Histology_Ross

Histology_Ross. FIGURE 5.16 • Molecular structure of zonula occludens. Diagram showing three transmembrane proteins involved in the formation of zonula occludens: occludin, claudin, and junctional adhesion molecule (JAM). Occludin and claudin have four transmembrane domains with two extracellular loops, but JAM has only a single transmembrane domain, and its extracellular portion possesses two immunoglobulinlike loops. Several major associated proteins of the occluding junction and their interactions with each other are visible. Note that one of the associated proteins, ZO-1, interacts with the cell cytoskeleton binding actin filaments. Several proteins are involved in the formation of zonula occludens strands.


Histology_Ross

Histology_Ross. Several proteins are involved in the formation of zonula occludens strands. Zonula occludens strands correspond to the location of the rows of transmembrane proteins. Three major groups of transmembrane proteins are found in the zonula occludens (Fig. 5.16; Table 5.3):  Occludin, a 60-kilodalton protein, was the first protein identified in the zonula occludens. It participates in maintaining the barrier between adjacent cells as well as the barrier between the apical and lateral domains. Occludin is present in most occluding junctions. However, several types of epithelial cells do not have occludin within their strands, but they still possess well-developed and fully functional zonulae occludentes.


Histology_Ross

Histology_Ross.  Claudins constitute a family of proteins (20 to 27 kilodaltons) that have recently been identified as integral components of zonula occludens strands. Claudins form the backbone of each strand. In addition, claudins (especially claudin-2 and claudin-16) are able to form extracellular aqueous channels for the paracellular passage of ions and other small molecules. About 24 different members of the claudin family have been characterized to date. Mutations in the gene encoding claudin-14 have been recently linked to human hereditary deafness. A mutated form of claudin-14 causes an increased permeability of zonula occludens in the organ of Corti (receptor of hearing), affecting generation of action potentals.


Histology_Ross

Histology_Ross.  Junctional adhesion molecule (JAM) is a 40kilodalton protein that belongs to the immunoglobulin superfamily (IgSF). JAM does not itself form a zonula occludens strand but is instead associated with claudins. It is involved in the formation of occluding junctions in endothelial cells as well as between endothelial cells and monocytes migrating from the vascular space to the connective tissue.


Histology_Ross

Histology_Ross. The extracellular portions of these transmembrane proteins function as a zipper and seal the intercellular space between two adjacent cells, thus creating a barrier against paracellular diffusion. The cytoplasmic portions of all three proteins contain a unique amino acid sequence that attracts regulatory and signaling proteins called PDZ-domain proteins. These proteins include the zonula occludens proteins ZO-1, ZO-2, and ZO-3 (see Fig. 5.16). Occludin and claudins interact with the actin cytoskeleton through ZO-1 and ZO-3. Regulatory functions during the formation of the zonula occludens have been suggested for all ZO proteins. In addition, ZO-1 is a tumor suppressor, and ZO-2 is required in the epidermal growth factor–receptor signaling mechanism. The ZO-3 protein interacts with ZO-1 and the cytoplasmic domain of occludins. The proteins localized in the region of the zonula occludens are summarized in Table 5.3. Many pathogenic agents, such as cytomegalovirus and cholera toxins, act


Histology_Ross

Histology_Ross. the cytoplasmic domain of occludins. The proteins localized in the region of the zonula occludens are summarized in Table 5.3. Many pathogenic agents, such as cytomegalovirus and cholera toxins, act on ZO-1 and ZO-2, causing the junction to become permeable.


Histology_Ross

Histology_Ross. The zonula occludens separates the luminal space from the intercellular space and connective tissue compartment. It is now evident that the zonula occludens plays an essential role in the selective passage of substances from one side of an epithelium to the other. The ability of epithelia to create a diffusion barrier is controlled by two distinct pathways for transport of substances across the epithelia (Fig. 5.17a):  The transcellular pathway occurs across the plasma membrane of the epithelial cell. In most of these pathways, transport is active and requires specialized energy-dependent membrane transport proteins and channels. These proteins and channels move selected substances across the apical plasma membrane into the cytoplasm and then across the lateral membrane below the level of the occluding junction into the intercellular compartment.


Histology_Ross

Histology_Ross.  The paracellular pathway occurs across the zonula occludens between two epithelial cells. The amount of water, electrolytes, and other small molecules transported through this pathway is contingent on the tightness of the zonula occludens. The permeability of an occluding junction depends on the molecular composition of the zonula occludens strands and thus the number of active aqueous channels in the seal (see the following section). Under physiologic conditions, substances transported through this pathway may be regulated or coupled to transcellular transport. Permeability of the zonula occludens depends not only on the complexity and number of strands but also on the presence of functional aqueous channels formed by various claudin molecules.


Histology_Ross

Histology_Ross. TABLE Major Proteins Localized Within the Zonula Occludens Junction 5.3 Zona Occludens Protein Associated Protein Partners Function Occludin Occludin, ZO-1, ZO-2, ZO-3, Is present in most occluding junctions; maintains barrier Vap33, actin between apical and lateral cell surface Claudin Claudin, ZO-1, JAM Forms backbone of zonula occludens strands; forms and regulates aqueous channels used for paracellular diffusion JAM JAM, ZO-1, claudin Present in endothelial cells; mediates interactions between endothelial cells and monocyte adhesions ZO-1 ZO-2, ZO-3, occludin, claudin, Important link in transduction of signals from all JAM, cingulin, actin, ZONAB, transmembrane proteins; interacts with actin flaments; ASIP, AF-6 has tumor-suppressor actin ZO-2 ZO-1, occluding, cingulin, 4.1R Required in the epidermal growth factor–receptor signaling mechanism ZO-3 ZO-1, occludin, actin Interacts with ZO-1, occludin, and actin flaments of cell cytoskeleton AF-6 RAS, ZO-1 Small protein involved in


Histology_Ross

Histology_Ross. in the epidermal growth factor–receptor signaling mechanism ZO-3 ZO-1, occludin, actin Interacts with ZO-1, occludin, and actin flaments of cell cytoskeleton AF-6 RAS, ZO-1 Small protein involved in molecular transport system and signal transduction Cingulin ZO-1, ZO-2, ZO-3, cingulin, Acidic, heat-stable protein that cross-links myosin II actin flaments into sedimentable complexes Symplekin CPSF-100 Dual-location protein: localized in zonula occludens and in the interchromatin particles of the karyoplasm ASIP/Par3 PKC Controls relocation of asymmetrically distributed proteins Rab3b GTPase Rab13 -PDE Rab8 G/C kinase, Sec4 Sec4 Rab8 GTPase required for polarized delivery of cargo vesicles to plasma membrane Sec6 Sec8 Participates in fusion of Golgi vesicle with the plasma membrane Sec8 Sec6 Inhibits basolateral translocation of LDLP receptors after formation of zonula occludens Members of the RAS oncogene family of proteins; control the assembly of protein complexes for docking of


Histology_Ross

Histology_Ross. Sec6 Inhibits basolateral translocation of LDLP receptors after formation of zonula occludens Members of the RAS oncogene family of proteins; control the assembly of protein complexes for docking of transport vesicles


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Histology_Ross. Observations of different kinds of epithelia reveal that the complexity and number of strands forming the zonulae occludentes varies. In epithelia in which anastomosing strands or fusion sites are sparse, such as certain kidney tubules, the intercellular pathway is partially permeable to water and solutes. In contrast, in epithelia in which the strands are numerous and extensively intertwined—for example, intestinal and urinary bladder epithelia—the intercellular region is highly impermeable.


Histology_Ross

Histology_Ross. However, in some epithelial cells, the number of strands does not directly correlate to the tightness of the seal. Differences in tightness between different zonulae occludentes could be explained by the presence of aqueous pores within individual zonula occludens strands (Fig. 5.17b). Recent experiments indicate that claudin-16 functions as an aqueous Mg2 channel between specific kidney epithelial cells. Similarly, claudin-2 is responsible for the presence of high-conductance aqueous pores in other kidney epithelia. Claudins not only form the backbone of the individual zonula occludens strand but also are responsible for the formation of extracellular aqueous channels. Thus, the combination and mixing ratios of claudins to occludins and other proteins found within individual paired zonula occludens strands determine tightness and selectivity of the seal between cells. PDZ domains of associated intracellular attachment proteins


Histology_Ross

Histology_Ross. FIGURE 5.17 • Two transcellular and paracellular pathways for transport of substances across the epithelia. a. The transcellular pathway occurs across the plasma membrane of the epithelial cell and represents an active transport system that requires specialized energy-dependent membrane transport proteins and channels. The paracellular pathway occurs across the zonula occludens between two epithelial cells. The amount of water, electrolytes, and other small molecules transported through this pathway is contingent on the tightness of the zonula occludens. b. Structure of the extracellular and cytoplasmic portions of tight-junction strands. Two zonula occludens strands from neighboring cells fuse together in a zipperlike fashion and create a barrier to movement between the cells. Aqueous pores allow water to move between the cells. The permeability of the barrier depends on the mixture of claudins and occludins in the zipper seal. The cytoplasmic portion of the strand attracts


Histology_Ross

Histology_Ross. pores allow water to move between the cells. The permeability of the barrier depends on the mixture of claudins and occludins in the zipper seal. The cytoplasmic portion of the strand attracts PDZ-domain proteins that function in cell signaling.


Histology_Ross

Histology_Ross. The zonula occludens establishes functional domains in the plasma membrane. As a junction, the zonula occludens controls not only the passage of substances across the epithelial layer but also the movement of lipid rafts containing specific proteins within the plasma membrane itself. The cell is able to segregate certain internal membrane proteins on the apical (free) surface and restrict others to the lateral or basal surfaces. In the intestine, for instance, the enzymes for terminal digestion of peptides and saccharides (dipeptidases and disaccharidases) are localized in the membrane of the microvilli of the apical surface. The NaK-ATPase that drives salt and transcellular water transport, as well as amino acid and sugar transport, is restricted to the lateral plasma membrane below the zonula occludens.


Histology_Ross

Histology_Ross. Anchoring junctions provide lateral adhesions between epithelial cells, using proteins that link into the cytoskeleton of adjacent cells. Two types of anchoring cell-to-cell junctions can be identified on the lateral cell surface:  zonula adherens (pl., zonulae adherentes), which inter acts with the network of actin filaments inside the cell; and  macula adherens (pl., maculae adherentes) or desmo some, which interacts with intermediate filaments In addition, two other types of anchoring junctions can be found where epithelial cells rest on the connective tissue matrix. These focal adhesions (focal contacts) and hemidesmosomes are discussed in the section on the basal domain (see pages 144 to 146). Cell adhesion molecules play important roles in cell-tocell and cell-to–extracellular matrix adhesions.


