EPITHELIAL TISSUES (cont.)
Cell Borders:
1. Cuticular = non-living proteinaceous secretion covering cellular surface (not
present in vertebrates, present in exoskeletons of invertebrates)
2. Striated (Brush) Border = composed of microvilli (small, fingerlike projections of
apical cell surface), acts to increase surface area for absorption. Present in tissues with
absorption as primary function (e.g., intestinal and kidney tubule epithelium).
3. Ciliated = fine hair-like processes of free apical surface, motile. Function to transport
material in one direction along the epithelial membrane surface; also may serve
receptor function (inner ear and retina). Found in respiratory and reproductive
tracts.
4. Stereociliated = long slender, sometimes branching processes from apical cell surface.
Not true cilia - structurally different and non-motile. Found only in ductus
epididymis
and vas deferens of male reproductive tract and hair cells of inner ear. May be both
absorptive and secretory in epididymis, sensory in hair cells.
SPECIALIZED EPITHELIUM = Neuroepithelium
1. Olfactory Epithelium - SEE HANDOUT
2. Taste Bud = similar to olfactory epithelium - same 3 cell types, arranged in barrel-
shaped clump. Taste Receptor Cells (instead of olfactory cells) have short microvilli
on apical end, basal end synapses with sensory fibers of facial and glossopharyngeal
nerves. Acts as a chemoreceptor - stimulated by dissolved substances.
EPITHELIAL GLANDS
Two types:
1) Exocrine = secretes through ducts to epithelial surface.
2) Endocrine = no ducts, secretes into bloodstream
- Both originate as outgrowths of epithelial membranes, except for goblet cells =
single-celled exocrine gland.
EXOCRINE GLANDS - have secretory units and duct(s).
- Secretory Unit = group of secretory epithelial cells enclosing a lumen
- Duct = epithelial-lined tube from secretory unit to surface
- Usually surrounded by CT capsule with septa extending into interior giving lobular appearance, lobes subdivided into lobules.
CRITERIA USED FOR CLASSIFICATION OF EXOCRINE GLANDS - SEE HANDOUT
ENDOCRINE GLANDS = no duct system, secretion diffuses from secretory cells into
blood vessels in surrounding CT. Secretory cells are intimately grouped around
wide capillary-like vessels. Secretions = Hormones (usually).
- Capsule = thin covering of CT enclosing the gland as a whole (characteristic of most
endocrine glands)
- Trabeculae (Septa) = projections or extensions of exterior capsule into interior of
gland, often divides the gland into lobes
- Supporting tissue within gland is sparse and associated with capillaries into which
secretory cells release hormones.
Two Histological Types of Endocrine Glands:
1) Cord-and-Clump Type = by far the most common; cells are arranged in
interconnecting cords and clumps between dilated blood capillaries; store hormone
intracellularly.
2) Follicle Type = thyroid glands; a group of cells forms a vesicle enclosing a central
cavity in which hormone is stored. Hormone is released as needed by passing back
through secretory cells and into blood capillaries lying between follicles.
- Some glands in the body are composite = having both endocrine and exocrine
functions.
EXAMPLES:
1) Liver - produces and secretes bile into duct system; releases glucose from glycogen
stores directly into blood.
2) Pancreas - secretes serous solution with digestive enzymes; releases insulin and
glucagon (involved in carbohydrate metabolism) into bloodstream
MYOEPITHELIAL CELLS (BASKET CELLS)
-
Present in some exocrine glands
- Located between secretory epithelial cells and basal lamina
- Appear as small dark nuclei at periphery of tubule or acinus, with little cytoplasm (cytoplasm extends as long, thin arms to encircle secretory endpiece)
- Contain microfilaments and are contractile
- Help eject secretion from gland
To Lecture 5