Lecture 6: Epithelial Cell Specializations Flashcards
3 sites of epithelial cell specializations
- intercellular surfaces: between cells, the occluding, adhering, and communicating junctions
- luminal surfaces: facing lumen, with cilia, microvilli, and stereocilia
- basal surfaces: basement membrane and hemidesmoses
components of junctional complexes
APICAL SIDE
1. tight junction/occluding junction
- adherens junction
- desmasome
- gap junction
- hemidesmosome
- basal lamina/basement membrane
BASAL SIDE
purpose of epithelial intercellular surfaces
permit epithelia to form a continuous cohesive layer so cells can communicate
tight junction/occluding junction/zonula occludens characteristics
fxn: seals intercellular spaces so luminal contents can’t penetrate between the cells (i.e. proteins)
location: directly below luminal surface
intracellular protein components: Z01, Z02, Z03, AF-6
intercellular protein components (like thread): occludin and claudin, seal 2 cells’ membranes to each other
adhering junctions characteristics
fxn: bind epthelial cells together and act as anchorage site for cytoskeleton of each cell
2 types:
a. zonula adherens
b. desmosomes (macula adherens)
zonula adherens characteristics
location: deep to zonula occludens
fxn: form strong bond between adjacent cells
composition: cadherins (transmembrane proteins); actin filaments (cytoplasmic face); catenins, vinculin, alpha-actinin (attachment proteins)
desmoses (macula (“spot”) adherens) characteristics
location: deep to adhering junctions; scattered or can be associated with zonula occludens or zonula adherens
fxn: provide structural integrity; **HAVE MANY in stratified squamous tissue; can connect plasma membranes of 2 adjacent epithelial cells
composition: desmocollins (intercellular); desmogleins (transmembrane); desmoplakin, keratin (plaque)
terminal bar
junctional complex including all 3 types of junctions: zonula occludens, zonula adherens, and desmosome/macula adherens
gap junction characteristics
location: below junctional complex
fxn: communication between cells; numerous in embryonic tissue; are gromet-like structures that extend from 1 cell membrane to another membrane, w/ open pores for ions, molecules; pores are regulated by pH, Ca++, cAMP
regulation: Ca++, pH, cAMP
composition: connexins (TM proteins); at least 24 known genes, but not all are alike in different tissues
particles of MW<1500 can flow through them
brings membranes very close together (2-4 nm) with connexion molecules
luminal (aka apical) surface specialization
simple cuboidal and simple columnar cell tissue
cilia (up to 300/cell) for movement
microvilli (up to 3000/cell) for increased surface area for absorption
cilia characteristics
example of luminal surface specialization
20 microtubule core axoneme, bound by plasma membrane; central pair surrounded by 9 doublets
have dynein arms that have ATPase activity for movement
inserts into a basal body composed of 9 triplets of microtubules
move by longitudinal movement of doublets relative to one another
fxns: clean airways; transport of ovum; line trachea, oviduct; help sperm flagella move
composed of: tubulin
microvilli characteristics
example of luminal surface specialization
1 micrometer in length
20-30 actin filaments (thin filaments)
covered in glocalyx, a glycoprotein
no ATP activity; contract & elongate to prevent clogging; no active motion
connect to terminal web of apical membrane at cell’s cytoskeleton
can have as many as 3000 microvilli on cell
stereocilia
in epididymus of male reproductive tract
longer than cilia
composed of actin! (so are misnamed)
incapable of active movement
facilitate absorption & movement (i.e. sperm)
2 types of basal surface cell specialization
- hemidesmosome
2. basement membrane
hemidesmosomes
location: basal plasma membrane
fxn: anchor cytoskeleton at base of cell to basement membrane
composition: integrins a6B4, which stick thru basal membrane; laminin 5; cytokeratins
basement membrane
fxn: connects cell to underlying connective tissue; physical support; selective barrier, i.e. vs fibroblasts; growth, differentiation; regeneration of epithelial tissue; ultrafiltration of kidney, blood-air barrier for lung, prevent tumor metastasis by being impenetrable
composition: ** type IV collagen ** specific to basement membrane; laminin (binds collagen to bm); proteoglycans; entactin (binds laminin to type IV collagen); fibronectin (binds integrin of plasma membrane, produced by fibroblasts of connective tissue); type VII collagen (binds basal lamina to subjacent collagen)
clinical correlations of specializations of epithelial cells
- epidemolysis bullosa
- bullous pemphigoid
- immotile cilia syndrome
- epitheleal cell tumors
epidemolysis bullosa
skin blisters from mutations in keratin 5 & 14 genes
bullous pemphigoid
autoantibodies against hemidesmosomes that cause blisters
immotile cilia syndrome
defect in dynein arms that affect cilia’s ability to move
can cause embryonic development issues
doesn’t allow sperm to move, so no fertilization occurs
epithelial cell tumors
if mechanisms for regulating epithelial turnover fail, tumors can develop
carcinomas: malignant tumors that arise from membranous epithelia; no defined layers of skin
adenocarcinomas: malignant tumors that arise from glandular epithelium
Mohs procedure
method to examine skin for cancerous tissue; remove skin ring by ring progressively until noncancerous tissue is encountered
possible only because junctional complexes hold epithelial tissue together and allow cutting of skin and its reformation via binding to actin filaments and cytoskeleton with cohesive junctions so skin can reform