Histology_Ross

Histology_Ross. Cell adhesion molecules play important roles in cell-tocell and cell-to–extracellular matrix adhesions. Transmembrane proteins known as cell adhesion mole cules (CAMs) form an essential part of every anchoring junction on both lateral and basal cell surfaces. The extracellular domains of CAMs interact with similar domains belonging to CAMs of neighboring cells. If the binding occurs between different types of CAMs it is described as heterotypic binding; homotypic binding occurs between CAMs of the same type (Fig. 5.18). CAMs have a selective adhesiveness of relatively low strength, which allows cells to easily join and dissociate.  FOLDER 5.3 Clinical Correlation: Junctional Complexes as a Target of Pathogenic Agents


Histology_Ross

Histology_Ross. Epithelia form a physical barrier that allows the body to main-tain internal homeostasis while protecting the organism from harmful pathogenic agents from the external environment. The easiest way for many viruses, bacteria, and parasites to suc-cessfully compromise the protective functions of the epithelial layer is to destroy the junctional complexes between epithelial cells. Several proteins found in junctional specializations of the cell membrane are affected by molecules produced or ex-pressed by these pathogenic agents. Bacteria. A common bacterium that causes food poi-soning, Clostridium perfringens, attacks the zonula occlu-dens junction. This microorganism is widely distributed in the external environment and is found within the intestinal flora of humans and many domestic animals. Food poison-ing symptoms are characterized by intense abdominal pain and diarrhea that begins 8 to 22 hours after eating foods contaminated by these bacteria. Symptoms usually abate within 24


Histology_Ross

Histology_Ross. Food poison-ing symptoms are characterized by intense abdominal pain and diarrhea that begins 8 to 22 hours after eating foods contaminated by these bacteria. Symptoms usually abate within 24 hours. The enterotoxin produced by C. perfrin-gens is a small 35-kilodalton protein whose carboxy termi-nus binds specifically to the claudin molecules of the zonula occludens. Its amino terminus forms pores within the apical domain of the plasma membrane. Binding to claudins prevents their incorporation into the zonula occlu-dens strands and leads to malfunction and breakdown of the junction. Dehydration that occurs with this type of food poisoning is a result of a massive movement of fluids via paracellular pathways into the lumen of the intestines. Helicobacter pylori, another bacterium, resides within the stomach and binds to the extracellular domains of zonula occludens proteins. During this process, the CagA surface-exposed 128-kilodalton protein produced by the bacteria is translocated


Histology_Ross

Histology_Ross. within the stomach and binds to the extracellular domains of zonula occludens proteins. During this process, the CagA surface-exposed 128-kilodalton protein produced by the bacteria is translocated from the microorganism into the cy-toplasm, where it targets both ZO-1 and JAM proteins. As a result, the zonula occludens barrier becomes disrupted, and its capacity for tyrosine kinase signaling diminishes, causing cytoskeletal rearrangements. H. pylori cause injury to the protective barrier of the stomach that may lead to the development of gastric ulcers and gastric carcinomas. Viruses. The one specific group of RNA viruses responsible for infant enteritis (inflammation of the intestines) uses the intracellular JAM signaling pathway. The attachment and endocytosis of the reovirus is initiated by the interaction of its viral attachment protein with a JAM molecule. This interaction activates nuclear factor-B pro-tein (NF B), which migrates into the nucleus and triggers a cascade of


Histology_Ross

Histology_Ross. by the interaction of its viral attachment protein with a JAM molecule. This interaction activates nuclear factor-B pro-tein (NF B), which migrates into the nucleus and triggers a cascade of cellular events leading to apoptosis. This is ev-idence that JAMs are being used as signal transduction molecules to convey impulses from the external environ-ment to the cell nucleus. Zonula occludens–associated proteins that contain the PDZ-expressed sequence are targets of oncogenic aden-ovirus and papillomavirus. The viral oncoproteins produced by these viruses bind via their PDZ binding domains to ZO-2 and multi-PDZ–containing protein-1 (MUPP-1). The oncogenic effect of these interactions is attributed, in part, to the sequestration and degradation of the zonula occludens and the tumor-suppressor proteins associated with the viruses. Parasites. The common house dust mite, Der-matophagoides pteronyssinus, also destroys zonula occludens junctions. It belongs to the arachnid family, which


Histology_Ross

Histology_Ross. proteins associated with the viruses. Parasites. The common house dust mite, Der-matophagoides pteronyssinus, also destroys zonula occludens junctions. It belongs to the arachnid family, which includes spiders, scorpions, and ticks. When its fecal pellets are inhaled with dust particles, serine and cysteine peptidases present in the pellets cleave occluding and ZO-1 protein, resulting in the breakdown of zonula occludens junctions in the respiratory epithelium. The loss of the pro-tective epithelial barrier in the lung exposes the lung to in-haled allergens and initiates an immune response that can lead to severe asthma attacks.


Histology_Ross

Histology_Ross. The cytoplasmic domains are linked through a variety of intracellular proteins to components of the cell cytoskeleton. Through the cytoskeleton connection, CAMs are able to control and regulate diverse intracellular processes associated with cell adhesion, cell proliferation, and cell migration. In addition, CAMs are implicated in many other cellular functions such as intercellular and intracellular communications, cell recognition, regulation of intercellular diffusion barrier, generation of immune responses, and apoptosis. From early embryonic development, every type of tissue at every stage of differentiation is defined by the expression of specific CAMs. Changes in the expression pattern of one or several CAMs may lead to pathologic changes during tissue differentiation or maturation. To date, about 50 CAMs have been identified, and they are classified on the bases of their molecular structure into four major families: cadherins, integrins, selectins, and the immunoglobin


Histology_Ross

Histology_Ross. To date, about 50 CAMs have been identified, and they are classified on the bases of their molecular structure into four major families: cadherins, integrins, selectins, and the immunoglobin superfamily (see Fig. 5.18).


Histology_Ross

Histology_Ross.  Cadherins are represented by transmembrane Ca2dependent CAMs localized mainly within the zonula adherens. At these sites, cadherins maintain homotypic interactions with similar proteins from the neighboring cell. They are associated with a group of intracellular proteins (catenins) that link cadherin molecules to actin flaments of the cell cytoskeleton. Through this interaction, cadherins convey signals that regulate mechanisms of growth and cell differentiation. Cadherins control cell-tocell interactions and participate in cell recognition and embryonic cell migration. E-cadherin, the most studied member of this family, maintains the zonula adherens junction between epithelial cells. It also acts as an important suppressor of epithelial tumor cells.  Integrins are represented by two transmembrane glycoprotein subunits consisting of 15 and 9 chains. This composition allows for the formation of different combinations of integrin molecules that are able to interact with


Histology_Ross

Histology_Ross. FIGURE 5.18 • Cell adhesion molecules (CAMs). Cadherin and immunoglobin superfamily (IgSF) CAMs exhibit homotypic binding in which two identical molecules from the neighboring cells interact. Binding that occurs between different types of CAMs (e.g., selectins and integrins) is considered heterotypic binding (no identical pair of molecules reacts with each other). various proteins (heterotypic interactions). Integrins interact with extracellular matrix molecules (such as collagens, laminin, and fibronectin) and with actin and intermediate flaments of the cell cytoskeleton. Through these interactions, integrins regulate cell adhesion, control cell movement and shape, and participate in cell growth and differentiation.


Histology_Ross

Histology_Ross.  Selectins are expressed on white blood cells (leukocytes) and endothelial cells and mediate neutrophil– endothelial cell recognition. This heterotypic binding initiates neutrophil migration through the endothelium of blood vessels into the extracellular matrix. Selectins are also involved in directing lymphocytes into accumulations of lymphatic tissue (homing procedure).


Histology_Ross

Histology_Ross.  Immunoglobulin superfamily (IgSF). Many molecules involved in immune reactions share a common precursor element in their structure. However, several other molecules with no known immunologic function also share this same repeat element. Together, the genes encoding these related molecules have been defined as the immunoglobulin gene superfamily. It is one of the largest gene families in the human genome, and its glycoproteins perform a wide variety of important biologic functions. IgSF members mediate homotypic cell-to-cell adhesions and are represented by the intercellular cell adhesion molecule (ICAM), cell–cell adhesion mole cule (C-CAM), vascular cell adhesion molecule (VCAM), Down syndrome cell adhesion molecule (DSCAM), platelet endothelial cell adhesion molecules (PECAM), junctional adhesion molecules (JAM), and many others. These proteins play key roles in cell adhesion and differentiation, cancer and tumor metastasis, angiogenesis (new vessel formation), inflammation,


Histology_Ross

Histology_Ross. adhesion molecules (JAM), and many others. These proteins play key roles in cell adhesion and differentiation, cancer and tumor metastasis, angiogenesis (new vessel formation), inflammation, immune responses, and microbial attachment, as well as many other functions.


Histology_Ross

Histology_Ross. The zonula adherens provides lateral adhesion between epithelial cells.


Histology_Ross

Histology_Ross. The integrity of epithelial surfaces depends in large part on the lateral adhesion of the cells with one another and their ability to resist separation. Although the zonula occludens involves a fusion of adjoining cell membranes, their resistance to mechanical stress is limited. Reinforcement of this region depends on a strong bonding site below the zonula occludens. Like the zonula occludens, this lateral adhesion device occurs in a continuous band or beltlike configuration around the cell; thus, the adhering junction is referred to as a zonula adherens. The zonula adherens is composed of the transmembrane cell adhesion molecule E-cadherin. On the cytoplasmic side, the tail of E-cadherin is bound to catenin (Fig. 5.19a). The resulting E-cadherin–catenin complex binds to vinculin and -actinin and is required for the interaction of cadherins with the actin filaments of the cytoskeleton. The extracellular components of the E-cadherin molecules from adjacent cells are linked by Ca2 ions


Histology_Ross

Histology_Ross. and is required for the interaction of cadherins with the actin filaments of the cytoskeleton. The extracellular components of the E-cadherin molecules from adjacent cells are linked by Ca2 ions or an additional extracellular link protein. Therefore, the morphologic and functional integrity of the zonula adherens is calcium dependent. Removal of Ca2 leads to dissociation of E-cadherin molecules and disruption of the junction. Recent studies indicate that the E-cadherin–catenin complex functions as a master molecule in regulating not only cell adhesion but also polarity, differentiation, migration, proliferation, and survival of epithelial cells.


