Histology Wk 10 Flashcards
Four basic tissue types
Epithelial:
-Aggregated polyhedral cells
-Extracellular Matrix- Small amount
-Main Functions- Lining of surface or body cavities; glandular secretion
Connective
Several types of fixed and wandering cells
Abundant amount
Support and protection of tissues/organs
Muscle
Elongated contractile cells
Moderate amount
Strong contraction; body movements
Nervous
Elongated cells with extremely fine processes
Very small amount
Transmission of nerve impulses
Parenchyma and stroma
Most organs can be divided into the parenchyma, which is composed of the cells responsible for the organ’s specialized functions, and the stroma, the cells of which have a supporting role in the organ.
Except in the brain and spinal cord, the stroma is always connective tissue.
CHARACTERISTIC FEATURES OF EPITHELIAL CELLS
The shapes and dimensions of epithelial cells are quite vari- able, ranging from tall columnar to cuboidal to low squa- mous cells.
Epithelial cell nuclei vary in shape and may be elliptic (oval), spherical, or flattened, with nuclear shape corresponding roughly to cell shape. Columnar cells generally have elongated nuclei, squamous cells have flattened nuclei, and cuboidal or pyramidal cells have more spherical nuclei.
Epithelial lining
The connective tissue that underlies the epithelia lining the organs of the digestive, respiratory, and urinary systems is called the lamina propria. The area of con- tact between the two tissues may be increased by small evagi- nations called papillae projecting from the connective tissue into the epithelium. Papillae occur most frequently in epithelial tissues subject to friction, such as the covering of the skin or tongue.
Polarity of epithelial cells
Epithelial cells generally show polarity, with organelles and membrane proteins distributed unevenly within the cell.
The region of the cell contacting the ECM and connective tissue is called the basal pole and the opposite end, usually facing a space, is the apical pole, with the two poles differing significantly in both structure and function. Regions of cuboidal or colum- nar cells that adjoin neighboring cells comprise the cells’ lateral surfaces; cell membranes here often have numerous folds which increase the area and functional capacity of that surface.
Basement membrane
The basal surface of all epithelia rests on a thin extracellular, felt-like sheet of macromolecules referred to as the basement membrane (Figure 4–1), a semipermeable filter for sub- stances reaching epithelial cells from below. Glycoproteins and other components in this structure can often be stained and visualized with the light microscope (Figure 4–2).
With the transmission electron microscope (TEM) two parts of the basement membrane may be resolved. Nearest the epithelial cells is the basal lamina, a thin, electron-dense, sheet-like layer of fine fibrils, and beneath this layer is a more diffuse and fibrous reticular lamina
Those of the basal lamina characteristically include the following:
TypeIVcollagen:Monomers of typeIV collagen self- assemble into a two-dimensional network of evenly spaced subunits resembling the mesh of a window screen.
■ Laminin: These are large glycoproteins that attach to transmembrane integrin proteins in the basal cell mem- brane and project through the mesh formed by the type IV collagen.
■ Nidogen and perlecan:Respectivelyashort,rod-like protein and a proteoglycan, both of these cross-link lami- nins to the type IV collagen network, helping to provide the basal lamina’s three-dimensional structure, to bind the epithelium to that structure, and to determine its porosity and the size of molecules able to filter through it.
Basal lamina
The more diffuse meshwork of the reticular lamina con- tains type III collagen and is bound to the basal lamina by anchoring fibrils of type VII collagen, both of which are pro- duced by cells of the connective tissue
Basal laminae often called external laminae but with sim- ilar composition also exist as thin sleeves surrounding muscle cells, nerves (Figure 4–3b), and fat-storing cells, where they serve as semipermeable barriers regulating macromolecular exchange between the enclosed cells and connective tissue.
LOOK AT TABLE 4-2
SPECIALIZATIONS OF THE APICAL CELL SURFACE
Microvilli
In epithelia specialized for absorption the apical cell surfaces are often filled with an array of projecting microvilli (L. villus, tuft), usually of uniform length. In cells such as those lining the small intestine, densely packed microvilli are visible as a brush or striated border projecting into the lumen (Figure 4–8). The average microvillus is about 1 μm long and 0.1 μm wide.
The thick glycoca- lyx covering microvilli of the intestinal brush border includes membrane-bound proteins and enzymes for digestion of cer- tain macromolecules.
Microvilli
Each microvillus contains bundled actin filaments capped and bound to the surrounding plasma membrane by actin-binding proteins (Figure 4–8d). Although microvilli are relatively stable, the microfilament arrays are dynamic and undergo various myosin-based movements, which help main- tain optimal conditions for absorption via numerous channels, receptors, and other proteins in the plasmalemma. The actin filaments insert into the terminal web of cortical microfila- ments at the base of the microvilli.
Stereocilia
absorptive epithelial cells lining the male reproductive system
stereocilia increase the cells’ surface area, facilitating absorption. More specialized stereocilia with a motion-detecting function are important components of inner ear sensory cells.
Stereocilia resemble microvilli in containing arrays of microfilaments and actin-binding proteins, with similar diam- eters, and with similar connections to the cell’s terminal web. However, stereocilia are typically much longer and less motile than microvilli, and may show branching distally
Cilia
Cilia are long, highly motile apical structures, larger than microvilli, and containing internal arrays of microtubules not microfilaments
Motile cilia are abundant on cuboidal or columnar cells of many epithelia. Typical cilia are 5-10 μm long and 0.2 μm in diameter, which is much longer and two times wider than a typical microvillus.
Each cilium has a core structure consisting of nine peripheral micro- tubule doublets arrayed around two central microtubules. This 9 + 2 assembly of microtubules is called an axoneme
Microtubules of axonemes
are continuous with those in basal bodies, which are apical cytoplasmic structures just below the cell membrane. Basal bodies have a structure similar to that of centrioles, with triplets of microtubules and dynamic tubulin protofila- ments forming rootlets anchoring the entire structure to the cytoskeleton.
Motion of cilia
Ciliary motion occurs through successive changes in the conformation of the axoneme, in which various acces- sory proteins make each cilium relatively stiff, but elastic.
Complexes with axonemal dynein bound to one microtu- bule in each doublet extend as “arms” toward a microtubule of the next doublet.
With energy from ATP dynein-powered sliding of adjacent doublets relative to each other bends the axoneme and a rapid series of these sliding movements pro- duces the beating motion of the cilium.
