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Histology: Epithelial Tissues & Glands
Terms in this set (65)
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.
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.
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.
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.
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.
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.
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.
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.
Urothelium is stratified and lines much of the urinary tract. The superficial cells are rounded or dome-shaped, and have specialized membrane features enabling them to withstand the hypertonic effects of urine and protect underlying cells from this toxic solution. Cells of this epithelium are also able to adjust their relationships with one another and undergo a transition in their appearance as the urinary bladder fills and the wall is distended. These unique features of transitional epithelium are discussed more extensively in Chapter 19. X400. H&E.
Cells of pseudostratified epithelia appear to be in several layers, but their basal ends all rest on the basement membrane. The pseudostratified columnar epithelium of the upper respiratory tract shown here contains many ciliated cells, as well as other cells with their nuclei at different levels. X400. H&E.
The simple columnar epithelium lining the large intestine shows many isolated goblet cells secreting mucus into the lumen. (a) With a stain for the oligosaccharide components of mucin glycoproteins, the cytoplasmic secretory granules of two goblet cells and secreted mucus are stained purple. X600. PAS-PT. (b) As shown ultrastructurally, goblet cells always have basal nuclei surrounded by RER (R), a large Golgi complex (G), and abundant apical cytoplasm filled with large secretory granules (SG). After exocytosis mucin components are hydrated and become mucus. A brush border of microvilli (M) is seen on neighboring columnar cells. X17,000.
During fetal development epithelial cells proliferate and penetrate the underlying connective tissue. These cells may—or may not—maintain a connection with the surface epithelium. The connection is maintained to form a duct in exocrine glands; it is lost as endocrine glands develop. Exocrine glands secrete substances to specific organs via duct systems. Endocrine glands produce hormones and are always rich in capillaries. Hormones are released outside the cells and picked up by these blood vessels for distribution throughout the body, where specific target cells are identified by receptors for the hormones. Endocrine glands can have secretory cells arranged as irregular cords (left) or as rounded follicles (right) with lumens for temporary storage of the secretory product.
Exocrine glands by definition have ducts that lead to another organ or the body surface. Inside the gland the duct runs through the connective tissue of septa and branches repeatedly, until its smallest branches end in the secretory portions of the gland.
Three basic types of secretion are used by cells of -exocrine glands, depending on what substance is being secreted.
(a) Merocrine secretion releases products, usually containing -proteins, by means of exocytosis at the apical end of the secretory cells. Most exocrine glands are merocrine.
(b) Holocrine secretion is produced by the disintegration of the secretory cells themselves as they complete -their terminal differentiation, which involves becoming filled with product. Sebaceous glands of hair follicles are the best examples of holocrine glands.
(c) Apocrine secretion involves loss of membrane-enclosed apical cytoplasm, usually containing one or more lipid droplets. Apocrine secretion, along with merocrine secretion, is seen in mammary glands.
In holocrine secretion, best seen in the sebaceous gland adjacent to hair follicles, entire cells fill with a lipid-rich product as they differentiate. Mature (terminally differentiated) cells separate and completely disintegrate, releasing the lipid that serves to protect and lubricate adjacent skin and hair. Sebaceous glands lack myoepithelial cells; cell proliferation inside a dense, inelastic connective tissue capsule continuously forces product into the duct. X200. H&E.
The secretory portions of a mammary gland demonstrate apocrine secretion, characterized by extrusion of the secretion product along with a bit of apical cytoplasm (arrows). The released portion of cell contains lipid droplet(s). Merocrine secretion also occurs from the same and other cells of the gland. X400. PSH.
The small serous acini of the exocrine pancreas each have 5-10 cells facing a very small central lumen. Each acinar cell is roughly pyramidal, with its apex at the lumen. (a) As seen by light microscopy, the apical ends are very eosinophilic due to the abundant secretory granules present there. The cells' basal ends contain the nuclei and an abundance of RER, making this area basophilic. A small duct (D) is seen, but lumens of acini are too small to be readily visible. The enclosed area is comparable to that shown in part b. X300. H&E. (b) A portion of one acinar cell is shown ultrastructurally, indicating the abundant RER (R), a Golgi complex (G), apical secretory granules (SG) and the small acinar lumen (L). X13,000.
Mucous cells of salivary glands are typically larger than serous cells, with flattened basal nuclei. Most of the cytoplasm is filled with secretory granules containing mucinogen like that of goblet cells. The RER and Golgi complexes of mucous cells produce heavily glycosylated glycoproteins with water-binding properties. The lumens (arrows) of mucous tubules are larger than those of serous acini. Much connective tissue surrounds the mucous tubules and ducts (D). X200. PT.
(a) The TEM shows two salivary gland cells containing secretory granules, with an associated myoepithelial cell (M). X20,000. (b) A myoepithelial cell immunostained brown with antibodies against actin shows its association with cells of an acinus stained by H&E. Contraction of the myoepithelial cell compresses the acinus and aids in the expulsion of secretory products into the duct. X200.
A Na+/K+-ATPase located in the plasma membrane of many cells uses ATP to transport or pump the cations Na+ and K+ through the membrane in opposite directions, against their concentrations gradients.
Ion and water transport across epithelia can occur in either direction, depending on which tissue is involved. (a) The direction of transport is from the lumen to the blood vessel, as in the gallbladder and intestine. This process is called absorption, and serves to concentrate bile and obtain water and ions in these organs.
