Lecture 6: Epithelial Cell Specializations

Question 1

3 sites of epithelial cell specializations

Answer 1

1. intercellular surfaces: between cells, the occluding, adhering, and communicating junctions
2. luminal surfaces: facing lumen, with cilia, microvilli, and stereocilia
3. basal surfaces: basement membrane and hemidesmoses

Question 2

components of junctional complexes

Answer 2

APICAL SIDE
1. tight junction/occluding junction

2. adherens junction
3. desmasome
4. gap junction
5. hemidesmosome
6. basal lamina/basement membrane
   BASAL SIDE

Question 3

purpose of epithelial intercellular surfaces

Answer 3

permit epithelia to form a continuous cohesive layer so cells can communicate

Question 4

tight junction/occluding junction/zonula occludens characteristics

Answer 4

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

Question 5

adhering junctions characteristics

Answer 5

fxn: bind epthelial cells together and act as anchorage site for cytoskeleton of each cell

2 types:

a. zonula adherens
b. desmosomes (macula adherens)

Question 6

zonula adherens characteristics

Answer 6

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)

Question 7

desmoses (macula (“spot”) adherens) characteristics

Answer 7

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)

Question 8

terminal bar

Answer 8

junctional complex including all 3 types of junctions: zonula occludens, zonula adherens, and desmosome/macula adherens

Question 9

gap junction characteristics

Answer 9

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

Question 10

luminal (aka apical) surface specialization

Answer 10

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

Question 11

cilia characteristics

Answer 11

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

Question 12

microvilli characteristics

Answer 12

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

Question 13

stereocilia

Answer 13

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)

Question 14

2 types of basal surface cell specialization

Answer 14

1. hemidesmosome

2. basement membrane

Question 15

hemidesmosomes

Answer 15

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

Question 16

basement membrane

Answer 16

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)

Question 17

clinical correlations of specializations of epithelial cells

Answer 17

1. epidemolysis bullosa
2. bullous pemphigoid
3. immotile cilia syndrome
4. epitheleal cell tumors

Question 18

epidemolysis bullosa

Answer 18

skin blisters from mutations in keratin 5 & 14 genes

Question 19

bullous pemphigoid

Answer 19

autoantibodies against hemidesmosomes that cause blisters

Question 20

immotile cilia syndrome

Answer 20

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

Question 21

epithelial cell tumors

Answer 21

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

Question 22

Mohs procedure

Answer 22

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