Histology_Ross

Histology_Ross. When examined with the TEM, the zonula adherens is characterized by a uniform 15to 20-nm space between the opposing cell membranes (Fig. 5.19b). The intercellular space is of low electron density, appearing almost clear, but it is evidently occupied by extracellular components of adjacent E-cadherin molecules and Ca2 ions. Within the confines of the zonula adherens, a moderately electron-dense material called fuzzy plaque is found along the cytoplasmic side of the membrane of each cell. This material corresponds to the location of the cytoplasmic component of the Ecadherin–catenin complex and the associated proteins (-actinin and vinculin) into which actin filaments attach. Evidence also suggests that the fuzzy plaque represents the stainable substance in light microscopy, the terminal bar. Associated with the electron-dense material is an array of 6-nm actin flaments that stretch across the apical cytoplasm of the epithelial cell, the terminal web.


Histology_Ross

Histology_Ross. FIGURE 5.19 • Zonula adherens. a. Molecular organization of zonula adherens. Actin filaments of adjacent cells are attached to the E-cadherin–catenin complex by -actinin and vinculin. The E-cadherin–catenin complex interacts with identical molecules embedded in the plasma membrane of the adjacent cell. Interactions between transmembrane proteins are mediated by calcium ions. b. Electron micrograph of the zonula adherens from Figure 5.14a at higher magnification. The plasma membranes are separated here by a relatively uniform intercellular space. This space appears clear, showing only a sparse amount of diffuse electron-dense substance, which represents extracellular domains of E-cadherin. The cytoplasmic side of the plasma membrane exhibits a moderately electron-dense material containing actin filaments. 100,000. The fascia adherens is a sheetlike junction that stabilizes nonepithelial tissues.


Histology_Ross

Histology_Ross. The fascia adherens is a sheetlike junction that stabilizes nonepithelial tissues. Physical attachments that occur between cells in tissues other than epithelia are usually not prominent, but there is at least one notable exception. Cardiac muscle cells are arranged end to end, forming threadlike contractile units. The cells are attached to each other by a combination of typical desmosomes, or maculae adherentes, and broad adhesion plates that morphologically resemble the zonula adherens of epithelial cells. Because the attachment is not ringlike but rather has a broad face, it is called the fascia adherens (Fig. 5.20). At the molecular level, the structure of the fascia adherens is similar to that of the zonula adherens; it also contains the zonula occludens ZO-1 protein found in the tight junctions of epithelial cells. The macula adherens (desmosome) provides a localized spotlike junction between epithelial cells.


Histology_Ross

Histology_Ross. The macula adherens (desmosome) provides a localized spotlike junction between epithelial cells. The macula adherens [L. macula, spot] represents a major anchoring cell-to-cell junction that provides a particularly strong attachment, as shown by microdissection studies. The macula adherens was originally described in epidermal cells and was called a desmosome [Gr. desmo, bond soma, body]. These junctions are localized on the lateral domain of the cell, much like a series of spot welds (see Fig. 5.14a), and they mediate direct cell-to-cell contact by providing anchoring sites for intermediate filaments. Increasing evidence suggests that the macula adherens, in addition to its structural function, participates in tissue morphogenesis and differentiation.


Histology_Ross

Histology_Ross. In simple epithelium formed by cuboidal or columnar cells, the macula adherens is found in conjunction with occluding (zonula occludens) and adhering (zonula adherens) junctions. Because the macula adherens occupies small, localized sites on the lateral cell surface, it is not a continuous structure around the cell, as is the zonula adherens. Thus, a section perpendicular to the surface of a cell that cuts through the entire lateral surface will often not include a macula adherens. The section will always, however, include the zonula adherens. In the area of the macula adherens, desmogleins and desmocollins provide the linkage between the plasma membranes of adjacent cells.


Histology_Ross

Histology_Ross. In the area of the macula adherens, desmogleins and desmocollins provide the linkage between the plasma membranes of adjacent cells. FIGURE 5.20 • Fascia adherens. Electron micrograph showing the end-to-end apposition of two cardiac muscle cells. The intercellular space appears as a clear undulating area. On the cytoplasmic side of the plasma membrane of each cell, there is a dense material similar to that seen in a zonula adherens containing actin filaments. Because the attachment site here involves a portion of the end face of the two cells, it is called a fascia adherens. 38,000.


Histology_Ross

Histology_Ross. Electron microscopy reveals that the macula adherens has a complex structure. On the cytoplasmic side of the plasma membrane of each of the adjoining cells is a disc-shaped structure consisting of very dense material called the desmosomal attachment plaque. This structure measures about 400 250 10 nm and anchors intermediate flaments (Fig. 5.21a). The filaments appear to loop through the attachment plaques and extend back out into the cytoplasm. They are thought to play a role in dissipating physical forces throughout the cell from the attachment site. At the molecular level, each attachment plaque is composed of several constitutive proteins, mainly desmoplakins and plakoglobins, which are capable of anchoring the intermediate filaments (Fig. 5.21b).


Histology_Ross

Histology_Ross. The intercellular space of the macula adherens is conspicuously wider (up to 30 nm) than that of the zonula adherens and is occupied by a dense medial band, the intermediate line. This line represents extracellular portions of transmembrane glycoproteins, the desmogleins and desmocollins, which are members of the cadherin family of Ca2-dependent cell adhesion molecules. In the presence of Ca2, extracellular portions of desmogleins and desmocollins bind adjacent identical molecules of neighboring cells (homotypic binding). X-ray crystallographic studies suggest that the extracellular binding domain of proteins from one cell interacts with two adjacent cadherin domains in an antiparallel orientation, thus forming a continuous cadherin zipper in the area of the desmosome (see Fig. 5.21b). The cytoplasmic portions of desmogleins and desmocollins are integral components of the desmosomal attachment plaque. They interact with the plakoglobins and desmoplakins that are involved in desmosome


Histology_Ross

Histology_Ross. cytoplasmic portions of desmogleins and desmocollins are integral components of the desmosomal attachment plaque. They interact with the plakoglobins and desmoplakins that are involved in desmosome assembly and the anchoring of intermediate flaments.


Histology_Ross

Histology_Ross. The cells of different epithelia require different types of attachments. In epithelia that serve as physiologic barriers, the junctional complex is particularly significant because it serves to create a long-term barrier, allowing the cells to compartmentalize and restrict the free passage of substances across the epithelium. Although it is the zonula occludens of the junctional complex that principally affects this function, it is the adhesive properties of the zonulae and maculae adherentes that guard against physical disruption of the barrier. In other epithelia, there is need for substantially stronger attachment between cells in several planes. In the stratified epithelial cells of the epidermis, for example, numerous maculae adherentes maintain adhesion between adjacent cells. In cardiac muscle, where there is a similar need for strong adhesion, a combination of the macula adherens and the fascia adherens serves this function.


Histology_Ross

Histology_Ross. Communicating junctions, also called gap junctions or nexuses, are the only known cellular structures that permit the direct passage of signaling molecules from one cell to another. They are present in a wide variety of tissues, including epithelia, smooth and cardiac muscle, and nerves. Gap junctions are important in tissues in which activity of adjacent cells must be coordinated, such as epithelia engaged in fluid and electrolyte transport, vascular and intestinal smooth muscle, and heart muscle. A gap junction consists of an accumulation of transmembrane channels or pores in a tightly packed array. It allows cells to exchange ions, regulatory molecules, and small metabolites through the pores. The number of pores in a gap junction can vary widely, as can the number of gap junctions between adjacent cells.


Histology_Ross

Histology_Ross. FIGURE 5.21 • Molecular structure of the macula adherens (desmosome). a. Electron micrograph of a macula adherens, showing the intermediate filaments (arrows) attaching into a dense, intracellular attachment plaque located on the cytoplasmic side of the plasma membrane. The intercellular space is also occupied by electron-dense material (arrowheads) containing desmocollins and desmogleins. The intercellular space above and below the macula adherens is not well defined because of extraction of the plasma membrane to show components of this structure. 40,000. (Courtesy of Dr. Ernst Kallenbach.) b. Schematic diagram showing the structure of a macula adherens. Note the intracellular attachment plaque with anchored intermediate filaments. The extracellular portions of desmocollins and desmogleins from opposing cells interact with each other in the localized area of the desmosome, forming the cadherin “zipper.”


Histology_Ross

Histology_Ross. A variety of methods are used to study structure and function of gap junctions. Various procedures have been used to study gap junctions, including the injection of dyes and fluorescent or radiolabeled compounds and measuring the flow of an electric current between cells. In dye studies, a fluorescent dye is injected with a micropipette into one cell. After a short period, the dye can be readily visualized in immediately adjacent cells. Electrical conductance studies show that neighboring cells joined by gap junctions exhibit a low electrical resistance between them and current flow is high; therefore, gap junctions are also called low-resistance junctions.


Histology_Ross

Histology_Ross. Current molecular biology techniques allow for isolation of cDNA clones encoding a family of gap junction proteins (connexins) and expressing them in tissue culture cells. Connexins expressed in transfected cells produce gap junctions, which can be isolated and studied by molecular and biochemical methods as well as by the improved imaging techniques of electron crystallography and atomic force microscopy. Gap junctions are formed by 12 subunits of the connexin protein family.


Histology_Ross

Histology_Ross. Gap junctions are formed by 12 subunits of the connexin protein family. When viewed with the TEM, the gap junction appears as an area of contact between the plasma membranes of adjacent cells (Fig. 5.22a). High-resolution imaging techniques such as cryo-electron microscopy have been used to examine the structure of gap junctions. These studies reveal groups of tightly packed channels, each formed by two half-channels called connexons embedded in the facing membranes. These channels are represented by pairs of connexons that bridge the extracellular space between adjacent cells. The connexon in one cell membrane is precisely aligned to dock with a corresponding connexon on the membrane of an adjacent cell, thus, as the name implies, allowing communication between the cells.


Histology_Ross

Histology_Ross. Each connexon contains six symmetrical subunits of an integral membrane protein called connexin (Cx) that is paired with a similar structure from the adjacent membrane. Therefore, the entire channel consists of 12 subunits. The subunits are configured in a circular arrangement to surround a 10-nm-long cylindrical transmembrane channel with a diameter of 2.8 nm (Fig. 5.22b). About 21 members of the connexin family of proteins have been identified. All traverse the lipid bilayer four times (i.e., they have four transmembrane domains). Most connexons pair with identical connexons (homotypic interaction) on the adjacent plasma membrane. These channels allow molecules to pass evenly in both directions; however, heterotypic channels can be asymmetrical in function, passing certain molecules faster in one direction than in another.


Histology_Ross

Histology_Ross. FIGURE 5.22 • Structure of a gap junction. a. Electron micrograph showing the plasma membranes of two adjoining cells forming a gap junction. The unit membranes (arrows) approach one another, narrowing the intercellular space to produce a 2-nm-wide gap. 76,000. b. Drawing of a gap junction showing the membranes of adjoining cells and the structural components of the membrane that form channels or passageways between the two cells. Each passageway is formed by a circular array of six subunits, dumbbell-shaped transmembrane proteins that span the plasma membrane of each cell. These complexes, called connexons, have a central opening of about 2-nm in diameter. The channels formed by the registration of the adjacent complementary pairs of connexons permit the flow of small molecules through the channel but not into the intercellular space. Conversely, substances in the intercellular space can permeate the area of a gap junction by flowing around the connexon complexes, but they cannot


Histology_Ross

Histology_Ross. the channel but not into the intercellular space. Conversely, substances in the intercellular space can permeate the area of a gap junction by flowing around the connexon complexes, but they cannot enter the channels. c. The diameter of the channel in an individual connexon is regulated by reversible changes in the conformation of the individual connexins.


Histology_Ross

Histology_Ross. Conformational changes in connexins leading to opening or closing gap junction channels have been observed with atomic force microscopy. Earlier electron microscopy studies of isolated gap junctions suggested that the gap junction channels are opened and closed by twisting of the connexin subunits (Fig. 5.22c). Recent atomic force microscopy (AFM) studies provide a dynamic view of the conformational changes that take place in connexons. Channels in gap junctions can fluctuate rapidly between an open and a closed state through reversible changes in the conformation of individual connexins. The conformational change in connexin molecules that triggers closure of gap junction channels at their extracellular surface appears to be induced by Ca2 ions (Fig. 5.23). However, other calcium-independent gating mechanisms responsible for closing and opening of the cytoplasmic domains of gap junction channels have also been identified.


Histology_Ross

Histology_Ross. Mutations in connexin genes are major pathogenic factors in several diseases. For instance, a mutation in the gene encoding connexin-26 (Cx26) is associated with congenital deafness. The gap junctions formed by Cx26 are found in the inner ear and are responsible for recirculating K in the cochlear sensory epithelium. Other mutations affecting Cx46 and Cx50 genes have been identified in patients with inherited cataracts. Both proteins are localized within the lens of the eye and form extensive gap junctions between the epithelial cells and lens fibers. These gap junctions play a crucial role in delivering nutrients to and removing metabolites from the avascular environment of the lens. A summary of the features of all of the junctions discussed in this chapter is found in Table 5.4. Morphologic Specializations of the Lateral Cell Surface Lateral cell surface folds (plicae) create interdigitating cytoplasmic processes of adjoining cells.


Histology_Ross

Histology_Ross. Morphologic Specializations of the Lateral Cell Surface Lateral cell surface folds (plicae) create interdigitating cytoplasmic processes of adjoining cells. The lateral surfaces of certain epithelial cells show a tortuous boundary as a result of infoldings or plicae along the border of each cell with its neighbor (Fig. 5.24). These in-foldings increase the lateral surface area of the cell and are particularly prominent in epithelia that are engaged in fluid and electrolyte transport, such as the intestinal and


Histology_Ross

Histology_Ross. FIGURE 5.23 • Atomic force microscopic (AFM) image of a gap junction. This image shows the extracellular surface of a plasma membrane preparation from the HeLa cell line. Multiple copies of the connexin-26 gene were incorporated into the HeLa cell genome to achieve overexpression of the connexin protein. Connexin-26 proteins self-assemble into functional gap junctions, and they were observed with AFM in two different buffer solutions. a. Gap junction containing individual connexons in a calcium-free buffer solution. 500,000. Inset shows a single connexon at higher magnification. Note the clear profiles of individual connexin molecules assembled into the connexon. The open profile of the channel is also visible. 2,000,000. b. The same preparation of connexons in a buffer containing Ca2 . 500,000. Inset: Note that the conformational change of the connexin molecules has caused the channel to close and has reduced the height of the connexon. 2,000,000. (Courtesy of Dr. Gina E. Sosinsky.)


Histology_Ross

Histology_Ross. Inset: Note that the conformational change of the connexin molecules has caused the channel to close and has reduced the height of the connexon. 2,000,000. (Courtesy of Dr. Gina E. Sosinsky.) gallbladder epithelium. In active fluid transport, sodium ions are pumped out of the cytoplasm at the lateral plasma membrane by NaK-ATPase localized in that membrane. Anions then diffuse across the membrane to maintain electrical neutrality, and water diffuses from the cytoplasm into the intercellular space, driven by the osmotic gradient between the salt concentration in the intercellular space and the concentration in the cytoplasm. The intercellular space distends because of the accumulating fluid moving across the epithelium, but it can distend only to a limited degree because of junctional attachments in the apical and basal portions of the cell. Hydrostatic pressure gradually builds up in the intercellular space and drives an essentially isotonic fluid from the space into the underlying


Histology_Ross

Histology_Ross. in the apical and basal portions of the cell. Hydrostatic pressure gradually builds up in the intercellular space and drives an essentially isotonic fluid from the space into the underlying connective tissue. The occluding junction at the apical end of the inter-cellular space prevents fluid from moving in the opposite direction. As the action of the sodium pump depletes the cytoplasm of salt and water, these are replaced by diffusion across the apical plasma membrane, whose surface area is greatly increased by the presence of microvilli, thus allowing the continuous movement of fluid from the lumen to the connective tissue as long as the Na/K-ATPase is active.


Histology_Ross

Histology_Ross. The basal domain of epithelial cells is characterized by several features:  The basement membrane is a specialized structure lo cated next to the basal domain of epithelial cells and the underlying connective tissue stroma.  Cell-to-extracellular matrix junctions anchor the cell to the extracellular matrix; they are represented by focal adhesions and hemidesmosomes.  Basal cell membrane infoldings increase the cell sur adjacent cells and extracellular matrix proteins.


Histology_Ross

Histology_Ross. The term basement membrane was originally given to an amorphous, dense layer of variable thickness at the basal surfaces of epithelia. Although a prominent structure referred to as basement membrane is observed with hematoxylin and eosin (H&E) stain in a few locations such as the trachea (Fig. 5.25) and, occasionally, the urinary bladder and ureters, basement membrane requires special staining to be seen in the light microscope. This requirement is caused, in part, by its thinness and by the effect of the eosin stain, which makes it indistinguishable from the immediately adjacent connective tissue. In the trachea, the structure that is often described as basement membrane includes not only the true basement membrane but also an additional layer of closely spaced and aligned collagen fibrils that belong to the connective tissue.


Histology_Ross

Histology_Ross. In contrast to H&E (Fig. 5.26a), the periodic acid–Schiff (PAS) staining technique (Fig. 5.26b) results in a positive reaction at the site of the basement membrane. It appears as a thin, well-defined magenta layer between the epithelium and the connective tissue. The stain reacts with the sugar moieties of proteoglycans, accumulating in sufficient amounts and density to make the basement membrane visible in the light microscope. Techniques involving the chapter 5 Epithelial Tissue TH E BASAL DOMAI N AN D ITS SPECIALIZATIONS TABLE Summary of Junctional Features 5.4 Associated Major Intracellular Link Extracellular Cytoskeleton Attachment Classification Proteins Ligands Components Proteins Functions Zonula Occludins, Occludins, Actin ZO-1, ZO-2, Seals adjacent occludens claudins, claudins, flaments ZO-3, AF6, cells together, (tight junction) JAMs JAMs in cingulin controls adjacent cell symplectin passage of ASIP/Povr 3 molecules Rab 36, 13, 8 between them Sec 4, 6, 8 (permeability),


Histology_Ross

Histology_Ross. flaments ZO-3, AF6, cells together, (tight junction) JAMs JAMs in cingulin controls adjacent cell symplectin passage of ASIP/Povr 3 molecules Rab 36, 13, 8 between them Sec 4, 6, 8 (permeability), defnes apical domain of plasma membrane, involved in cell signaling Zonula E-cadherin-E-cadherin-Actin -Actinin, Couples the adherens catenin catenin flaments vinculin actin cytoske-complex complex in leton to the adjacent cell plasma mem-brane at regi-ons of cell-cell adhesion Macula Cadherins (e.g., Desmogleins, Intermediate Desmoplakins, Couples the adherens desmogleins, desmocollins flaments plakoglobins intermediate (desmosome) desmocollins) in adjacent flaments to cell the plasma membrane at regions of cell-cell adhesion Focal Integrins Extracellular Actin Vinculin, talin, Anchors the adhesion matrix flaments -actinin, actin cytoske-proteins (e.g., paxillin leton to the fbronectin) extracellular matrix, detects and transdu-ces signals from outside the cell Hemides-Integrins


Histology_Ross

Histology_Ross. the adhesion matrix flaments -actinin, actin cytoske-proteins (e.g., paxillin leton to the fbronectin) extracellular matrix, detects and transdu-ces signals from outside the cell Hemides-Integrins Extracellular Intermediate Desmoplakin-Anchors the mosome (64 matrix protein flaments like proteins, intermediate integrin), (e.g., (possible microtu-BP 230 flaments to collagen laminin-5, bules and actin plectin, the extrace-XVII collagen-IV) flaments via inter-erbin llular matrix action with plectin) Gap junction Connexin Connexin in None Not Known Creates a (nexus) adjacent cell conduct between two adjacent cells for passage of small ions and informational micro-molecules Occluding Junction(cell-to-cell) Anchoring Junctions(cell-to-cell) Communicating Junction (cell-to-cell) Anchoring Junctions(cell-to–extracellular matrix) hhhh


Histology_Ross

Histology_Ross. FIGURE 5.24 • Lateral interdigitations. This electron micrograph shows infoldings or interdigitations at the lateral surfaces of two adjoining intestinal absorptive cells. 25,000. reduction of silver salts by the sugars blacken the basement membrane and are also used to demonstrate this structure. Although the basement membrane is classically described as exclusively associated with epithelia, similar PAS-positive and silver-reactive sites can be demonstrated surrounding peripheral nerve supporting cells, adipocytes, and muscle cells (Fig. 5.27); this helps to delineate them from the surrounding connective tissue in histologic sections. Connective tissue cells other than adipocytes do not show a similar PAS-positive or silver reaction. That most connective tissue cells are not surrounded by basement membrane material is consistent with their lack of adhesion to the connective tissue fibers. In fact, they must migrate within the tissue under appropriate stimuli to function.


