L2 : Microscopic Examination of Epithelial Tissues

• Epithelial Tissues - cover external body surfaces, provide lining of the internal surfaces
• Cells in epithelial tissues are arranged very closely with little intercellular substance between, arranged as sheets covering or lining surfaces, or as masses of cells in glands
• categories of epithelial tissues :

  1. simple epithelium
  2. stratified epithelium
  3. glandular or pyramidal epithelium
• Epithelium - greek words :

  Epi means upon , Thele means nipple
• Epithelial Tissues characteristics :

  Cells : closely aggregated polyhedral cells

  Extracellular matrix : small amount or very thin layer
• Epithelial tissues vascularity:

  Avascular - no blood supply

  Vascular - have blood supply
• Epithelial tissues functions :

  • Covering, lining, and protecting (e.g epidermis)
  • Absorption (e.g intestinal lining)
  • Secretion (e.g., parenchymal cells of glands)
• Secretion is different from excretion
• Basement membrane - extracellular part, no cells ; entire structure beneath epithelial cells
• Cell junctions :

  1. Zonula
  2. Tight junction (zonula ocludens)
  3. Adherens junction (zonula adherens)
  4. Desmosome (macula adherens)
  5. Hemidesmosome
  6. Gap junction (nexus)
• Zonula - indicates that. the junction forms a band encircling each cell
• Tight junction (zonula occludens) - forms a seal between adjacent cells to control the passage of molecules
• Adherens junction (zonula adherens) - strengthens and stabilizes nearby tight junctions, holds cells together
• Desmosome (macula adherens) - dark shaped, maintains integrity of the epithelium
• Hemidesmosome - anchora cytoskeleton to the basal lamina
• Gap junction (nexus) - allows direct transfer to small molecules and ions from one cell to another; also called communication junction
• Types of epithelium tissues : simple cuboidal

  
• Type of epithelium tissues : simple columnar

  
• Type of epithelium tissues : simple squamous

  
• Type of epithelium tissues : stratified squamous

  
• Specialozed structures :

  • microvilli
  • cillia
• Microvilli - used for absorption; lining the small intestines, uniform in shape and size; appear. as brush border or stereotyped; contain microfilaments
• Cillia - “Hair-like” ; structures for propelling fluis (mucus) ; longer than microvilli ; contain internal arrays of microtubules
• Simple Epithelium - composed of one layer of cells, all attached to the basement membrane
• Simple squamous epithelium - composed of one layer of cells, all attached to the basement membrane

  
• Simple cuboidal epithelium - found in glands and ducts, covering the ovary, thyroid, and kidney tubules 

  
• Simple columnar epithelium - brick like cells, elongated nucleus

  
• Pseudostratified columnar epithelium - not all cells reach the surface but all are attached to the basal lamina ; can be ciliated or non-ciliated


  pseudostratified columnar epithelium :

  
• Goblet cells :

  Structure - column-shaped cells found in respiratory and intestinal tracts

  Function - secretes mucin, creating a protective mucus layer
• mucin - main component of mucus
• pseudostratified ciliated columnar epithelium - can be ciliated or non-ciliated
• stratified epithelium - composed of more that one layer of cells
• only the basal cells are attached to the basement membrane
• types of stratified epithelium :

  1. stratified squamous epithelium
  2. stratifed cuboidal epithelium
  3. stratified columnar epithelium
  4. transitional epithelium
• 2 types of stratified squamous epithelium :

  1. keratinized
  2. non-keratinized
• stratified squamous epithelium keratinized - found in the epidermis of the skin ; DRY
• stratified squamous epithelium non-keratinized : found in the opening of the mouth, esophagus, and vagina ; moist
• stratified cuboidal epithelium - found in sweat gland duct
• stratified columnar epithelium - found in male urethra, conjuctiva, and excretory ducts
• transitional epithelium - found in the excretory passage of urniary tract (e.g., ureter, urinary bladder, and urethra) ; located in the lining of the urinary system ; large and rounded surface