Epithelial classification

Epithelial classification

1. Shape

   • squamous
     • flattenend
     • irregular shape
     • central nucleus
     • stratified vs simple
       • simple squamous
       • stratified squamous
         • wet
           • apical cells are shed through desquamation
             • basal cells divide through mitosis and move upwards towards surface tropical desquamated cells
           • non keratinised
             • found in locations subject to abrasion but not dehydration
               • e.g mouth, anus, vagina
         • dry
           • keratinised to prevent water loss
             • cells on surface die and become keratinised to form tough horny waterproof layer
   • cubiodal
     • cube shaped
   • columnar
     • elongated, rectangle shape
     • surface modifications
       • microvilli
         • brush border - mucus adheres to it
           • goblet cells release mucus in Gi tract
           • secretes enzymes in gi tract - important for maintaining homeostasis
         • increase surface area for absorption

2. Layers

   • simple
     • single layer of cells - all cells in direct contact with basement membrane
       • allows efficient diffusion/absorption/excretion
         • do not offer effective protection
   • stratified
     • multiple layers of cells
       • cells in contact with basement membrane = basal
         • cells in contact with lumen = apical
       • allow protection from abrasion, infection, caustic content
         • not found in areas of body where absorption/secretion/diffusion needs to occur
   • pseudostratified
     • varying heights but all in contact with BM
   • transitional
     • can adapt to changes in volume
     • many tight junctions to prevent leakage
     • apical layer cubical but will change shape with stretching

3. Function

   • Exchange
     • simple squamous
     • contain pores
     • located in lungs and lining blood vessels
   • transportation
     • simple columnar or cuboidal
     • tight junctions to limit but not entirely eliminate movement of substances between cells
     • located in intestine and kidney
   • ciliated
     • simple columnar or cubiodal
     • located in trachea and female reproductive tract
   • protection
     • stratified - squamous in upper layers, polygonal in deeper layers
     • cells connected tightly by desmosomes
     • mouth and skin
   • secretory
     • stratified columnar to polygonal
     • exocrine and endocrine
     • extensive ER