(b) Transport of water in the other direction from the capillaries into a lumen, as in the choroid plexus and sweat glands, is often part of secretion and serves to expel water from the interstitial fluid into specialized aqueous fluids in these tissues. No matter whether an epithelium is absorbing or secreting water, apical occluding junctions are necessary to maintain tight compartmentalization.
A diagram and TEM of epithelial cells highly specialized for absorption: cells of proximal convoluted tubule of the kidney. Typically, long invaginations of the basal cell membrane outline regions with mitochondria (M). Interdigitations from neighboring cells are also present laterally. Immediately below the microvilli (MV) are many pinocytotic vesicles, which may fuse with lysosomes as shown or mediate transcytosis by secreting their contents at the basolateral membrane. Junctional complexes between individual cells separate the apical and basolateral compartments. Sodium ions diffuse passively through the apical membranes of renal epithelial cells and are actively transported out of the cells by Na+/K+-ATPase located in the basolateral membrane. Immediately below the basal lamina is a capillary (C) that removes water absorbed across the epithelium. X9600.
Mesothelium: generic term of the layers covering the internal structures of the body an example of the mesothelium is the peritoneum. Mesothelium is the stuff in the middle. Is a layer of simple squamous epithelium.
Simple cuboidal epithelium of kidney tubules
Simple columnar epithelium of gall bladder
Simple columnar epithelium of small intestine with villi
Simple columnar epithelium of the uterine (Fallopian) tube
Stratified squamous epithelium of the vagina
Epidermis of skin
Pseudostratified respiratory epithelium
Transitional epithelium in the urinary tract
Basement membrane vs. basal lamina
basal lamina contains glycosaminoglycans, collagen, laminin, fibronectin; it is produced by an epithelial cell
Schematic of Glands
Classification of glands according to branching patterns and structural complexity
Simple tubular glands of the large intestine left = longitudinal section
Simple tubular glands of the large intestine right = cross section
parenchyma comprises the functional (secratory in the case of glands)parts of an organ
stoma is the structural portion of the organ
Simple coiled tubular gland: sweat gland
Exocrine gland organization
basket-shaped, smooth muscle-like cells
surround exocrine gland acini and contract in order to express acinar contents (e.g. mammary gland, sweat gland)
confocal image of myoepithelial cells, using antibody (immunohistochemical stain)
Types of secretions of glands
Serous - proteinaceous secretions - watery consistency (e.g. parotid gland) will look a little pinker filled cell in H&E
Mucous - glycoproteinaceous secretions: complex -looks pale in Hand E polysaccharides named mucus - viscous, gel-like consistency (e.g. sublingual gland)
Mixed serous + mucous - (e.g. submandibilar gland)
**all three are found in the parotid gland
Parotid gland (serous)
Sublingual gland (mucous)
several openings under the tounge
empty, PAS positive
H&E you would'nt seem much
enzymes for digestion are also secreted here
Submandibular gland (mixed)
Clear area is mucous secreting
Dark area is serous secreting (demi-moon)
Large open regular duct with cuboidal cells is a duct
Mechanisms of Secretion
Merocrine - most common; exocytosis of material in secretory vesicles or granules; found in most serous glands (e.g. exocrine pancreas, salivary glands, goblet cells of the gut wall)
Apocrine - whole apical portion of the secretory cell is shed along with the secretion (e.g. mammary gland, prostate)
Holocrine - entire cell is sloughed off along with its secretory contents (e.g. sebaceous glands)
Paracrine - secretion reaches adjacent epithelial cells via extracellular spaces (e.g. somatostatin-secreting cells of islets of Langerhans in pancreas) LOCAL EFFECT ONLY
Combined merocrine & apocrine gland (goblet cells of the large intestine)
merocrine secretion, if stressed apocrine
Combined merocrine & apocrine gland (goblet cells of the large intestine)
Combined merocrine & apocrine gland (goblet cells of the large intestine)
Mammary gland(apocrine secretion)
may look different dependent of age or stage of life
this one is mostly active cells and little stroma so its a pregnant lady
if you see lots of milk it is a lactating woman stroma>parenchyma pre pubecscent
Prostate gland(apocrine secretion)
not a typical prostate gland, prostate is much more dense
Sebaceous gland (holocrine secretion)
mason's trichrome stain (stains collagen blue)
simple branched acinar gland
typical is active cells at the and stratification towards the duct where they die
after holocrine secretion remember cells die
Structural types of endocrine glands
Adrenal gland - cortex(steroid secreting cells)
can see if zoomed in the capillary networks, they are the really small red dots
Endocrine pancreas (islets of Langerhans)
exocrine secratory acini ducts are viewable
islte of langerhans is much more dense (nuclei)
Endocrine pancreas (islets of Langerhans)
exocrine secratory ducts are viewable
brown dots are the antibody stain binding to the langerhans, which secretes insulin into the blood. counterstained
Recommended textbook explanations
Lisa A. Urry, Michael L. Cain, Peter V Minorsky, Steven A. Wasserman
Fundamentals of Biochemistry: Life at the Molecular Level
Charlotte W. Pratt, Donald Voet, Judith G. Voet
Biocalculus: Calculus for the Life Sciences
Campbell Biology (AP Edition)
Cain, Campbell, Minorsky, Reece, Urry, Wasserman
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