig) superfamily and integrins to stabilize leukocyte attachment, leading to extravasation (see Figure 1-12).

N-CAM (for neural cell adhesion molecule) belongs to the Ig superfamily and mediates homophilic and heterophilic interactions. In contrast to cadherins and selectins, members of the Ig superfamily are Ca²⁺-independent cell adhesion molecules and are encoded by a single gene. Members of the Ig superfamily are generated by the alternative messenger RNA (mRNA) splicing and have differences in glycosylation.

A conserved feature shared by all members of the Ig superfamily is an extracellular segment with one or more folded domains characteristic of immunoglobulins (Figure 1-10). Of particular interest is CD4, a member of the Ig superfamily and the receptor for the human immunodeficiency virus type 1 (HIV-1) in a subclass of lymphocytes known as T cells or helper cells. We discuss the significance of several members of the Ig superfamily in Chapter 10, Immune-Lymphatic System.

Figure 1-10 Immunoglobulin superfamily

Other members of the Ig superfamily play important roles in the homing process during inflammation. Examples include intercellular adhesion molecules 1 and 2 (ICAM-1 and ICAM-2) on endothelial cell surfaces. ICAM-1 is expressed when an inflammation is in progress to facilitate the transendothelial migration of leukocytes (see Chapter 6, Blood and Hematopoiesis).

Integrins (Figure 1-11) differ from cadherins, selectins, and members of the Ig superfamily in that integrins are heterodimers formed by two associated α and β subunits encoded by different genes. There are about 22 integrin heterodimers consisting of 17 forms of α subunits and 8 forms of β subunits.

Figure 1-11 Integrins

Figure 1-12 Homing, a process involving selectins and integrins

Almost every cell expresses one or several integrins. Similar to cadherins, the cytoplasmic domain of β integrins is linked to actin filaments through connecting proteins (talin, vinculin, and α-actinin).

The extracellular domain of integrins binds to the tripeptide RGD (Arg-Gly-Asp) sequence present in laminin and fibronectin, two major components of the basement membrane, a specific type of extracellular matrix. Laminin and fibronectin interact with various collagen types (including type IV collagen), heparan sulfate proteoglycan perlecan, and entactin (also called nidogen).

The integrin–extracellular matrix relationship is critical for cell migration to precise sites during embryogenesis and can be regulated when cell motility is required. In addition to their role in cell-matrix interactions, integrins also mediate cell-cell interaction. Integrins containing β₂ subunits are expressed on the surface of leukocytes and mediate cell-cell binding. An example is the α₁β₂ integrin heterodimer that binds to ligands on endothelial cell surfaces during the integrin phase (extravasation) of homing (Figure 1-12).

Integrins respond to intercellular events by changing their adhesive conformation with respect to molecules of the extracellular matrix. This response is known as inside-out signaling. In addition, integrins mediate a complex intracellular cascade in response to extracellular events.

ADAM proteins

The reversal of integrin-mediated cell binding to the extracellular matrix can be disrupted by proteins called ADAM (for a disintegrin and metalloprotease). ADAMs have pivotal roles in fertilization, angiogenesis, neurogenesis, heart development, cancer and Alzheimer’s disease (see Chapter 8, Nervous Tissue).

A typical ADAM protein (Figure 1-13) contains an extracellular domain and an intracellular domain. The extracellular domain consists of several portions including a disintegrin domain and a metalloprotease domain.

Figure 1-13 ADAM protein, a sheddase

1. A disintegrin domain binds to integrins and competitively prevents integrin-mediated binding of cells to laminin, fibronectin, and other extracellular matrix proteins.

2. A metalloprotease domain degrades matrix components and enables cell migration.

A significant function of ADAMs is protein ectodomain shedding, consisting of the proteolytic release of the ectodomain of a membrane protein cleaved adjacent to the plasma membrane. ADAMs are members of the family of sheddases. Ectodomain shedding targets for cleavage the proinflammatory cytokine tumor necrosis factor–α (TNF-α) and all ligands of the epidermal growth factor receptor. A released soluble ectodomain of a cytokine or growth factor can function at a distance from the site of cleavage (paracrine signaling). Ectodomain shedding of a receptor can inactivate the receptor by functioning as a decoy sequestering soluble ligands away from the plasma membrane-bound unoccupied receptor.

