Glands

• Glands

  ->Epithelial cells specialized for the function of secretion.

  ->Molecules stored in secretory granules
• Glandular epithelium function

  • Cells may synthesize, store and secrete proteins, lipids, carbohydrates and protein complexes
• Classification of glandular epithelium

  • According to the morphology
  • According to the mode of secretion
  • According to the nature of secretion
• Endocrine glands

  Secretions are released into adjacent blood vessel
• Exocrine glands

  Secretions are secreted unto a surface or into ducts that are connected to a surface
• Paracrine glands

  Secretions reach adjacent target cells by diffusion
• Autocrine glands

  Secretions are targeted for receptors on the same cell that produce the secretions
• Types of exocrine glands

  • Unicellular gland: Single secretory cell, scattered in the midst of non secretory lining epithelia cells.
  • Multicellular glands: Consists of more than one cell, classified by structure of the gland.
• Exocrine glands general structure

  • Stroma: capsule and septa divide gland into lobes and lobules
  • Parenchyma: cells that secrete
  • Acinus: cluster of secretory cells surrounding a duct that drains the acinus cells
• Classification of exocrine glands

  • Shape of secretory portion (tubular vs acinar/alveolar vs specialised-branched/coiled)
  • Structure of ducts (simple vs compound)
• Types of exocrine glands

  • Simple (not branched)
  • Compound (2 or more branches)
  • Tubular
  • Acinar/alveolar
  • Specialized
• Simple glands

  • Simple tubular glands (e.g. glands of colon)
  • Simple coiled tubular glands (e.g. eccrine sweat glands )
  • Simple branched tubular glands (e.g. stomach and uterus glands)
  • Simple branched alveolar glands (e.g. sebaceous glands )
  • Simple acinar/alveolar (e.g. mucus glands along urethra. Not common in adult humans.)
• Compound glands

  • Compound tubular glands (e.g. Brunner’s gland in duodenum)
  • Compound alveolar/acinar glands (e.g. Exocrine pancreas)
  • Compound tubuloalveolar glands (e.g. Salivary glands)
• Classification of glands - Mode of secretion

  • Merocrine: Secretory products are delivered in membrane bound vesicles to apical surface of cells and by exocytosis released from the cell. E,g, pancreas, sweat glands
  • Holocrine: Secretory products accumulate within maturing cells which undergoes cell death. Leading to release of cell debris and secretory products into lumen of gland. E.g. sebaceous glands thin skin
  • Apocrine: Secretory product accumulates in the apical portion and are released with a thin layer of cytoplasm. E.g. mammary glands, armpit and groin area
• Classification of glands - Nature of secretion

  • