Histology: Epithelial Tissues & Glands

Cuboidal or pyramidal cells of epithelia generally have spherical nuclei, while nuclei of squamous epithelial cells are flattened. An extracellular basement membrane (red) always lies at the interface of epithelial cells and connective tissue. Nutrients for epithelial cells must diffuse across the basement membrane. Nerve fibers normally penetrate this structure, but small blood capillaries (being epithelial themselves) normally never enter epithelia.

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This section of kidney shows the well-stained basement membranes (arrows) of epithelia forming structures within the large, round renal glomerulus and its surrounding tubules. In kidney glomeruli the basement membrane, besides having a supporting function, has a highly developed role as a filter that is key to renal function. X100. Picrosirius-hematoxylin (PSH).

Image: This section of kidney shows the well-stained basement membranes (arrows) of epithelia forming structures within the large, round renal glomerulus and its surrounding tubules. In kidney glomeruli the basement membrane, besides having a supporting function, has a highly developed role as a filter that is key to renal function. X100. Picrosirius-hematoxylin (PSH).

The ultrastructural components of the basement membrane are revealed by TEM. The dense basal lamina (BL) may appear with thin clear zones on each side and is anchored to a thicker, more diffuse reticular lamina (RL) containing collagen III reticular fibers. Hemidesmosomes (H) bind the basal surface of the epithelial cell (C) to the basal lamina. X54,000.

Image: The ultrastructural components of the basement membrane are revealed by TEM. The dense basal lamina (BL) may appear with thin clear zones on each side and is anchored to a thicker, more diffuse reticular lamina (RL) containing collagen III reticular fibers. Hemidesmosomes (H) bind the basal surface of the epithelial cell (C) to the basal lamina. X54,000.

Most cuboidal or columnar epithelial cells have four major types of intercellular junctional complexes, as shown schematically here. At the apical end, tight junctions (zonulae occludens) and adherent junctions (zonulae adherens) are typically close together and each forms a continuous ribbon around the cell. Multiple ridges of the tight junction prevent passive flow of material between the cells but are not very strong; the adhering junctions immediately below them serve to stabilize and strengthen the circular occluding bands and help hold the cells together.
Both desmosomes and gap junctions are spot-like, not circular, structures between two cells. Bound to intermediate filaments inside the cells, desmosomes form very strong attachment points that supplement the zonulae adherens and play a major role to maintain the integrity of an epithelium. Gap junctions, each a patch of many connexons in the adjacent cell membranes, have little strength but serve as intercellular channels for flow of molecules. All of these junctional types are also found in certain other cell types besides epithelia. Hemidesmosomes bind epithelial cells to the underlying basal lamina.

Image: Most cuboidal or columnar epithelial cells have four major types of intercellular junctional complexes, as shown schematically here. At the apical end, tight junctions (zonulae occludens) and adherent junctions (zonulae adherens) are typically close together and each forms a continuous ribbon around the cell. Multiple ridges of the tight junction prevent passive flow of material between the cells but are not very strong; the adhering junctions immediately below them serve to stabilize and strengthen the circular occluding bands and help hold the cells together.
Both desmosomes and gap junctions are spot-like, not circular, structures between two cells. Bound to intermediate filaments inside the cells, desmosomes form very strong attachment points that supplement the zonulae adherens and play a major role to maintain the integrity of an epithelium. Gap junctions, each a patch of many connexons in the adjacent cell membranes, have little strength but serve as intercellular channels for flow of molecules. All of these junctional types are also found in certain other cell types besides epithelia. Hemidesmosomes bind epithelial cells to the underlying basal lamina.

Ultrastructural view of the apical region near microvilli (MV) of two epithelial cells, revealing a junctional complex with a tight junction (TJ) or zonula occludens, an adherent junction (AJ) or zonula adherens, and a desmosome (D) associated with intermediate filaments (IF). The functions and major protein components of these junction types are summarized in Table 4-2. X195,000.

Image: Ultrastructural view of the apical region near microvilli (MV) of two epithelial cells, revealing a junctional complex with a tight junction (TJ) or zonula occludens, an adherent junction (AJ) or zonula adherens, and a desmosome (D) associated with intermediate filaments (IF). The functions and major protein components of these junction types are summarized in Table 4-2. X195,000.