A defect in TNF receptor 1 (TNFR1) shedding, determined by a mutation in the receptor cleavage site, causes aperiodic febrile syndrome because of continuous availability of TNFR1 for TNF-α binding. Consequently, recurring fever occurs by increased inflammatory responses.

CELL JUNCTIONS

Although cell adhesion molecules are responsible for cell-cell adhesion, cell junctions are necessary for providing stronger stability. In addition, the movement of solutes, ions, and water through an epithelial layer occurs across and between individual cell components. The transcellular pathway is controlled by numerous channels and transporters. The paracellular pathway is regulated by a continuous intercellular contact or cell junctions. A deficiency in the cell junctions accounts for acquired and inherited diseases caused by inefficient epithelial barriers.

Cell junctions are symmetrical structures formed between two adjacent cells. There are three major classes of symmetrical cell junctions (Figure 1-14; see Box 1-C):

1. Tight junctions

2. Anchoring junctions

3. Gap or communicating junctions

Figure 1-14 Anchoring and communicating junctions

Box 1-C Cell junctions

• Cell junctions can be classified as symmetrical and asymmetrical. Symmetrical junctions include the tight junctions, the belt desmosome (zonula adherens), desmosomes (macula adherens), and gap junctions. The hemidesmosome is an asymmetrical junction

• Tight junctions contain occludin and claudin, belonging to the protein family of tetraspanins because four segments of each protein span the plasma membrane. An additional component is the afadin-nectin protein complex.

Junctional adhesion molecules (JAMs), zonula occludens (ZO) proteins ZO-1, ZO-2, and ZO-3 and F-actin are additional protein components. Tight junctions form a circumferential gasket that controls the paracellular pathway of molecules.

• Zonula adherens (belt desmosome) consists of a plaque that contains desmoplakin, plakoglobin (γ catenin), and plakophilin. Cadherins, mainly desmocollins and desmogleins dimers, and the afadin-nectin complex extend from the plaque to the extracellular space. A catenin complex links actin filaments to the plaque. Similar to tight junctions, the belt desmosome forms a circumferential gasket at the apical region of epithelial cells.

• Macula adherens (spot desmosome) are structurally comparable with the zonula adherens except that the afadin-nectin and catenin complexes are absent and intermediate filaments (tonofilaments), instead of actin filaments, are attached to the plaque.

• Hemidesmosomes consist of an inner membrane plate—to which tonofilaments attach—and an outer membrane plaque, linked by integrin α₆β₄ and laminin 5 to the basal lamina.

• Tight junctions, the belt desmosome, spot desmosomes, and hemidesmosomes are anchoring junctions. Gap junctions are not anchoring junctions. Instead, gap junctions are communicating junctions linking adjacent cells. The basic unit of a gap junction is the connexon, formed by 6 connexin molecules surrounding a central channel.

Tight junctions (also called occluding junctions) (Figure 1-15) have two major functions:

1. They determine epithelial cell polarity by separating the apical domain from the basolateral domain and preventing the free diffusion of lipids and proteins between them.

2. They prevent the free passage of substances across an epithelial cell layer (paracellular pathway barrier).

Figure 1-15 Molecular organization of tight junctions

Cell membranes of two adjacent cells come together at regular intervals to seal the apical intercellular space. These areas of close contact continue around the entire surface of the cell like a belt, forming anastomosing strips of the trans-membrane proteins occludin and claudin. Occludin and claudin belong to the family of tetraspanins with four transmembrane domains, two outer loops, and two short cytoplasmic tails.

