Comes from various sources, ectodermal or endodermal material. Cell sheet lines a surface or body cavity. One side, called freesurface or Apical, is exposed to
- animal interior (forming the lumen) or
- exterior of its body cavity.
The other side rests on the basal layer.
Epithelial tissue is not penetrated by blood vessels.
Classified on two features:
- simple, (a single layer of cells),
- stratified, (more than one cell layer.)
Cell shape at free surface:
- squamous (broad and flat)
- cuboidal (spherish)
- columnar (tall and thin)
- Simple squamous epithelium
- usually lines body cavities and vessels,alveoli, glomeruli of kidney; in blood and lymph vessels called endothelium; in body cavities called mesothelium (serosae): parietal serous membranes line body wall, visceral serous membranes cover organ
- Simple cuboidal epithelium
- in ducts like kidney and salivary glands.
- Simple columnar epithelium
- nonciliated type lines digestive tract, ciliated type lines some regions of uterine tubes and lungs
- Stratified squamous epithelium
- (important) lines mouth, esophagus,and vagina. Cells sometimes dead, flat and keratinized, making them resistant to abrasion. Stratified squamous epithelium changes to columnar squamous epithelium progressively down esophagus to the stomach.
- from epithelium. Below this is dermis, thicker and with blood vessels.
Two specialized epithelia:
- Pseudostratified epithelia
- lines the trachea (where it is ciliated)and the male urethra (where it is non ciliated), looks stratified but not.
- Transitional epithelia
- found only in bladder and urinary system. As it stretches it appears to go from 6 to 3 cell layers deep.
- Glandular epithelia
- (gland: group of cells that excretes something.. mostly derived from epithelium. Glands are classified into endocrine and exocrine by where they excrete.
- Endocrine glands
- secrete hormones into the blood without use of ducts.
- Exocrine glands
- secrete onto the body surface or into a cavity, thru a duct. Exocrine substances include sweat, mucous, oil, and saliva. An exocrine gland is the liver, which secretes bile.