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chapter 4- Epithelium

Terms in this set (70)

each cilium has a core structure consisting of nine peripheral microtubule doublets arrayed around two central microtubules. (9 + 2 assembly)

(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.
-Epithelia with two or more layers of cells
-almost all are stratified squamous, (outer cell layers are thin and flattened)
-2 types: keratinized (ex skin) and non keratinized
-large cells on basal layers, cells get smaller as you go up (in skin last layers are dead)

-Cells (in stratified squamous epithelia) move gradually from the basal to the surface layers, changing shape and becoming filled with keratin intermediate filaments.
-Protect underlying tissues from excess water loss (dehydration) and microbial invasion.

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, that is, 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.

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.