Lining epithelia

• Epithelial cells


  • Derive from all three embryonic germ layers
  • Closely apposed and anchored to a basement membrane
  • Have different features and functions in apical, lateral and basal domains
  • Display specialisations on the free edge (microvilli, cilia) and junctions in the baso-lateral membrane
  • Avascular, lacking blood vessels
• Lining epithelia


  • Cover the external surface of the body and line the internal cavities, protecting from mechanical, physical and chemical injuries
  • Allow metabolic exchanges (secretion, absorption, gas exchange)
  • Receive sensitive stimulation
• Epithelial tissue types


  • Simple (one layer)
  • Stratified (more than one layer)
• Simple epithelia


  Mostly concerned with absorption, secretion and filtration
• Stratified epithelia


  Mainly protect tissues/organs
• Stratified epithelial types


  • Stratified squamous (keratinised and non-keratinised)
  • Transitional
  • Stratified cuboidal
  • Stratified columnar
• Simple squamous epithelium


  • Single layer of flattened cells
  • Allows passage of material by diffusion and filtration
  • May secrete lubricating substances
• Simple cuboidal epithelium


  • Single layer of cubelike cells
  • Main functions are secretion and absorption
• Simple columnar epithelium


  • Single layer of tall cells
  • May contain mucus-secreting goblet cells
  • Main functions are absorption and secretion
• Pseudostratified epithelium


  • Single layer of cells of different heights
  • May contain goblet cells
  • Main functions are secretion and propulsion of mucus
• Stratified squamous epithelium


  • Thick membrane composed of several cell layers
  • Basal cells are cuboidal or columnar and metabolically active
  • Surface cells are squamous
  • Keratinised type has surface cells full of keratin and dead
  • Main function is protection of underlying tissues
• Transitional epithelium


  • Resembles both stratified squamous and stratified cuboidal
  • Basal cells are cuboidal or columnar
  • Surface cells are dome-shaped or squamous, depending on organ stretch
  • Permits distension of urinary organs
• Stratified cuboidal epithelium


  • Located in duct of glands like sweat, sebaceous, and exocrine pancreas
• Stratified columnar epithelium


  • Present in larynx mucosa, male urethra, conjunctiva, and duct of large glands
• Epidermis


  • Basal layer is mitotic, continuously replacing cells
  • Spinous layer has flattened cells
  • Stratum granulosum has cells with keratin granules
  • Stratum lucidum has dead cells with indistinct boundaries
  • Stratum corneum has 25+ layers of dead squamous cells
• Epidermolysis bullosa simplex


  Pathology caused by mutations in keratin, characterized by blisters induced by mechanical stress
• Other epidermal cell types

  • Melanocytes
  • Langerhans cells
  • Merkel cells
• Melanocytes


  • Produce and distribute the pigment melanin
  • Melanin provides photoprotection
• Langerhans cells


  • Cells of the immune system
  • Activate T lymphocytes against encountered pathogens or harmful proteins
• Merkel cells


  • Tactile cells with synaptic contacts to somatosensory nerve fibers
  • Associated with light touch, shape and texture discrimination
• Secretion


  • Molecules to be secreted can be stored in membrane-bound secretory granules (vesicles)
  • Exocrine glands deliver products outside through excretory ducts
  • Endocrine glands produce hormones and neurotransmitters and release them into the blood
• Exocrine gland classification parameters

  • Number of cells (unicellular or multicellular)
  • Site (intramural or extramural)
  • Shape of terminal secretory unit (tubular or alveolar)
  • Branching of secretory units and ducts (simple or compound)
  • Mode of secretion (merocrine, apocrine, holocrine)
  • Nature of secretion (serous, mucous, mixed)
• Merocrine secretion


  Secretory product is delivered in membrane-bound vesicles and extruded by exocytosis
• Apocrine secretion


  Secretory product is released in the apical portion of the cell, surrounded by a thin layer of cytoplasm within an envelope of plasma membrane
• Holocrine secretion


  Secretory product accumulates within the maturing cell, which undergoes destruction and discharges both products and cell debris
• Myoepithelium


  • Specialised squamous epithelium with contractile power
  • Contains actin, myosin and cytokeratin, but not muscle cells
  • Surrounds glandular acini and ducts of many glands
• Exocrine pancreas


  • Compound tubulo-acinar gland composed of serous secretory cells
  • Acinus is the simplest secretory unit, a blind sac
  • Acinar lumen continues into intercalated ducts, then intralobular ducts, and finally the major pancreatic duct (Wirsung duct) that enters the duodenum
  • Groups of acini form lobules separated by connective tissue septa