Lecture 8: epithelial specialisa0ons Surface specialisations
Epithelial surfaces:
Intercellular: between adjacent cells
Luminal (apical):facing tube lumen, outside
Basal: in contact with underlying supportive tissue
Each exhibit a variety of specialisations
Intercellular surfaces –cell/cell connections
Luminal surfaces –cilia, microvilli and stereocilia
Basal surfaces –below epithelium, flow of nutrients/metabolites to and from
epithelium
Intercellular surface specialisa0ons Cell Junctions – allow communication between cells
»Occluding Junctions (tight junctions)
»Adhering Junctions (zonula adherens) and Desmosomes
»Gap Junctions: communicating junctions that allow direct
passage of materials through pores
Junctional Complex: the combination of occluding
junctions, adhering junctions and desmosomes
Junc0onal complex: components Occluding Junctions (tight junctions)
Located immediately beneath luminal surface of simple columnar epithelium (eg
intestinal lining). Seal intercellular spaces preventing movement of luminal contents in
between cells. Zonula occludens
Blood brain barrier in cerebral capillaries
Important proteins: occludins
Adhering Junctions (zonula adherens)
Tightly bind cells of epithelium together. Act as anchorage sites for cytoskeleton of
each cell; all cytoskeletons linked together
Deep to tight junctions, zonula adherens forms continuous bands around cells
Important proteins: cadherins
Desmosomes
Small patches or spots of adhering junctions deep to zonula adherens
Important proteins: desmoglein, desmoplakin
Hemidesmosomes
Present on inner aspect of basal plasma membrane adjacent to basal membrane.
Provide a means of anchoring cells to BM and underlying tissue via the cytoskeleton
Important proteins: Integrins, laminin
Intercellular junctions: Junctional complex
3D arrangement of junctional complex
and its components
Complex prevents access of luminal
contents to intercellular spaces
Apical basal polarity (transport
functions)
Intestinal columnar epithelium; EM x 125000
TJ-Tight Junction
ZA –Zonula adherens
D –Desmosomes
HD Hemidesmosomes
Mv–Microvillus
Mf –Microfilaments
CJ –Communicating junction
IF -Intermediate Filament
TW –Terminal Web
Intercellular junctions: Junctional complex
TJ-Tight Junction
ZA –Zonula adherens
C –Electron dense line of cadherin molecules
P –Plaque P – Electron dense plaque
D –Desmosomes
D2 –Larger Desmosome
H –Hemidesmosomes
IF -Intermediate Filament
TW –Terminal Web
LL –Lamina lucida
LD –Lamina Densa
Transmembrane proteins: cadherins and
integrins
Anchoring proteins: actinin and catenins,
desmoplakin, plakoglobin, plectin
Cytoskeleton Actin filaments and
intermediate filaments of keratin
Intercellular surfaces –Communicating/Gap junctions
Intestinal epithelium EM x 80,000
Each gap junction is 2-4nm diameter
Contains numerous pores allowing
passage of cations and small molecules
Exchange sites
Important in myocardial function
Each pore consists of Connexons
Transmembrane channels containing 6
transmembrane proteins (connexins)
Luminal surface: cilia
Respiratory tract; Toluidine blue x 800
Motile structures which project from apical surface of
certain epithelial cells
Cells may have up to 300 cilia
10µm long
Respiratory tract and Fallopian tubes
Bounded by plasma membrane
Central core (axoneme): 20 microtubles arranged in pairs
Basal body: 9 microtuble triplets
Dynein arms: conversion of ATP to ADP
Kartagener syndrome (rare): ciliary dyskinesia
EM
x 20000
Luminal surface: microvilli
Small intestine; H&E x 320
In absorptive epithelia, the apical surface is involuted
many times to form microvilli (brush border)
0.5 –1µm in length
In small intestine and proximal renal tubules the epithelial
cells have up to 3000 regular microvilli per cell
Found principally at sites of high absorptive or secretory
activity (e.g stomach and small intestine)
EM x 4000
EM x 30,000
Luminal surface: stereocilia
Epididymis; H&E x 320
Extremely long microvilli
Composed of an actin microfilament skeleton similar to microvilli
Found in male reproductive tract (epididymis) where its believed they facilitate
absorption