Occludin interacts with four major zonula occludin (ZO) proteins: ZO-1, ZO-2, ZO-3, and afadin. Claudin (Latin claudere, to close), a family of 16 proteins forming linear fibrils in the tight junctions, confers barrier properties on the paracellular pathway. A mutation in the gene encoding claudin 16 is the cause of a rare human renal magnesium wasting syndrome characterized by hypomagnesemia and seizures.

Two members of the Ig superfamily, nectins and junctional adhesion molecules (JAMs), are present in tight junctions. Both form homodimers (cis homodimers) and then trans homodimers across the intercellular space. Nectins are connected to actin filaments through the protein afadin. The targeted deletion of the afadin gene in mice results in embryonic lethality. A mutation in the nectin-1 gene is responsible for cleft lip/palate and ectodermal dysplasia (CLEPD1) of skin, hairs, nails, and teeth in humans. Nectin-2–deficient male mice are sterile.

Tight junctions can be visualized by freeze-fracturing a network of branching and anastomosing sealing strands. We discuss in Chapter 2, Epithelial Glands, the procedure of freeze-fracturing for the study of cell membranes.

Anchoring junctions are found below the tight junctions, usually near the apical surface of an epithelium. There are three classes of anchoring junctions (see Figures 1-14, 1-16, 1-18, and 1-19):

1. The zonula adherens or belt desmosome

2. The macula adherens or spot desmosome

3. The hemidesmosome

Figure 1-16 Zonula adherens (belt desmosome)

Figure 1-17 Desmogleins in skin disease: Pemphigus foliaceus

Figure 1-18 Macula adherens (spot desmosome)

Figure 1-19 Hemidesmosome

Similar to the tight junctions, the zonula adherens is a beltlike junction. The zonula adherens (Figure 1-16) is associated with actin microfilaments. This association is mediated by the interaction of cadherins (desmocollins and desmogleins) with catenins (α, β, and γ). The main desmogleins expressed in the epidermis of the skin are desmoglein 1 and desmoglein 3 (Figure 1-17).

The macula adherens (also called desmosome) is a spotlike junction associated with keratin intermediate filaments (also known as tonofilaments) extending from one spot to another on the lateral and basal cell surfaces of epithelial cells (Figure 1-18). Spot desmosomes provide strength and rigidity to an epithelial cell layer. Spot desmosomes are also present in the intercalated disks linking adjacent cardiocytes in heart (see Chapter 7, Muscle Tissue) and in the meninges lining the outer surfaces of the brain and spinal cord.

In contrast to occluding junctions, adjacent cell membranes linked by zonula and macula adherens are separated by a relatively wide intercellular space. This space is occupied by the glycosylated portion of proteins of the cadherin family, desmogleins and desmocollins, anchored to cytoplasmic plaques containing desmoplakin, plakoglobin (γ-catenin), and plakophilin. The cytoplasmic plaques are attached to the cytosolic face of the plasma membrane. The interlocking of similar cadherins binds two cells together by Ca²⁺-dependent homophilic or heterophilic interaction, as we have already seen. Inherited disorders of some of the desmosomal components are indicated in Figure 1-18.

The human desmosomal cadherins genes include four desmogleins and three desmocollins. Their cytoplasmic regions interact with plakoglobin and plakophilin. Desmoplakin interacts with the intermediate filaments keratin in epidermis, desmin in the intercalated disks, and vimentin in the meninges. Desmoglein 1 and desmoglein 3 maintain the cohesiveness of the epidermis, a stratified squamous epithelium. Autoantibodies to desmoglein 1 cause a blistering disease (disruption of cell adhesion) of the skin called pemphigus foliaceus (see Figure 1-17).

Hemidesmosomes are asymmetrical structures anchoring the basal domain of an epithelial cell to the underlying basal lamina (Figure 1-19).

Hemidesmosomes have a different organization compared with a macula adherens or desmosome. A hemidesmosome consists of the following:

1. An inner cytoplasmic plate associated with intermediate filaments (also called keratins or tonofilaments)

2. An outer membrane plaque linking the hemidesmosome to the basal lamina by anchoring filaments (composed of laminin 5) and integrin α₆β₄

Although hemidesmosomes look like half-desmosomes, none of the biochemical components present in the desmosome is found in hemidesmosomes. Hemidesmosomes increase the overall stability of epithelial tissues by linking intermediate filaments of the cytoskeleton with components of the basal lamina. We consider additional details of the hemidesmosomes and their role in autoimmune diseases of the skin when we discuss the structure of intermediate filaments in the cytoskeleton section.

Gap junctions are symmetrical communicating junctions formed by integral membrane proteins called connexins. Six connexin monomers associate to form a connexon, a hollow cylindrical structure that spans the plasma membrane. The end-to-end alignment of connexons in adjacent cells provides a direct channel of communication (1.5 to 2 nm in diameter) between the cytoplasm of two adjacent cells (Figure 1-20). Connexons have a clustering tendency and can form patches about 0.3 mm in diameter.

Figure 1-20 Gap junctions

These junctions facilitate the movement of molecules 1.2 nm in diameter (for example, Ca²⁺ and cyclic adenosine monophosphate [cAMP]) between cells. The connexon axial channels close when the concentration of Ca²⁺ is high. This junction is responsible for the chemical and electrical “coupling” between adjacent cells. A typical example is cardiac muscle cells connected by gap junctions to enable the transmission of electrical signals.

Clinical significance: Connexin mutations in human disease

Several diseases occur when genes encoding connexins are mutated. Mutations in the connexin 26 (Cx26) gene, highly expressed in cells of the cochlea, are associated with deafness.

Mutations in the connexin 32 (Cx32) gene are found in X-linked Charcot-Marie-Tooth demyelinating neuropathy resulting in progressive degeneration of peripheral nerves, characterized by distal muscle weakness and atrophy and impairment of deep tendon reflexes. Connexin 32 protein is expressed in Schwann cells, which are involved in the production of rolled myelin tubes around the axons in the peripheral nervous system (see Chapter 8, Nervous Tissue). Gap junctions couple different parts of the rolled myelin tubes of the same Schwann cell, rather than different cells. A loss of the functional axial channels in myelin leads to the demyelinating disorder. Mutations in the connexin 50 (Cx50) gene are associated with congenital cataracts, leading to blindness.

Bone cells (osteoblasts/osteocytes) are connected by gap junctions and express connexin 43 (Cx43) and connexin 45 (Cx45) proteins. A deletion of the Cx43 gene determines skeletal defects and delays in mineralization.

BASEMENT MEMBRANE

Integrins mediate cell-matrix interactions by their binding affinity to the RGD domain in laminin and fibronectin (see Figure 1-11). Laminin and fibronectin are distinct proteins of the extracellular matrix and are associated with collagens, proteoglycans, and other proteins to organize a basement membrane, the supporting sheet of most epithelia.

The basement membrane consists of two components (Figure 1-21):

1. The basal lamina, a sheetlike extracellular matrix in direct contact with epithelial cell surfaces. The basal lamina results from the self-assembly of laminin molecules with type IV collagen, entactin, and proteoglycans.

2. A reticular lamina—formed by collagen fibers—supports the basal lamina and is continuous with the connective tissue.

Figure 1-21 Basement membrane

The basal and reticular laminae can be distinguished by electron microscopy. Under the light microscope, the combined basal and reticular laminae receive the name of basement membrane, which can be recognized by the periodic acid–Schiff (PAS) stain (see Figure 1-21; see Box 1-D).

Box 1-D Periodic acid–Schiff (PAS) reaction

• PAS is a widely used histochemical technique to show 1,2-glycol or 1,2-aminoalcohol groups, such as those present in glycogen, mucus, and glycoproteins.

• Periodic acid, an oxidant, converts these groups to aldehydes. The Schiff reagent, a colorless fuchsin, reacts with the aldehydes to form a characteristic red-purple (magenta) product.

• Some important PAS-positive structures are the basement membrane, glycocalyx, mucus produced by goblet cells, stored glycoprotein hormones in cells of the pituitary gland, and collagens.