Histology_Ross

Histology_Ross. The basal lamina is the structural attachment site for overlying epithelial cells and underlying connective tissue. Former descriptions of basal lamina were based on the investigation of specimens routinely prepared for electron microscopy. The examination of the site of epithelial base- FIGURE 5.25 • Tracheal basement membrane. Photomicrograph of an H&E–stained section of the pseudostratified ciliated epithelium of the trachea. The basement membrane appears as a thick homogeneous layer immediately below the epithelium. It is actually a part of the connective tissue and is composed largely of densely packed collagen fibrils. 450.


Histology_Ross

Histology_Ross. ment membranes with the EM reveals a discrete layer of electron-dense matrix material 40to 60-nm thick between the epithelium and the adjacent connective tissue (Fig. 5.28) called the basal lamina or, sometimes, lamina densa. When observed at high resolution, this layer exhibits a network of fine, 3to 4-nm filaments composed of laminins, a type IV collagen molecule, and various associated proteoglycans and glycoproteins. Between the basal lamina and the cell is a relatively clear or electron-lucent area, the lamina lucida (also about 40 nm wide). The area outlined by the lamina lucida contains extracellular portions of CAMs, mainly fbronectin and laminin receptors. These receptors are members of the integrin family of transmembrane proteins. With the development of new EM preparation techniques, the lamina lucida appears to be an artifact of fixation; in the living state, the basal lamina is composed of a single layer of the lamina densa.


Histology_Ross

Histology_Ross. If the tissue specimen for EM is fixed using low-temperature, high-pressure freezing (HPF) methods (without chemical fixatives), it retains much more of the tissue than specimens routinely fixed with glutaraldehyde. EM examination of such specimens reveals that the basal lamina is composed only of the lamina densa. No lamina lucida is detected. The lamina lucida may thus be an artifact of chemical fixation that appears as the epithelial cells shrink away from a high concentration of macromolecules deposited next to the basal domain of the epithelial cells. It probably results from the rapid dehydration that occurs during tissue processing for electron microscopy. Other structures visible with traditional electron microscopy are also not visible when tissues are prepared by the HPF method (Fig. 5.29).


Histology_Ross

Histology_Ross. FIGURE 5.26 • Photomicrographs showing serial sections of intestinal glands of the colon. The glands in this specimen have been cross-sectioned and appear as round profiles. a. This specimen was stained with H&E. Note that neither the basement membrane nor the mucin that is located within the goblet cells is stained. 550. b. This section was stained by the PAS method. It reveals the basement membrane as a thin, magenta layer (arrows) between the base of the epithelial cells of the glands and the adjacent connective tissue. The mucin within the goblet cells is also PAS positive. 550.


Histology_Ross

Histology_Ross. FIGURE 5.27 • Smooth muscle external lamina. This photomicrograph is stained by the PAS method and counterstained with hematoxylin (pale nuclei). The muscle cells have been cut in cross section and appear as polygonal profiles because of the presence of PAS-positive basement membrane material surrounding each cell. The cytoplasm is not stained. As the plane of section passes through each smooth muscle cell, it may or may not pass through the portion of the cell that includes the nucleus. Therefore, in some of the polygonal profiles, nuclei can be seen; in other profiles, no nuclei are seen. 850. The basal lamina in nonepithelial cells is referred to as the external lamina.


Histology_Ross

Histology_Ross. The basal lamina in nonepithelial cells is referred to as the external lamina. Muscle cells, adipocytes, and peripheral nerve supporting cells exhibit an extracellular electron-dense material that resembles the basal lamina of epithelium. This material also corresponds to a PAS-positive staining reaction, as described earlier (see Fig. 5.27). Although the term basement membrane is not ordinarily applied to the extracellular stainable material of these nonepithelial cells in light microscopy, the terms basal lamina or external lamina are typically used at the EM level. The basal lamina contains molecules that come together to form a sheetlike structure.


Histology_Ross

Histology_Ross. The basal lamina contains molecules that come together to form a sheetlike structure. Analyses of basal laminae derived from epithelia in many locations (kidney glomeruli, lung, cornea, lens of the eye) indicate that they consist of approximately 50 proteins that can be classified into four groups: collagens, laminins, glycoproteins, and proteoglycans. These proteins are synthesized and secreted by the epithelial cells and other cell types that possess an external lamina.  Collagens. At least three types of collagen species are present in the basal lamina; they represent a fraction of the approximately 28 types of collagen found in the body.


Histology_Ross

Histology_Ross. The major component, comprising 50% of all basal used inconsistently in the literature. Some authors use basement membrane when referring to both light and electron microscopic images. Others dispense with the term basement membrane altogether and use basal lam-ina in both light and electron microscopy. Because the term basement membrane originated with light mi-croscopy, it is used in this book only in the context of light microscopic descriptions and only in relation to epithelia. tural content to denote the layer present at the interface of connective tissue with epithelial cells. In this context, the light microscopy term basement membrane actually de-scribes basal lamina and the underlying reticular lamina combined. The term external lamina is used to identify basal lamina when it forms a peripheral cellular invest-ment, as in muscle cells and peripheral nerve supporting cells.  FOLDER 5.4 Functional Considerations: Basement Membrane and Basal Lamina Terminology


Histology_Ross

Histology_Ross.  FOLDER 5.4 Functional Considerations: Basement Membrane and Basal Lamina Terminology The terms basement membrane and basal lamina are The EM term basal lamina is reserved for the ultrastruc- FIGURE 5.28 • Electron micrograph of two adjoining epithelial cells with their basal lamina. The micrograph shows only the basal portions of the two cells and parts of their nuclei (N). The intercellular space is partially obscured by lateral interdigitations between the two cells (arrows). The basal lamina (BL) appears as a thin layer that follows the contours of the basal domain of the overlying cell. Below the basal lamina are numerous cross-sectioned collagen (reticular) fibrils. 30,000.


Histology_Ross

Histology_Ross. lamina proteins, is type IV collagen. The molecular characteristics and function of type IV collagen in forming a scaffold of basal lamina is described in the next section. The presence of different type IV collagen isoforms provides specificity to the basal lamina associated with different tissues. Two nonfibrillar types of collagens, type XV collagen and type XVIII collagen, are also found in the basal lamina. Type XV collagen plays an important role in stabilizing the structure of the external lamina in skeletal and cardiac muscle cells, whereas type XVIII collagen is mainly present in vascular and epithelial basal laminae and is believed to function in angiogenesis. In addition, type VII collagen forms anchoring fibrils that link the basal lamina to the underlying reticular lamina (described below).


Histology_Ross

Histology_Ross.  Laminins. These cross-shaped glycoprotein molecules (140 to 400 kilodaltons) are composed of three polypeptide chains. They are essential in initiating the assembly of the basal lamina. Laminins possess binding sites for different integrin receptors in the basal domain of the overlying epithelial cells. They are involved in many cellto–extracellular matrix interactions. They also play roles in the development, differentiation, and remodeling of epithelium. There are approximately 15 different variations of laminin molecules.  Entactin/nidogen. This small, rodlike sulfated glycoprotein (150 kilodaltons) serves as a link between laminin and the type IV collagen network in almost all basal laminae. Each entactin molecule is organized into distinct domains that bind calcium, support cell adhesion, promote neu


Histology_Ross

Histology_Ross. FIGURE 5.29 • Electron micrograph of epithelial cells preserved by low-temperature, high-pressure freezing. This electron micrograph shows basal domain of an epithelial cell obtained from human skin. The specimen was prepared by low-temperature, high-pressure freezing, which retains more tissue components than does chemical fixation. Note that a separate lamina densa or lamina lucida is not seen in this preparation. The lamina lucida is most likely an artifact that appears as the epithelial cell shrinks away from a high concentration of macromolecules just basal of the epithelial cell. This region of highly concentrated macromolecules precipitates into the artifact known as the lamina densa. BL, basal lamina; HD, hemidesmosome; CF, collagen fibrils. 55,000. (Courtesy of Douglas R. Keene.) trophil chemotaxis and phagocytosis, and interact with laminin, perlecan, fibronectin, and type IV collagen.


Histology_Ross

Histology_Ross.  Proteoglycans. Most of the volume of the basal lamina is probably attributable to its proteoglycan content. Proteoglycans consist of a protein core to which heparan sulfate (e.g., perlecan, agrin), chondroitin sulfate (e.g., bamacan), or dermatan sulfate side chains are attached. Because of their highly anionic character, these molecules are extensively hydrated. They also carry a high negative charge; this quality suggests that proteoglycans play a role in regulating the passage of ions across the basal lamina. The most common heparan sulfate proteoglycan found in all basal laminae is the large multidomain proteoglycan perlecan (400 kilodaltons). It provides additional cross-links to the basal lamina by binding to laminin, type IV collagen, and entactin/nidogen. Agrin (500 kilodaltons) is another important molecule found almost exclusively in the glomerular basement membrane of the kidney. It plays a major role in renal filtration as well as in cell-to–extracellular matrix


Histology_Ross

Histology_Ross. is another important molecule found almost exclusively in the glomerular basement membrane of the kidney. It plays a major role in renal filtration as well as in cell-to–extracellular matrix interactions.


Histology_Ross

Histology_Ross. The molecular structure of type IV collagen determines its role in the formation of the basal lamina network suprastructure. The type IV collagen molecule is similar to other collagens in that it contains three polypeptide chains. Each chain has a short amino-terminus domain (7S domain), a long middle collagenous helical domain (which interacts with the remaining two chains in the fully assembled molecule), and a carboxy-terminus globular noncollagenous domain (NC1 domain). The six known chains of type IV collagen molecules (1 to 6) form three sets of triple helical molecules known as collagen protomers. They are designated as [1(IV)]22(IV); 3(IV)4(IV)5(IV), and [5(IV)]26(IV) protomers (see Table 6.2).


Histology_Ross

Histology_Ross. Protomer assembly begins when the three NC1 domains assemble to form an NC1 trimer (Fig. 5.30). The next step in the assembly of the basal lamina structure is the formation of type IV collagen dimer molecules. This is achieved when two NC1 trimers interact to form an NC1 hexamer. Next, four dimers join in the region of the 7S domain to form a tetramer. The 7S domain of the tetramer (called the 7S box) determines the geometry of the tetramer. Finally, the type IV collagen scaffold is formed when other collagen tetramers interact end to end with each other. This scaffold forms the suprastructure of the basal lamina. Assembly of this suprastructure is genetically determined. Those containing [1(IV)]22(IV) protomers are found in all basal laminae. Those containing 3(IV)4 (IV)5(IV) protomers occur mainly in the kidney and lungs, and those containing [5(IV)]26(IV) protomers are restricted to the skin, esophagus, and Bowman capsule in the kidney.


