Epithelia Flashcards
Epithelium _____ rests on a basement membrane
Epithelium ALWAYS rests on a basement membrane
Epithelial cell vascularization
Epithelium is avascular, and must rely on capillaries and extracellular fluid in the underlying connective tissue for nutrient and gas exchange.
Basic epithelial structure
Classifying epithelia
Epithelia are classified by two major characteristics: (a) the shape of the cells that make up the epithelium and (b) their organization with respect to each other and the basement membrane. To fully classify an epithelium, you need at least two descriptive terms, one describing each of these features (e.g., “stratified squamous epithelium”).
Squamous epithelia generally _______, while cuboidal and columnar epithelia _______.
Squamous epithelia generally perform a barrier function, while cuboidal and columnar epithelia function in absorption and/or secretion.
If there are multiple layers of cells between the basement membrane and the outside world, it is referred to as _______.
If there are multiple layers of cells between the basement membrane and the outside world, it is referred to as stratified.
pseudostratified epithelium
A pseudostratified epithelium is one that appears to have multiple layers of cells based on the location of the nuclei. In fact, each cell is anchored to the basement membrane and there is only one layer of cells, so it’s not really stratified – pseudostratified epithelia are actually simple epithelia
Transitional epithelium
Transitional epithelium has cuboidal cells attached to the basement membrane, and squamous cells at the surface. This type of epithelium is unique to the urinary tract.
Keratinizing vs non-keratinizing stratified squamous epithelia
Keratinizing epithelia of the skin will produce keratin at the surface, where as the non-keratinizing epithelia of the uterine cervix will not.
The majority of epithelial specializations of interest in histology are found at the ____ surface of ____ epithelia.
The majority of epithelial specializations of interest in histology are found at the apical surface of columnar epithelia.
One example of this is pseudostratification. Microvilli, found in the small intestine and the kidney tubules, are another.
Cilia
Cilia are visible under light microscopy, as in the image above, although it requires high magnification to see them. They are roughly equal in height to the nucleus, and are made up of microtubules (as opposed to microvilli, which are composed of actin filaments).
Cilia are motile; the presence of cilia in an epithelium indicates that its function is to move something in the lumen from point A to point B.
Ciliated epithelia can be found in the respiratory tract and Fallopian tube.
Stereocilia are apical structures that resemble cilia and are visible under the light microscope, but are nonmotile and function in absorption, more like microvilli. They are found in the male reproductive tract.
The three major cytoskeletal components
actin filaments, intermediate filaments, and microtubules
Actin
Actin is thin and flexible. They typically form bundles or gels that are much stronger, and can exert substantial forces within the cell, or between cells.
Microtubules
Microtubules consist of repeating units of two proteins, alpha and beta tubulin, arranged end to end in a cylindrical array. Microtubules are thicker and stiffer than actin, and usually radiate from a single central point outwards.
Microtubules have important transport functions in moving organelles through the cell. Microtubules also rearrange to segregate chromosomes in mitosis.
Intermediate filaments
Intermediate filaments are intermediate in size between actin and microtubules. They are flexible, rope-like polymers that give cells mechanical strength. In epithelia, in general, there are ropes of intermediate filaments throughout the cytoplasm and contacts between the cells where these filaments are “sutured” together (more on these in a moment). In this way, these filaments provide the entire tissue with mechanical strength.
Include keratins and lamins, among other proteins.
Cadherins
Calcium-dependent adhesion proteins that are found in cell-cell interactions between the same type of cells. The major component of adherens junctions.
Cadherin proteins can interact with proteins of the actin cytoskeleton but also proteins that regulate signaling. In turn, signaling pathways can also regulate the strength of adhesion (so called “inside-out” signaling).
Other major type of “anchoring junction” between adjacent epithelial cells. Adherens junctions connect to actin filaments in the cytoskeleton of the adjacent cells, while desmosomes connect to intermediate filaments.
Contain specialized forms of cadherins (desmoglein and desmocollin) that interact with other adapter proteins that can recruit intermediate filaments rather than actin filaments
Critical for tissue integrity under stress.
