Epithelium
Characteristics
free surface lining that can arise from any germ layer
contiguous cell sheets; all cells are on basement membrane
generally avascular
regenerative
Polarized – apical, basal, lateral
Function
Protection
Surface Transport
Permiability Barrier (via junctions)
Secretion
Absoption (GI)
Sensory
Cytogenetic (testes)
Classifications
Layers: 1 = simple; >1 = stratified; simple but look stratified = pseudostratified
Shape: flat=squamous cube=cuboidal column=columnar
Modification: ciliated, keritinized, etc.
Some examples of Tissue Types with function:
(1)
Simple Squamous
vascular system lining (endothelium) – exchange, barrier in CNS
lining of body cavities (mesothelium) – exchange, lubrication
Bowman's capsule in kidney – barrier
lining of respiratory spaces – absorption
(2)
Simple Cuboidal
small ducts of exocrine glands – absorption and conduit
germinal surface of ovary – barrier
kidney tubules – exchange, absorption, secretion
(3)
Simple Columnar
small intestine and colon – secretion and absorption
stomach – secretion
gallbladder – absorption
(4) P
seudostratified Columnar
trachea and bronchi – secretion and conduit
ductus deferens and epididymis – secretion and conduit
(5)
Stratified Squamous
epidermis, oral and esophagial lining, vaginal lining, etc. - barrier (protection)
(6)
Stratified Cuboidal
glands, ducts, anorectal junction – barrier (protection)
(7)
Stratified Columnar
largest glands, anorectal junction – barrier (protection)
(8)
Transitional
renal calyces, ureter, bladder, urethra – barrier (protection) and distensibility
Specialization of Epithelium
(1) Apical Specialization
microvilli
– increase surface area for absorption
stereocilia
– transduce inner ear signals and function to absorb in epididymis and vas deferens
cilia
– produce coordinated movement
(2) Lateral Specialization
(Terminal Bar)
Zona Occludens
(tight Junction) intramembranous ridges – permeability seal for cellular selective permiability
Zona Adherens
belt of CAM glycoproteins
Macula Adherens
(desmosomes) – snap button (cytoskeleton tonofilaments)
Gap Junction
(Nexus) – open by means of connexons (paired transmembrane proteins); channels=electrical/metabolic coupling
(3) Basal Specialization
Basal Infoldings
– interdigitation for ion tranport
Hemidesmosomes
– anchor stratified squamous (like desmosomes)
Basement Membrane
– both epithelium and connective tissue:
Basal lamina – lamina densa and lucida
Reticular Lamina – reticular fibers (collagenIII) from connective tissue
Glands
Derivation: epithelial invagination into connective tissue stroma
Exocrine if luminal connection remains, Endocrine if not
Classification
Number of Cells
– unincellular (goblet), multicellular (pancreas)
Duct System
– simple, compound
Secretory Morphology
– tubular vs alveolar vs tubuloalveolar
Secretion
– mucous, serous, seromucous
Secretory Mode
– merocrine (product only), apocrine (apical cytoplasm secreted with product), holocrine (whole cell secreted)