Histology_Ross

Histology_Ross. Basal lamina self-assembly is initiated by the polymerization of laminins on the basal cell domain and interaction with the type IV collagen suprastructure.


Histology_Ross

Histology_Ross. The constituents of the basal lamina come together in a process of self-assembly to form a sheetlike structure. This process is initiated by both type IV collagen and laminins. The primary sequence of these molecules contains information for their self-assembly (other molecules of the basal lamina are incapable of forming sheetlike structures by themselves). Studies using cell lines have shown that the first step in self-assembly of the basal lamina is calcium-dependent polymerization of laminin molecules on the basal cell surface domain (Fig. 5.31). This process is aided by CAMs (integrins). At the same time, the type IV collagen suprastructure becomes associated with laminin polymers. These two structures are joined together primarily by entactin/nidogen bridges and are additionally secured by other proteins (perlecan, agrin, fibronectin, and so forth). The scaffold of type IV collagen and laminins provides the site for other basal lamina molecules to interact and form the fully


Histology_Ross

Histology_Ross. secured by other proteins (perlecan, agrin, fibronectin, and so forth). The scaffold of type IV collagen and laminins provides the site for other basal lamina molecules to interact and form the fully functional basal lamina.


Histology_Ross

Histology_Ross. FIGURE 5.30 • Formation of the type IV collagen suprastructure. Each type IV collagen molecule has three domains: an amino-terminus (7S domain), a middle collagenous helical domain, and a carboxy-terminus (NC1 domain). The NC1 domain initiates assembly of the type IV collagen protomer, which consists of three molecules. Protomer formation proceeds like a zipper from the NC1 domain toward the 7S domain, resulting in a fully assembled protomer. The next step in assembly is the dimerization of type IV collagen protomers. Two type IV collagen protomers become connected via their NC1 domains, and their two NC1 trimers join together to form an NC1 hexamer. Next, four dimers join together at their 7S domains to form tetramers connected by the 7S box. These tetramers interact to form the type IV collagen suprastructure via their interactions with the 7S domains of other tetramers and also by lateral associations between type IV collagen protomers.


Histology_Ross

Histology_Ross. A layer of reticular fibers underlies the basal lamina.


Histology_Ross

Histology_Ross. There is still lack of agreement about the extent to which the basal lamina seen with the EM corresponds to the structure described as the basement membrane in the light microscope. Some investigators contend that the basement membrane includes not only the basal lamina but also a secondary layer of small-unit fibrils of type III collagen (reticular fbers) that forms the reticular lamina. The reticular lamina, as such, belongs to the connective tissue and is not a product of the epithelium. The reticular lamina was once regarded as the component that reacted with silver, whereas the polysaccharides of the basal lamina and the ground substance associated with the reticular fibers were thought to be the components stained with the PAS reaction. However, convincing arguments can be made for the basal lamina reacting with both PAS and silver in several sites. In normal kidney glomeruli, for example, no collagen (reticular) fibers are associated with the basal lamina of the epithelial


Histology_Ross

Histology_Ross. for the basal lamina reacting with both PAS and silver in several sites. In normal kidney glomeruli, for example, no collagen (reticular) fibers are associated with the basal lamina of the epithelial cells (Fig. 5.32), although a positive reaction occurs with both PAS staining and silver impregnation. Also, in the spleen, where the basal lamina of the venous sinuses forms a unique pattern of ringlike bands rather than a thin, sheathlike layer around the vessel, exactly corresponding images are seen with the PAS and silver techniques as well as with the EM (Fig. 5.33).


Histology_Ross

Histology_Ross. Several structures are responsible for attachment of the basal lamina to the underlying connective tissue. On the opposite side of the basal lamina, the connective tissue side, several mechanisms provide attachment of the basal lamina to the underlying connective tissue:  Anchoring fbrils (type VII collagen) are usually found in close association with hemidesmosomes. They extend from the basal lamina either to the structures called anchoring plaques in the connective tissue matrix or loop back to the basal lamina (Fig. 5.34). The anchoring fibrils entrap type III collagen (reticular) fibers in the underlying connective tissue, which ensures sound epithelial anchorage. Anchoring fibrils are critical to the function of the anchoring junctions; mutations in the collagen VII gene result in dystrophic epidermolysis bullosa, an inherited blistering skin disease in which the epithelium is detached below the basement membrane.


Histology_Ross

Histology_Ross.  Fibrillin microfbrils are 10 to 12 nm in diameter and attach the lamina densa to elastic fibers. Fibrillin microfibrils are known to have elastic properties. A mutation in the fibrillin gene (FBN1) causes Marfan’s syndrome and other related connective tissue disorders.  Discrete projections of the lamina densa on its connective tissue side interact directly with the reticular lamina to form an additional binding site with type III collagen. An interwoven network of proteins provides the bases for a variety of basal lamina functions. In recent years, the basal lamina has been recognized as an important regulator of cell behavior rather than just a structural feature of the epithelial tissue. Organ-specific molecules have been identified in the basal lamina. Although morphologically single molecule of laminin integrin receptors


Histology_Ross

Histology_Ross. FIGURE 5.31 • Molecular components of the basal lamina. To produce a basal lamina, each epithelial cell must first synthesize and secrete its molecular components. The assembly of the basal lamina occurs outside the cell at its basal domain. The calcium-dependent polymerization of laminin molecules that occurs at the basal cell surface initiates basal lamina formation. Laminin polymers are next anchored to the cell surface by integrin receptors. At the same time, the type IV collagen suprastructure is assembled (see Fig. 5.30) in close proximity to laminin polymers. These two structures are connected by entactin or nidogen bridges and are additionally secured by other proteins (i.e., perlecan). The primary scaffold of type IV collagen connected to laminin polymers provides the site for other basal lamina molecules to interact and form the fully functional basal lamina.


Histology_Ross

Histology_Ross. LEnPEnBLPPNLL En P En BL P P N L FIGURE 5.32 • Basal lamina in the kidney glomerulus. Electron micrograph of a kidney glomerular capillary showing the basal lamina (BL) interposed between the capillary endothelial cell (En) and the cytoplasmic processes (P; podocytes) of epithelial cells. The epithelial cell is located on the outer (abluminal) surface of the endothelial cell. 12,000. Inset. Relationship at higher magnification. Note that the endothelial cells and epithelial cells are separated by the shared basal lamina and that no collagen fibrils are present. N, nucleus of epithelial cell; L, lumen of capillary. 40,000.


Histology_Ross

Histology_Ross. FIGURE 5.33 • Demonstration of basement membrane material in splenic vessels. a. Photomicrograph of a silver preparation revealing two longitudinally sectioned venous sinuses in the spleen. These blood vessels are surrounded by a modified basement membrane, which takes the form of a ringlike structure, much like the hoops of a barrel, rather than a continuous layer or lamina. The rings are blackened by the silver and appear as bands where the walls of the vessel have been tangentially sectioned (arrows). To the right, the cut has penetrated deeper into the vessel and shows the lumen (L). Here the cut edges of the rings are seen on both sides of the vessel. In the lower vessel, the cut rings have been sectioned in a virtually perpendicular plane, and the rings appear as a series of dots. 400. b. Electron micrograph of the wall of a venous sinus, showing a longitudinally sectioned endothelial cell (EnC). The nucleus (N) of the cell is protruding into the lumen (L). The basal lamina


Histology_Ross

Histology_Ross. 400. b. Electron micrograph of the wall of a venous sinus, showing a longitudinally sectioned endothelial cell (EnC). The nucleus (N) of the cell is protruding into the lumen (L). The basal lamina material (asterisks) has the same homogeneous appearance as seen by electron microscopy in other sites except that it is aggregated into ringlike structures rather than into a flat layer or lamina. Moreover, its location and plane of section correspond to the silver-reactive, dotlike material in the panel above. 25,000.


Histology_Ross

Histology_Ross. all basal laminae appear similar, their molecular composition including all of its specialized tissues, such as bone and car-and functions are unique to each tissue. The following are tilage (with the exception of adipose tissue, in that its cells various functions now attributed to the basal lamina. possess an external lamina)—can be viewed as a single,  Structural attachment. As noted, the basal lamina serves continuous compartment. In contrast, epithelia, muscles, and nerves are separated from adjacent connective tissue as an intermediary structure in the attachment of cells to by intervening basal or external laminae. For any substance the adjacent connective tissue. Epithelial cells are anchored to move from one tissue to another (e.g., from one com- into the basal lamina by cell-to–extracellular matrix juncpartment to another), it must cross such a lamina.


Histology_Ross

Histology_Ross. tions, and the basal lamina is attached to underlying con nective tissue by anchoring fibrils and fibrillin microfibrils.  Filtration. The movement of substances to and from the  Compartmentalization. Structurally, basal and external connective tissue is regulated in part by the basal lamina, largely through ionic charges and integral spaces. Filtration laminae separate or isolate the connective tissue from epis well characterized in the kidney, in which the plasma ithelia, nerve, and muscle tissues. Connective tissue— projections of lamina densa


Histology_Ross

Histology_Ross. FIGURE 5.34 • Schematic diagram and electron micrograph of the basal portion of epithelial cell. a. This diagram shows the cellular and extracellular components that provide attachment between epithelial cells and the underlying connective tissue. On the connective tissue side of the basal lamina, anchoring fibrils extend from the basal lamina to the collagen (reticular) fibrils of the connective tissue, providing structural attachment at this site. On the epithelial side, laminin (green), collagen XVII (red), and integrins (yellow) are present in the lamina lucida and lamina densa and provide adhesion between the basal lamina and the intracellular attachment plaques of hemidesmosomes. b. This high-magnification electron micrograph of human skin shows the basal portion of human epithelial cells with underlying basal lamina. The electron-lucent space, the lamina lucida located just below the basal cell membrane, is occupied by anchoring filaments formed by the laminin-5 and type XVII


Histology_Ross

Histology_Ross. cells with underlying basal lamina. The electron-lucent space, the lamina lucida located just below the basal cell membrane, is occupied by anchoring filaments formed by the laminin-5 and type XVII collagen molecules. Anchoring filaments are responsible for attaching the basal cell membrane to the basal lamina. The looplike fibers originating from the basal lamina represent anchoring fibrils of type VII collagen that link the basal lamina with the reticular fibers (type III collagen) and with anchoring plaques located within the extracellular matrix. 200,000. (Courtesy of Douglas R. Keene.) filtrate must cross the compound basal laminae of capillar-processes of a cell use the basal lamina that remains after cell ies and adjacent epithelial cells to reach the urinary space loss, thus helping to maintain the original tissue architec within a renal corpuscle. ture. For example, when nerves are damaged, new neuro Tissue scaffolding. The basal lamina serves as a guide or muscular


Histology_Ross

Histology_Ross. helping to maintain the original tissue architec within a renal corpuscle. ture. For example, when nerves are damaged, new neuro Tissue scaffolding. The basal lamina serves as a guide or muscular junctions from a growing axon will be established scaffold during regeneration. Newly formed cells or growing only if the external lamina remains intact after injury. The basal laminae also allow cells to migrate under physiologic Focal adhesions create a dynamic link between the actin conditions but act as barriers against tumor cell invasion.


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Histology_Ross.  Regulation and signaling. Many molecules that reside in the basal lamina interact with cell surface receptors, influencing epithelial cell behavior during morphogenesis, fetal development, and wound healing by regulating cell shape, proliferation, differentiation, and motility as well as gene expression and apoptosis. For instance, the basal lamina of endothelial cells has recently been found to be involved in the regulation of tumor angiogenesis. The organization of cells in epithelium depends on the support provided by the extracellular matrix on which the basal surface of each cell rests. Anchoring junctions maintain the morphologic integrity of the epithelium–connective tissue interface. The two major anchoring junctions are:  focal adhesions, which anchor actin filaments of the cytoskeleton into the basement membrane; and hemidesmosomes, which anchor the intermediate filaments of the cytoskeleton into the basement membrane.


Histology_Ross

Histology_Ross. In addition, transmembrane proteins located in the basal cell domain (mainly related to the integrin family of adhesion molecules) interact with the basal lamina. cytoskeleton and extracellular matrix proteins. Focal adhesions form a structural link between the actin cytoskeleton and extracellular matrix proteins. They are responsible for attaching long bundles of actin filaments (stress fibers) into the basal lamina (Fig. 5.35a). Focal adhesions play a prominent role during dynamic changes that occur in epithelial cells (e.g., migration of epithelial cells in wound repair). Coordinated remodeling of the actin cytoskeleton and the controlled formation and dismantling of focal adhesions provide the molecular bases for cell migration. Focal adhesions are also found in other nonepithelial cells such as fibroblasts and smooth muscle cells.


Histology_Ross

Histology_Ross. In general, focal adhesions consist of a cytoplasmic face to which actin flaments are bound, a transmembrane connecting region, and an extracellular face that binds to the proteins of the extracellular matrix. The main family of transmembrane proteins involved in focal adhesions are integrins, which are concentrated in clusters within the areas where the junctions can be detected. On the cytoplasmic face, integrins interact with actin-binding proteins (-actinin, vinculin, talin, paxillin) as well as many regulatory proteins such as focal adhesion kinase or tyrosine kinase (Fig. 5.35b). On the extracellular side, integrins bind to extracellular matrix glycoproteins, usually laminin and fibronectin.


Histology_Ross

Histology_Ross. FIGURE 5.35 • Molecular structure of focal adhesions. a. Diagram showing the molecular organization of focal adhesions. On the cytoplasmic side, note the arrangement of different actin-binding proteins. These proteins interact with integrins, the transmembrane proteins, the extracellular domains of which bind to proteins of the extracellular matrix (e.g., fibronectin). b. This image was obtained from the fluorescence microscope and shows cells cultured on the fibronectin-coated surface stained with fluoresceinlabeled phalloidin to visualize actin filaments (stress fibers) in green. Next, using indirect immunofluorescence techniques, focal adhesions were labeled with primary monoclonal antibody against phosphotyrosines and visualized with secondary rhodamine-labeled antibody (red). The phosphotyrosine is a product of the tyrosine kinase reaction in which tyrosine molecules of the associated proteins are phosphorylated by this enzyme. Tyrosine kinase is closely associated with focal


Histology_Ross

Histology_Ross. phosphotyrosine is a product of the tyrosine kinase reaction in which tyrosine molecules of the associated proteins are phosphorylated by this enzyme. Tyrosine kinase is closely associated with focal adhesion molecules, so the area where focal adhesions are formed is labeled red. Note the relationship of focal adhesions and actin filaments at the periphery of the cell. 3,000. (Courtesy of Dr. Keith Burridge.)


Histology_Ross

Histology_Ross. Focal adhesions play an important role in sensing and transmitting signals from the extracellular environment into the interior of the cell. Focal adhesions are also important sites of signal detection and transduction. They are able to detect contractile forces or mechanical changes in the extracellular matrix and convert them into biochemical signals. This phenomenon, known as mechanosensitivity, allows cells to alter their adhesion-mediated functions in response to external mechanical stimuli. Integrins transmit these signals to the interior of the cell, where they affect cell migration, differentiation, and growth. Recent studies indicate that focal adhesion proteins also serve as a common point of entry for signals resulting from stimulation of various classes of growth factor receptors. Hemidesmosomes occur in epithelia that require strong, stable adhesion to the connective tissue.


Histology_Ross

Histology_Ross. Hemidesmosomes occur in epithelia that require strong, stable adhesion to the connective tissue. A variant of the anchoring junction similar to the desmosome is found in certain epithelia subject to abrasion and mechanical shearing forces that would tend to separate the epithelium from the underlying connective tissue. Typically, it occurs in the cornea, the skin, and the mucosa of the oral cavity, esophagus, and vagina. In these locations, it appears as if half the desmosome is present, hence the name hemidesmosome. Hemidesmosomes are found on the basal cell surface, where they provide increased adhesion to the basal lamina (Fig. 5.36a).


Histology_Ross

Histology_Ross. When observed with the EM, the hemidesmosome exhibits an intracellular attachment plaque on the cytoplasmic side of the basal plasma membrane. The protein composition of this structure is similar to that of the desmosomal plaque, as it contains a desmoplakin-like family of proteins capable of anchoring intermediate filaments of the cytoskeleton. Three major proteins have been identified in the plaque:  Plectin (450 kilodaltons) functions as a cross-linker of the intermediate filaments that bind them to the hemidesmosomal attachment plaque. Recent studies indicate that plectin also interacts with microtubules, actin filaments, and myosin II. Thus, plectin cross-links and integrates all elements of the cytoskeleton.


Histology_Ross

Histology_Ross.  BP 230 (230 kilodaltons) attaches intermediate filaments to the intercellular attachment plaque. The absence of functional BP 230 causes bullous pemphigoid, a disease characterized clinically by blister formation. High levels of antibodies directed against components of the hemidesmosome, including antibodies against BP 230 and type XVII collagen, are detected in people with this disease. For this reason, BP 230 is called bullous pemphigoid antigen 1 (BPAG1), and the collagen XVII molecule is called bullous pemphigoid antigen 2 (BPAG2) or BP 180.  Erbin (180 kilodaltons) mediates association of BP 230 with integrins.


Histology_Ross

Histology_Ross. FIGURE 5.36 • Molecular structure of hemidesmosome. a. Electron micrograph of the basal aspect of a gingival epithelial cell. Below the nucleus (N), intermediate filaments are seen converging on the intracellular attachment plaques (arrows) of the hemidesmosome. Below the plasma membrane are the basal lamina (BL) and collagen (reticular) fibrils (most of which are cut in cross section) of the connective tissue. 40,000. b. Diagram showing the molecular organization of a hemidesmosome. The intracellular attachment plaque is associated with transmembrane adhesion molecules, such as the family of integrins and transmembrane type XVII collagen, and contains plectin, BP 230, and erbin. Note that the intermediate filaments seem to originate or terminate in the intracellular attachment plaque. Extracellular portions of integrins bind to laminin-5 and type IV collagen. With the help of anchoring fibrils (type VII collagen), laminin, and integrin, the attachment plaque is secured to the


Histology_Ross

Histology_Ross. Extracellular portions of integrins bind to laminin-5 and type IV collagen. With the help of anchoring fibrils (type VII collagen), laminin, and integrin, the attachment plaque is secured to the reticular fibers (type III collagen) of the extracellular matrix.


Histology_Ross

Histology_Ross. In contrast to the desmosome, whose transmembrane proteins belong to the cadherin family of calcium-dependent molecules, the majority of transmembrane proteins found in the hemidesmosome belong to the integrin class of cell matrix receptors. These include:  46 integrin, a heterodimer molecule containing two polypeptide chains. Its extracellular domain enters the basal lamina and interacts with type IV collagen supra structure containing laminins (laminin-5), entactin/nidogen or the perlecan. On the extracellular surface of the hemidesmosome, laminin-5 molecules form threadlike anchoring flaments that extend from the integrin molecules to the structure of the basement membrane (Fig. 5.36b). Interaction between laminin-5 and 64 integrin is essential for hemidesmosome formation and for the maintenance of epithelial adhesion. Mutation of the genes encoding laminin-5 chains results in junctional epidermolysis bullosa, another hereditary blistering skin disease.


Histology_Ross

Histology_Ross.  Type XVII collagen (BPAG2, BP 180), a transmembrane molecule (180 kilodaltons), that regulates expression and function of laminin-5. In experimental models, type XVII collagen inhibits migration of endothelial cells during angiogenesis and regulates keratinocyte migration in the skin (see Fig. 5.36b).  CD151 (32 kilodaltons), a glycoprotein that participates in the clustering of integrin receptors to facilitate cellto–extracellular matrix interactions. Despite their similarity in names, the terms anchoring filaments and anchoring fibrils do not describe the same structure. Anchoring flaments are formed mainly by laminin-5 and type XVII collagen molecules. They attach the basal cell membrane of epithelial cells into the underlying basal lamina. Anchoring fbrils are formed by type VII collagen and attach the basal lamina to the underlying reticular fibers (see page 140). Morphologic Modifications of the Basal Cell Surface


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Histology_Ross. Morphologic Modifications of the Basal Cell Surface Many cells that transport fluid have infoldings at the basal cell surface. They significantly increase the surface area of the basal cell domain, allowing for more transport proteins and channels to be present. These basal surface modifications are prominent in cells that participate in active transport of molecules (e.g., in proximal and distal tubules of the kidney; Fig. 5.37) and in certain ducts of the salivary glands. Mitochondria are typically concentrated at this basal site to provide the energy requirements for active transport. The mitochondria are usually oriented vertically within the folds. The orientation of the mitochondria, combined with the basal membrane infoldings, results in a striated appearance along the basal aspect of the cell when observed in the light microscope. Because of this phenomenon, the salivary gland ducts that possess these cells are referred to as striated ducts.