The basal lamina has specific functions in different tissues. The double basal lamina of the renal corpuscle constitutes the most important element of the glomerular filtration barrier during the initial step in the formation of urine (see Chapter 14, Urinary System).

In skeletal muscle, the basal lamina maintains the integrity of the tissue, and its disruption gives rise to muscular dystrophies (see Chapter 7, Muscle Tissue).

During the migration of primordial germinal cells, basal lamina components guide the migrating cells toward the gonadal ridge in preparation for the development of the gonads. The basal lamina not only provides support to epithelia, but also participates in other non–epithelial cell functions.

Laminin (Figure 1-22) is a cross-shaped protein consisting of three chains: the α chain, the β chain, and the γ chain. Laminin molecules can associate with each other to form a meshlike polymer. Laminin and type IV collagen are the major components of the basal lamina, and both are synthesized by epithelial cells resting on the lamina.

Figure 1-22 Laminin and fibronectin

Laminin has binding sites for nidogen (also called entactin), proteoglycans (in particular, heparan sulfate perlecan), α-dystroglycan (see Chapter 7, Muscle Tissue), and integrins.

Fibronectin (see Figure 1-22) consists of two protein chains cross-linked by disulfide bonds. Fibronectin is the main adhesion molecule of the extracellular matrix of the connective tissue and is produced by fibroblasts. Fibronectin has binding sites for heparin present in proteoglycans, several types of collagens (types I, II, III, and V), and fibrin (derived from fibrinogen during blood coagulation).

Fibronectin circulating in blood is synthesized in the liver by hepatocytes. It differs from fibronectin produced by fibroblasts in that it lacks one or two repeats (designated EDA and EDB for extra domain A and extra domain B) as a result of alternative mRNA splicing. Circulating fibronectin binds to fibrin, a component of the blood clot formed at the site of blood vessel damage. The RGD domain of immobilized fibronectin binds to integrin expressed on the surface of activated platelets, and the blood clot enlarges. We return to the topic of blood coagulation or hemostasis in Chapter 6, Blood and Hematopoiesis.

How cells interact with one another and with the basal lamina

Figure 1-23 summarizes the highlights of cell adhesion molecules and cell junctions. An epithelium is a continuous sheet of polarized cells supported by a basement membrane. The polarized nature of an epithelium depends on the tight junctions that separate the polarized cells into apical and basolateral regions. Tight junctions control the paracellular pathway of solutes, ions, and water. Tight junctions form a belt around the circumference of each cell.

Figure 1-23 Summary of cell junctions and cell adhesion molecules

Endothelial cells, the constituents of a simple squamous epithelium, are linked by tight and spot desmosomes tightly regulated to maintain the integrity of the endothelium and protect the vessels from unregulated permeability, inflammation, and reactions leading to blood coagulation in the lumen (see Chapter 12, Cardiovascular System). Leukocytes reach the site of infection by attaching to endothelial cell surfaces and migrate across the endothelium into the underlying tissues by a mechanism called diapedesis. Leukocytes find their way through endothelial cell-cell junctions after docking to activated or resting endothelial cells by the endothelial cell adhesion molecules ICAM-1 and VCAM-1 (see Figure 1-10). ICAM-1 and VCAM-1 bind to β₂ and β₁ integrin subunits in leukocytes (see Figure 1-12).

The cohesive nature of the epithelium depends on three factors: cell junctions, cell adhesive molecules in general, and the interaction of integrins with the extracellular matrix, produced to a large extent by fibroblasts. The basal lamina is essential for the differentiation of epithelial cells during embryogenesis.

Note in Figure 1-23 that:

1. The basal domain of epithelial cells interacts with the basal lamina through hemidesmosomes and integrins. Hemidesmosomes, so called because of their appearance as half-desmosomes in electron micrographs, are anchored to the basal lamina ouside the cell and to a network of keratin intermediate filaments inside the cell through a plate-plaque complex. Mutations in hemidesmosome components cause severe skin blistering as a result of a rupture of the anchoring molecular integrity.

