epithelium

epithelium

1. surface epithelium

   • *lining on tubes & cavities  *covering of the body surface 
   • simple
     • squamous
       • *nucleus: flat & elongated
       • *cytoplasm: acidophilic , small amount with few organelles
       • *sites: 1.lung alveoli 2.endothelium of blood vessels
       • *function: it provides a smooth & thin surface to facilitate filtration & diffusion of water,electrolytes & transport of gases
     • cubuidal
       • *nucleus: central & rounded
       • *cytoplasm: acidophilic or basophilic acc. to its activity
       • *sites: 1.thyroid follicles 2.some kidney tubules 3.ovarian surface
       • *function: 1.absorption 2.secretion
     • columnar
       • *nucleus: basal & oval
       • *cytoplasm: acidophilic or basophilic
       • *function: 1.secretory as epithelium of the stomach 2. absorptive as epithelium of small intestine 3.movement of ova or sperms in uterus or fallopian tubes (ciliated)
       • *sites:1.without brush border as epithelium of stomach 2.with brush border a. cilia as epithelium of uterus & fallopian tubes b. microvilli as epithelium of small intestine
     • pseudostratified
       • it’s formed of one sheet of cells which vary in height & width  all cells rest on the basement membrane (simple)
       • *nucleus: nuclei lie at different levels 1.tall:apical 2.short:basal
       • *function: 1.protection 2. movement
       • *sites: 1.ciliated as epithelium of trachea 2.with stereocilia as epithelium of epididymis
   • stratified
     • the basal layer is usually formed of columnar cells  the middle layers mostly formed of polyhedral cells  the superficial layer with different shapes of cells 
     • all types of stratified epithelium perform protective functions so they are usually found in the surfaces subjected to frictional forces and other potential hazards 
     • بنقسمهم علي حسب اخر طبقه اللي هي superficial layers 
     • squamous
       • basal: cuboidal or columnar
         • middle: 5-7 layers are polygonal
           • superficial: flat
       • *sites
         • 1.keratinized covers the skin (epidermis)
           • superficial layers are apoptotic cells
             • which lose their nuclei & cytoplasm is replaced with keratin 
         • 2.non keratinized lines wet surface as mouth, oral pharynx, esophagus & vagina
           • superficial cells are squamous & nucleated
     • cuboidal
       • it’s a thin type of stratified epithelium 
       • *sites: lines large ducts of exocrine glands as salivary & sweat glands
     • columnar
       • *sites: lines large ducts of exocrine glands , part of male urethra (penile part)& conjunctiva
     • transitional
       • *sites: it’s exclusively found in urinary passages including ureter, renal pelvis, renal calyces, urinary bladder & first part of urethra
         • it’s specialized to accommodate greater stretch so the number of its layer is changeable
           • 4-8 layers in the empty bladder
           • 2-3 when the bladder is full
       • superficial cells are cuboidal

2. myoepithelium

   • have the ability to contract like the muscle as they contain myofilaments 
   • *sites: around secretory exocrine acini (usually bet. the secretory cells & their basal lamina)
   • *function : they help releasing products to the lumen of the acini (follicle)

3. neuroepithelium

   • they are modified epithelial cells for preparation of nerve stimuli
   • *sites: 1. tongue (taste buds) 2.inner ear a. organ of corti for hearing b. macula & crista ampularis (vestibular apparatus) for equilibrium 3. nose (olfactory mucosa)

4. glandular epithelium

   • endoocrine
     • directly to the blood
       • product hormones
         • as thyroid gland & pituitary gland
   • exocrine
     • through a duct
       • as salivary gland & sweat gland
   • mixed
     • as pancreas