Epithelial tissue

Epithelial tissue

1. Characteristics

   • Cellularity: almost all cells, not much free space, cells pressed up against each other
   • Polarity: apical surface = free, basement membrane, basal lamina
   • Cell contacts = cell adhesion proteins connecting epithelial cells and the cells with the basal lamina
   • Supported by CT
   • Avascular but innervated. Not much room for blood vessels (must get nutrition from above or below) but has some nerve endings
     • Gases and nutrients reach the epithelial cells from blood by diffusing across basement membrane
     • Most active cells closest to basement membrane
   • Regeneration: can be very active
   • Where it's found

2. Types of epithelial tissue

   • Simple
     • Simple squamous: found where there is little wear and tear. Function = diffusion, filtration, secretion. Found in inner linings of vessels, kidneys, inner linings of parietal and visceral serosa
     • Simple cuboidal: Function = active transport, secretion, absorption. Found in small glands, ducts, kidney tubules, ovarian surface, liver, thyroid, mammary glands, bronchioles, salivary gland ducts
     • Simple columnar: Function = absorption, secretion of mucus (microvilli increase SA). Found in the linings of the digestive tract. Contain goblet cells.
   • Stratified
     • Stratified squamous: Underneath layers more cuboidal. Found where a little more protection is needed. Function = protection against abrasion, diffusion, filtration. Non-keratinized type found in esophagus, mouth, and vagina. Keratinized type found in epidermis.
     • Stratified cuboidal: Function = secretion, protection. Found in sweat glands, overian follicles, seminiferous tubules.
   • Pseudostratified
     • Function = mucus cells coat passageways. Ciliated type found in trachea. The cilia sweep the mucus up.
   • Transitional
     • Function = distend to stretch out when organ fills up. Found in ureters, urethra, and urinary bladder