General Biology/Tissues and Systems/Epithelial Tissue

=Epithelial tissue= 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 The other side rests on the basal layer.
 * animal interior (forming the lumen) or
 * exterior of its body cavity.

Epithelial tissue is not penetrated by blood vessels.

Two categories:
 * sheets
 * glands

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.
 * Epidermis : from epithelium. Below this is dermis, thicker and with blood vessels.

Two specialized epithelia:
 * pseudostratified
 * transitional


 * 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.