Just below the apical microvilli (MV) of this epithelial cell, a cryofracture plane splitting fused cell membranes reveals the fused strands of transmembrane proteins forming the tight junction (zonula occludens). X100,000.

$Image: Just below the apical microvilli (MV) of this epithelial cell, a cryofracture plane splitting fused cell membranes reveals the fused strands of transmembrane proteins forming the tight junction (zonula occludens). X100,000.$

(a) A diagram of a gap junction shows the structural elements that allow the exchange of nutrients and signal molecules between cells without loss of material into the intercellular space. The communicating channels are formed by pairs of abutting particles (connexons), which are in turn each composed of six protein subunits (connexins) that span the lipid bilayer of each cell membrane. The channel formed by paired connexons (arrow) is about 1.5 nm in diameter, limiting the size of transmitted molecules. (b) A cryofracture preparation of a gap junction, showing the patch of aggregated transmembrane protein complexes, the connexons. X150,000.

$Image: (a) A diagram of a gap junction shows the structural elements that allow the exchange of nutrients and signal molecules between cells without loss of material into the intercellular space. The communicating channels are formed by pairs of abutting particles (connexons), which are in turn each composed of six protein subunits (connexins) that span the lipid bilayer of each cell membrane. The channel formed by paired connexons (arrow) is about 1.5 nm in diameter, limiting the size of transmitted molecules. (b) A cryofracture preparation of a gap junction, showing the patch of aggregated transmembrane protein complexes, the connexons. X150,000.$

Absorptive cells lining the small intestine demonstrate the highly uniform microvilli of a striated or brush border particularly well. (a) A high-magnification light microscope shows many parallel microvilli and their connections to the terminal web (TW) in the underlying cytoplasm. X6500. (b) SEM of a sectioned epithelial cell shows both the internal and surface structure of individual microvilli and the association with actin filaments and intermediate filaments of the terminal web (TW). X7000. (c) TEM of microvilli sectioned longitudinally and transversely (inset) reveals the microfilament arrays that form the core of these projections. The terminal web (TW) of the cytoskeleton is also seen. The glycocalyx (G) extending from glycoproteins and glycolipids of the microvilli plasmalemma contains certain enzymes for late stages of macromolecule digestion. X15,000.
(d) The diagram shows a few microfilaments in a microvillus, with various actin-binding proteins important for F-actin assembly, capping, cross-linking, and movement. Like microfilaments in other regions of the cytoskeleton, those of microvilli are highly dynamic, with treadmilling and various myosin-based interactions. Myosin motors import various -microvilli components along the actin filaments.

Image: Absorptive cells lining the small intestine demonstrate the highly uniform microvilli of a striated or brush border particularly well. (a) A high-magnification light microscope shows many parallel microvilli and their connections to the terminal web (TW) in the underlying cytoplasm. X6500. (b) SEM of a sectioned epithelial cell shows both the internal and surface structure of individual microvilli and the association with actin filaments and intermediate filaments of the terminal web (TW). X7000. (c) TEM of microvilli sectioned longitudinally and transversely (inset) reveals the microfilament arrays that form the core of these projections. The terminal web (TW) of the cytoskeleton is also seen. The glycocalyx (G) extending from glycoproteins and glycolipids of the microvilli plasmalemma contains certain enzymes for late stages of macromolecule digestion. X15,000.
(d) The diagram shows a few microfilaments in a microvillus, with various actin-binding proteins important for F-actin assembly, capping, cross-linking, and movement. Like microfilaments in other regions of the cytoskeleton, those of microvilli are highly dynamic, with treadmilling and various myosin-based interactions. Myosin motors import various -microvilli components along the actin filaments.

At the apical ends of the tall epithelial cells lining organs such as the epididymis (shown here) are numerous very long stereocilia, which increase the surface area available for absorption. Stereocilia are much longer than microvilli and often have distal branching. X400. H&E.

Image: At the apical ends of the tall epithelial cells lining organs such as the epididymis (shown here) are numerous very long stereocilia, which increase the surface area available for absorption. Stereocilia are much longer than microvilli and often have distal branching. X400. H&E.

