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 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. 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.