Histology_Ross

Histology_Ross. FIGURE 5.37 • Basal infoldings. Electron micrograph of the basal portion of a kidney tubule cell showing the infolding of the plasma membrane. Note the aligned mitochondria. The infoldings of adjoining cells result in the interdigitations of cytoplasm between the two cells. 25,000. Typically, glands are classified into two major groups according to how their products are released (Table 5.5):  Exocrine glands secrete their products onto a surface directly or through epithelial ducts or tubes that are connected to a surface. Ducts may convey the secreted material in an unaltered form or may modify the secretion by concentrating it or adding or reabsorbing constituent substances.  Endocrine glands lack a duct system. They secrete their products into the connective tissue, from which they enter the bloodstream to reach their target cells. The products of endocrine glands are called hormones.


Histology_Ross

Histology_Ross. In some epithelia, individual cells secrete a substance that does not reach the bloodstream but rather affects other cells within the same epithelium. Such secretory activity is referred to as paracrine. The secretory material reaches the target cells by diffusion through the extracellular space or immediately subjacent connective tissue. TABLE Types of Glands 5.5 Exocrine Glands Endocrine Paracrine Merocine Apocrine Holocrine Glands Glands Cells of exocrine glands exhibit different mechanisms of secretion. The cells of exocrine glands have three basic release mechanisms for secretory products (see Table 5.5):  Merocrine secretion. This secretory product is delivered in membrane-bounded vesicles to the apical surface of the cell. Here vesicles fuse with the plasma membrane and extrude their contents by exocytosis. This is the most common mechanism of secretion and is found, for example, in pancreatic acinar cells.


Histology_Ross

Histology_Ross.  Apocrine secretion. The secretory product is released in the apical portion of the cell, surrounded by a thin layer of cytoplasm within an envelope of plasma membrane. This mechanism of secretion is found in the lactating mammary gland, where it is responsible for releasing large lipid droplets into the milk. It also occurs in the apocrine glands of skin, ciliary (Moll’s) glands of the eyelid, and the ceruminous glands of the external auditory meatus.  Holocrine secretion. The secretory product accumulates within the maturing cell, which simultaneously undergoes programmed cell death. Both secretory products and cell debris are discharged into the lumen of the gland. This mechanism is found in sebaceous glands of skin and the tarsal (Meibomian) glands of the eyelid. Exocrine glands are classified as either unicellular or multicellular.


Histology_Ross

Histology_Ross. Exocrine glands are classified as either unicellular or multicellular. Unicellular glands are the simplest in structure. In unicellular exocrine glands, the secretory component consists of single cells distributed among other nonsecretory cells. A typical example is the goblet cell, a mucus-secreting cell positioned among other columnar cells (Fig. 5.38). Goblet cells are located in the surface lining and glands of the intestines and in certain passages of the respiratory tract. Multicellular glands are composed of more than one cell. They exhibit varying degrees of complexity. Their structural organization allows subclassification according to the arrangement of the secretory cells (parenchyma) and the presence or absence of branching of the duct elements. The simplest arrangement of a multicellular gland is a cellular sheet in which each surface cell is a secretory cell. For example, the lining of the stomach and its gastric pits is a sheet of mucus-secreting cells (Fig. 5.39).


Histology_Ross

Histology_Ross. FIGURE 5.38 • Unicellular glands. Photomicrograph of intestinal epithelium showing single goblet cells (arrows) dispersed among absorptive cells. Each goblet cell may be regarded as a unicellular gland—the simplest exocrine type gland. 350. FIGURE 5.39 • Mucus-secreting surface cells of stomach. Photomicrograph of stomach surface. The epithelial cells lining the surface are all mucus-secreting cells, as are the cells lining the gastric pits (P). The cells of the gastric pit form simple tubular glands. 260.


Histology_Ross

Histology_Ross. Other multicellular glands typically form tubular invaginations from the surface. The end pieces of the gland contain the secretory cells; the portion of the gland connecting the secretory cells to the surface serves as a duct. If the duct is unbranched, the gland is called simple; if the duct is branched, it is called compound. If the secretory portion is shaped like a tube, the gland is tubular; if it is shaped like a flask, the gland is alveolar or acinar; if the tube ends in a saclike dilation, the gland is tubuloalveolar. Tubular secretory portions may be straight, branched, or coiled; alveolar portions may be single or branched. Various combinations of duct and secretory portion shapes are found in the body. Classification and description of exocrine glands may be found in Table 5.6. Mucus and serous glands are so named because of the type of secretion produced.


Histology_Ross

Histology_Ross. Mucus and serous glands are so named because of the type of secretion produced. The secretory cells of exocrine glands associated with the various body tubes (e.g., the alimentary canal, respiratory passages, and urogenital system) are often described as being mucus, serous, or both.


Histology_Ross

Histology_Ross. Mucus secretions are viscous and slimy, whereas serous secretions are watery. Goblet cells, secretory cells of the sublingual salivary glands, and surface cells of the stomach are examples of mucus-secreting cells. The mucus nature of the secretion results from extensive glycosylation of the constituent proteins with anionic oligosaccharides. The mucinogen granules, the secretory product within the cell, are therefore PAS positive (see Fig. 5.26a). However, they are water soluble and lost during routine tissue preparation. For this reason, the cytoplasm of mucus cells appears to be empty in H&E–stained paraffin sections. Another characteristic feature of a mucus cell is that its nucleus is usually flattened against the base of the cell by accumulated secretory product (Fig. 5.40).


Histology_Ross

Histology_Ross. In contrast to mucus-secreting cells, serous cells produce poorly glycosylated or nonglycosylated protein secretions. The nucleus is typically round or oval (Fig. 5.41). The apical cytoplasm is often intensely stained with eosin if its secretory granules are well preserved. The perinuclear cytoplasm often appears basophilic because of an extensive rough endoplasmic reticulum, a characteristic of protein-synthesizing cells. Serous cell–containing acini (sing., acinus) are found in the parotid gland and pancreas. Acini of some glands, such as the submandibular gland, contain both mucus and serous cells. In routine tissue preparation, the serous cells are more


Histology_Ross

Histology_Ross. FIGURE 5.40 • Mucus-secreting compound gland. Photomicrograph showing two small lobes of a mucus-secreting gland associated with the larynx. Each displays the beginning of a duct (D) into which mucin is secreted (arrows). The individual secretory cells that form the acinus (A) are difficult to define. Their nuclei (arrowheads) are flattened and located in the very basal portion of the cell, a feature typical of mucus-secreting glands. The cytoplasm is filled with mucin that has been retained during preparation of the tissue and appears stained. 350.


Histology_Ross

Histology_Ross. chapter 5 Epithelial Tissue G LAN D S TABLE Classification of Multicellular Glands5.6 Classification Typical Location Features Simple tubular Large intestine: intestinal Secretory portion of the gland glands of the colon is a straight tube formed by the secretory cells (goblet cells) Simple coiled Skin: eccrine sweat gland Coiled tubular structure is tubular composed of the secretory portion located deep in the dermis Simple branched Stomach: mucus-secreting Branched tubular glands with tubular glands of the pylorus wide secretory portion are formed by the secretory cells and produce a viscous mucous secretion Simple acinar Urethra: paraurethral Simple acinar glands develop and periurethral glands as an outpouching of the transitional epithelium and are formed by a single layer of secretory cells Branched acinar Stomach: mucus-secreting Branched acinar glands with glands of cardia secretory portions are formed by mucus-secreting cells; the short, single-duct portion opens directly


Histology_Ross

Histology_Ross. cells Branched acinar Stomach: mucus-secreting Branched acinar glands with glands of cardia secretory portions are formed by mucus-secreting cells; the short, single-duct portion opens directly into the lumen Compound Duodenum: submucosal Compound tubular glands with tubular glands of Brunner coiled secretory portions are located deep in the submucosa of the duodenum Compound Pancreas: excretory portion Compound acinar glands with acinar alveolar-shaped secretory units are formed by pyramid-shaped serous-secreting cells Compound Submandibular salivary Compound tubuloacinar glands tubuloacinar gland, mammary gland, can have both mucous branched lacrimal gland tubular and serous branched acinar secretory units; they have serous end-caps (demilunes) Simple GlandsCompound Glands hh


Histology_Ross

Histology_Ross. FIGURE 5.41 • Serous-secreting compound gland. Photomicro-graph of pancreatic acinus (A; outlined by the dotted line) with its duct (D). The small round objects within the acinar cells represent the zymogen granules, the stored secretory precursor material. 320. removed from the lumen of the acinus and are shaped as crescents or demilunes (half-moons) at the periphery of the mucus acinus. Most epithelial cells have a finite life span less than that of the whole organism.


Histology_Ross

Histology_Ross. Surface epithelia and epithelia of many simple glands belong to the category of continuously renewing cell populations. The rate of cell turnover (i.e., the replacement rate) is characteristic of a specific epithelium. For example, the cells lining the small intestine are renewed every 4 to 6 days in humans. The replacement cells are produced by mitotic activity of self-maintaining adult stem cells. They are located in sites called niches. In the small intestine, niches of adult stem cells are located in the lower portion of the intestinal glands (crypts; Fig. 5.42). They then migrate and differentiate into four principal cell types. Enterocytes (columnar absorptive cells), goblet cells (mucus-secreting), and enteroendocrine cells (regulatory and hormone-secreting) continue to differentiate and mature while they migrate up along the villi to the surface of the intestinal lumen. The migration of these new cells continues until they reach the tips of the villi, where they undergo


Histology_Ross

Histology_Ross. and mature while they migrate up along the villi to the surface of the intestinal lumen. The migration of these new cells continues until they reach the tips of the villi, where they undergo apoptosis and slough off into the lumen. The fourth cell type, Paneth cells, migrate downward and reside at the bottom of the crypt. The transcription factor Math1 expressed in the intestinal epithelium determines the fate of the cell. The cells committed to the secretory lineage (i.e., they will differentiate into goblet, enteroendocrine, and Paneth cells) have increased expression of Math1. Inhibition of Math1 expression characterizes the default developmental pathway into absorptive intestinal cells (enterocytes).


Histology_Ross

Histology_Ross. Similarly, the stratifed squamous epithelium of skin is replaced in most sites during a period of approximately 28 days. Cells in the basal layer of the epidermis, appropriately named the stratum basale (germinativum), undergo mitosis to provide for cell renewal. As these cells differentiate, they are pushed toward the surface by new cells in the basal layer. Ultimately, the cells become keratinized and slough off. In both of the above examples, a steady state is maintained within the epithelium, with new cells normally replacing ex-foliated cells at the same rate. In other epithelia, particularly in more complex glands, individual cells may live for a long time, and cell division is rare after the mature state is reached. These epithelial cells are characteristic of stable cell populations in which relatively little mitotic activity occurs such as in the liver. However, loss of significant amounts of liver tissue through physical