Epithelial cells lining the respiratory tract have many very well-developed cilia. (a) By light microscopy cilia (C) on the columnar cells appear as a wave of long projections, interrupted by nonciliated, mucus-secreting goblet cells (G). X400. Toluidine blue. (b) SEM of the apical surfaces of this epithelium shows the density of the cilia (C) and the scattered goblet cells (G). X300.
(c) TEM of cilia (C) sectioned longitudinally reveals the central and peripheral microtubules of the axonemes, with cross sections (inset) clearly showing the 9 + 2 array of the microtubule doublets. At the base of each cilium is a basal body (B) anchoring the axoneme to the apical cytoplasm. Much shorter microvilli (MV) can be seen between the cilia. X59,000. Inset: X80,000.

Image: Epithelial cells lining the respiratory tract have many very well-developed cilia. (a) By light microscopy cilia (C) on the columnar cells appear as a wave of long projections, interrupted by nonciliated, mucus-secreting goblet cells (G). X400. Toluidine blue. (b) SEM of the apical surfaces of this epithelium shows the density of the cilia (C) and the scattered goblet cells (G). X300.
(c) TEM of cilia (C) sectioned longitudinally reveals the central and peripheral microtubules of the axonemes, with cross sections (inset) clearly showing the 9 + 2 array of the microtubule doublets. At the base of each cilium is a basal body (B) anchoring the axoneme to the apical cytoplasm. Much shorter microvilli (MV) can be seen between the cilia. X59,000. Inset: X80,000.

(a) A diagram of a cilium with the axoneme consisting of two central microtubules surrounded by nine peripheral microtubular doublets associated with other proteins. In the doublets, microtubule A is complete, consisting of 13 protofilaments, whereas microtubule B shares some of A's protofilament heterodimers. The axoneme is elastic but relatively stiff, with its structure maintained by nexins linking the peripheral doublets and other protein complexes forming a sheath and radial spokes between the doublets and the central microtubules.
The axoneme is continuous with a basal body located in the apical cytoplasm. Basal bodies are structurally very similar to centrioles, consisting of nine relatively short microtubular triplets linked together in a pinwheel-like arrangement. A dynamic pool of tubulin and other proteins exists distally in cilia, and proteins are transported into and out of the structure by kinesin and cytoplasmic dynein motors moving along the peripheral doublets of microtubules.
(b) Ciliary movement involves a rapid series of changes in the shape of the axoneme. Along the length of each doublet, a series of paired "arms" with axonemal dynein is bound to microtubule A, with each pair extended toward microtubule B of the next doublet. When activated by ATP, the dynein arms briefly bind the neighboring microtubule and the doublets slide past each other slightly. The sliding motion is restricted by nexin cross-links between the doublets, causing the axoneme to bend. A rapid succession of this movement along the axoneme produces ciliary motion.

Image: (a) A diagram of a cilium with the axoneme consisting of two central microtubules surrounded by nine peripheral microtubular doublets associated with other proteins. In the doublets, microtubule A is complete, consisting of 13 protofilaments, whereas microtubule B shares some of A's protofilament heterodimers. The axoneme is elastic but relatively stiff, with its structure maintained by nexins linking the peripheral doublets and other protein complexes forming a sheath and radial spokes between the doublets and the central microtubules.
The axoneme is continuous with a basal body located in the apical cytoplasm. Basal bodies are structurally very similar to centrioles, consisting of nine relatively short microtubular triplets linked together in a pinwheel-like arrangement. A dynamic pool of tubulin and other proteins exists distally in cilia, and proteins are transported into and out of the structure by kinesin and cytoplasmic dynein motors moving along the peripheral doublets of microtubules.
(b) Ciliary movement involves a rapid series of changes in the shape of the axoneme. Along the length of each doublet, a series of paired "arms" with axonemal dynein is bound to microtubule A, with each pair extended toward microtubule B of the next doublet. When activated by ATP, the dynein arms briefly bind the neighboring microtubule and the doublets slide past each other slightly. The sliding motion is restricted by nexin cross-links between the doublets, causing the axoneme to bend. A rapid succession of this movement along the axoneme produces ciliary motion.