2. Integrins interact directly with laminin and fibronectin, in particular the RGD domain to which integrins bind. Inside the cell, integrins interact with actin micro filaments. Integrins connect the extracellular environment to the intracellular space. We have seen that some ADAM proteins can use their disintegrin domain to prevent integrin binding to extracellular matrix ligands.

3. Collagens and proteoglycans do not interact directly with the basal domain of epithelial cells. Instead, this interaction is mediated by laminin and fibronectin, which contain specific binding sites for collagens, the proteoglycan perlecan, and nidogen.

4. The lateral domains of adjacent epithelial cells communicate by gap junctions (not shown in Figure 1-23). In contrast to tight junctions and belt and spot desmosomes, gap junctions are not anchoring devices. They consist of intercellular channels connecting the cytoplasm of adjacent cells. They are communicating junctions.

5. Cadherins and the afadin-nectin complex are present in tight junctions and zonula adherens. Actin microfilaments are associated with these two junctions.

CYTOSKELETON

Cytoskeleton is a three-dimensional network of proteins distributed throughout the cytoplasm of eukaryotic cells.

The cytoskeleton has roles in:

1. Cell movement (crawling of blood cells along blood-vessel walls, migration of fibroblasts during wound healing, and movement of cells during embryonic development)

2. Support and strength for the cell

3. Phagocytosis

4. Cytokinesis

5. Cell-cell and cell–extracellular matrix adherence

6. Changes in cell shape

The components of the cytoskeleton were originally identified by electron microscopy. These early studies described a system of cytoplasmic “cables” that fell into three size groups, as follows:

1. Microfilaments (7 nm thick)

2. Intermediate filaments (10 nm thick)

3. Microtubules (25 nm in diameter)

Biochemical studies, involving the extraction of cytoskeletal proteins from cells with detergents and salts and in vitro translation of specific mRNA, showed that each class of filaments has a unique protein organization. When cytoskeletal proteins were purified, they were used as antigens for the production of antibodies. Antibodies are used as tools for the localization of the various cytoskeletal proteins in the cell. The immunocytochemical localization of cytoskeletal proteins (Figure 1-24) and cell treatment with various chemical agents disrupting the normal organization of the cytoskeleton have been instrumental in understanding the organization and function of the cytoskeleton.

Figure 1-24 Immunocytochemistry

Microfilaments

The main component of microfilaments is actin. Actin filaments are composed of globular monomers (G-actin, 42 kd), which polymerize to form helical and asymmetrical filaments (F-actin).

Actin is a versatile and abundant cytoskeletal component forming static and contractile bundles and filamentous networks specified by actin-binding proteins and their distinctive location and function in a cell. F-actin bundles are present in the microvilli of the intestinal (Figure 1-25) and renal epithelial cells (brush border) and the stereocilia from the hair cells of the inner ear.

Figure 1-25 F-actin bundles form the core of intestinal microvilli

We have already seen that the intracellular portion of the cell adhesion molecules cadherins and integrin β₁ interacts with F-actin through linker proteins (see Figures 1-8 and 1-11). As discussed in Chapter 6, Blood and Hematopoiesis, actin—together with spectrin—forms a filamentous network on the inner face of the red blood cell membrane that is crucial for maintaining the shape and integrity of red blood cells. Spectrin is a tetramer consisting of two distinct polypeptide chains (α and β).

Growth of actin filaments may occur at both ends; however, one end (the “barbed end” or plus end) grows faster than the other end (the “pointed end” or minus end). The names correspond to the arrowhead appearance of myosin head bound at an angle to actin. Actin filaments can branch in the leading edge (lamellipodia) of cells involved in either motility or interaction with other cell types. F-actin branching is initiated from the side of a preexisting actin filament by Arp2/3 (for actin-related protein), an actin nucleating complex of seven proteins (Figure 1-26). Formin regulates the assembly of unbranched actin in cell protrusions such as the intestinal microvilli (see Figure 1-25).