Hemidesmosomes
Hemidesmosomes are protein complexes that mediate the stable attachment of epithelial cells to the underlying basement membrane in tissues such as the skin shown here. They provide a link between the extracellular matrix and the intermediate filament cytoskeleton.
The major adhesion proteins in hemidesmosomes are integrins, which attach to extracellular matrix proteins (laminins) in the basement membrane.
Tight junctions
Form a permeability barrier between cells
Tight junction composition
The principal proteins of the tight junction are claudin and occludin. These proteins interact together to form very tight seals, with selective permeability properties for some ions and small molecules.
The C-termini of the proteins interact with adapter proteins in the cell that link to the actin cytoskeleton.
Due to expression of different tight junction proteins, the permeability of tight junctions to ions, small molecules and water varies greatly among different epithelial tissues.
Basement membrane
The basement membrane actually consists of two layers, the basal lamina (what the epithelial cells actually attach to) and the reticular lamina underneath it. The resolution of the light microscope is insufficient to resolve the two layers, so they appear as a single structure.
The basal lamina is composed of proteins and proteoglycans. The connective tissue below consists of cells such as fibroblasts and the proteins secreted by the fibroblasts such as collagen.
Components of basement membrane
The major macromolecular components of the basal lamina are laminin and type IV collagen.
Laminins are large glycoprotein molecules that self-assemble to form a lacelike sheet immediately below the basal poles of the epithelial cells, where they are held in place by integrins.
Monomers of type IV collagen contain three polypeptide chains and selfassemble further to form a felt-like sheet associated with the laminin layer.
M = mesothelium
C = connective tissue
SM = smooth muscle
The mesothelium here may be seen to be a simple squamous epithelium.
Endothelium
Note the straddeling of this blood vessel by basement membrane and the simple cuboidal morphology of the epithelial cells.
The Fallopian tube.
You can tell since there are only two ciliated epithelia in humans – the airway and the Fallopian tube – and of those only the Fallopian tube displays a simple columnar morphology.
The Respiratory tract
You can tell since there are only two ciliated epithelia in humans – the airway and the Fallopian tube – and of those only the resipratory tract displays a pseudostratified columnar morphology.
Cervix
Non-keratinizing stratified squamous epithelia
Non-keratinizing stratified squamous epithelium is found on moist surfaces that nonetheless are exposed to the outside world, such as the oral cavity, esophagus, cervix, and vagina.
Thin skin
Keratinizing stratified squamous epithelia
Note the stratum corneum, the top layer of dead cell skeletons containing no nuclei.
Salivary gland
Note the stratified cuboidal epithelium lining the gland.
Urothelium, with umbrella cells at the top.
Transitional epithelium is unique to the urinary tract. It “transitions” from a stratified cuboidal appearance at the base to stratified squamous at the surface.
Transitional epithelium is a classic example of form following function. The epithelium of the urinary tract must both protect the underlying tissue from the acidic, hyperosmolar urine in the lumen, and be able to adapt to changes in volume. To accomplish the former, the epithelial cells have a thicker plasma membrane than normal, and numerous tight junctions to prevent infiltration of fluid. The latter is accomplished by the umbrella cells (also known as dome cells), the cells nearest the surface. By internalizing and recycling their membrane, these cells can alter their shape to be round if the bladder is empty, and flat when it is distended.
Exocrine gland development vs Endocrine gland development
All glands are derived from epithelia.
Exocrine glands secrete their products via the apical surface into ducts which are continuous with the oustide world.
In contrast, endocrine glands are ductless, and secrete directly into the blood via the basolateral surface.
Arrangement of endocrine gland cells
Endocrine glands lack ducts and secrete directly into the bloodstream. To accomplish this, the cells are arranged around blood vessels (capillaries) and package their secreted products into cytoplasmic secretory granules, which are released into the capillary lumen.
Unlike exocrine glands, the products of endocrine glands (hormones) generally act at a distance from their source.