This is a single layer of thin cells, in which the cell nuclei (arrows) are the thickest and most visible structures. Simple epithelia are typically specialized as lining of vessels and -cavities, where they regulate passage of substances into the underlying tissue. The thin cells often exhibit transcytosis. Examples shown here are those lining the thin renal loops of Henle (a), covering the outer wall of the intestine (b), and lining the inner surface of the cornea (c). a, c X400; b X600. H&E.

Image: This is a single layer of thin cells, in which the cell nuclei (arrows) are the thickest and most visible structures. Simple epithelia are typically specialized as lining of vessels and -cavities, where they regulate passage of substances into the underlying tissue. The thin cells often exhibit transcytosis. Examples shown here are those lining the thin renal loops of Henle (a), covering the outer wall of the intestine (b), and lining the inner surface of the cornea (c). a, c X400; b X600. H&E.

Cells here are roughly as tall as they are wide. Their greater thickness allows cytoplasm to be rich in mitochondria and other organelles for a high level of active transport across the epithelium and other functions. Examples shown here are from a renal collecting tubule (a), a large thyroid follicle (b), and the thick mesothelium covering an ovary (c). All X400. H&E.

Image: Cells here are roughly as tall as they are wide. Their greater thickness allows cytoplasm to be rich in mitochondria and other organelles for a high level of active transport across the epithelium and other functions. Examples shown here are from a renal collecting tubule (a), a large thyroid follicle (b), and the thick mesothelium covering an ovary (c). All X400. H&E.

Cells here are always taller than they are wide, with apical cilia or microvilli, and are often specialized for absorption. Complexes of tight and adherent junctions, sometimes called "terminal bars" in light microscopic images, are present at the apical ends of cells. The examples shown here are from a renal collecting duct (a), the oviduct lining, with both secretory and ciliated cells (b), and the lining of the gall bladder (c). All X400. H&E.

Image: Cells here are always taller than they are wide, with apical cilia or microvilli, and are often specialized for absorption. Complexes of tight and adherent junctions, sometimes called "terminal bars" in light microscopic images, are present at the apical ends of cells. The examples shown here are from a renal collecting duct (a), the oviduct lining, with both secretory and ciliated cells (b), and the lining of the gall bladder (c). All X400. H&E.

Stratified squamous epithelia usually have protective functions: protection against easy invasion of underlying tissue by microorganisms and protection against water loss. These functions are particularly important in the epidermis (a) in which differentiating cells become keratinized, ie, filled with keratin and other substances, eventually lose their nuclei and organelles, and form superficial layers flattened squames that impede water loss. Keratinized cells are sloughed off and replaced by new cells from more basal layers, which are discussed fully with the skin in Chapter 18.
Nonkeratinized epithelia occur in many organs, such as the esophageal lining (b) or outer covering of the cornea (c). Here cells accumulate much less keratin and retain their nuclei but still provide protection against microorganisms.
Stratified cuboidal or columnar epithelia are fairly rare but occur in excretory ducts of certain glands, such as sweat glands (d) where the double layer of cells allows additional functions. All X400; (b) PT, (a, c, and d) H&E.

Image: Stratified squamous epithelia usually have protective functions: protection against easy invasion of underlying tissue by microorganisms and protection against water loss. These functions are particularly important in the epidermis (a) in which differentiating cells become keratinized, ie, filled with keratin and other substances, eventually lose their nuclei and organelles, and form superficial layers flattened squames that impede water loss. Keratinized cells are sloughed off and replaced by new cells from more basal layers, which are discussed fully with the skin in Chapter 18.
Nonkeratinized epithelia occur in many organs, such as the esophageal lining (b) or outer covering of the cornea (c). Here cells accumulate much less keratin and retain their nuclei but still provide protection against microorganisms.
Stratified cuboidal or columnar epithelia are fairly rare but occur in excretory ducts of certain glands, such as sweat glands (d) where the double layer of cells allows additional functions. All X400; (b) PT, (a, c, and d) H&E.

Histology: Epithelial Tissues & Glands

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