Epithelial Tissues
A. Types of Tissues
•
•
•
•
Nervous
Epithelial
Muscular
Connective
B. Epithelium or Epithelial Tissue
• Consists of sheets of closely aggregated
polyhedral cells with little EC substance
functions
➡ Covering, lining, and protecting surfaces
➡ Absorption
➡ Secretion
➡ Contractility (myoepithelial cells)
• Cells rest on a basement membrane
➡ thin, non-cellular region separating the
epithelium from underlying connective
tissue
➡ basement membrane can be seen
with a light microscope
★ Basal Lamina or Basement Membrane
• 2 zones on E/M:
➡ Lamina lucida/rara - inner pale
zone of very low density
➡ Lamina densa - outer zone of
greater density
• Principal components:
➡ type IV collagen
➡ laminin
➡ proteoglycans
• Primary function:
➡ support the epithelium
➡ serves as passive molecular sieve or ultrafilter
★ Germ Cell Origin
• Ectoderm
➡ gives rise to the:
✓ Corneal epithelium
✓ Epidermis of the skin
✓ Glandular appendages of the skin
- e.g. sweat, sebaceous and mammary glands
• Mesoderm
➡ Urinary tract
➡ Male and female reproductive tracts
➡ Endothelium - cells lining the blood and lymph vessels
➡ Mesothelium - cells lining the cavities
• Endoderm
➡ Intestinal glands
➡ Liver
➡ Pancreas
★ Classification of Epithelia
➡ based on the number of layers, shape of cells, specializations present on
their free surface
• Simple: if made up of 1 layer
• Stratified: if there are multiple layers
C. Simple Squamous Epithelium
•
•
•
•
Alveoli
Bowman’s Capsule
Endothelium
Mesothelium
✴ Simple Squamous Epithelium - Bowman’s capsule of glomerulus (kidney)
D. Simple Cuboidal Epithelium
•
•
•
•
•
Thyroid follicles
Surface of ovary
Choroid plexus of brain
Pigment epithelium of retina
Ducts of many glands
✴ Simple Cuboidal Epithelium - Thyroid follicle
E. Simple Columnar Epithelium
• lines the digestive tract from the cardia of stomach to the anus
• Uterus (endometrium) and fallopian tubes
✴ Simple Columnar Epithelium - Intestinal Epithelium
F. Pseudostratified Columnar Epithelium
• Trachea and bronchi
• Excretory dust to the parotid gland
• Male urethra
✴ Pseudostratified Columnar Ciliated Epithelium - Respiratory Epithelium
G. Stratified Squamous Epithelium
• Keratinizing Stratified Squamous Epithelium - skin
• Non-keratinizing Stratified Squamous Epithelium - oral cavity, vagina,
esophagus, cornea
✴ Stratified Squamous Keratinized Epithelium - Skin
✴ Stratified Squamous Non-keratinized Epithelium - Esophagus
H. Transitional Epithelium
• Urothelium
• Urinary tracts from the renal calyces to the urethra
✴ Transitional - Urinary Epithelium
๏ Epithelial Polarity
• cells of epithelia are structurally and functionally polarized to carry out function
of secretion, absorption, and regulation of trans-epithelial traffic of ions
and solutes.
• They have a:
✓ free or apical surface
✓ junctional specializations
✓ basal lamina
๏ Junctional Specializations
1. Zonula Occludens (ZO) or Tight Junction
• Most apical forming a continuous band encircling the cell apex together with
ZA
• Membrane of adjoining cells fuse and close off the intercellular space
• Cell attachment - strongest at the ZO
• Important Function: permeability barrier
2. Zonula Adherens (ZA) or Adherent Junction
• Band encircling the cell (below ZO)
• Rich in actin
• Important Functions:
✓ Responsible for binding of band to junctional membrane
✓ Stabilize the brush border of columnar epithelia
3. Desmosomes or Macula Adherens (D)
• Do not form continuous band encircling the cell apex
• Separate plagues arranged in a row around the cell
• Scattered on the lateral surface of the cell (below ZA)
4. Gap Junctions or Communicating Junction (GJ)
• No actual fusion of membrane
• Connexons - seen bridging the gap
• Permeability is affected by pH and calcium concentration
• Function: permits passage of small molecules between cells (ions and amino
acids)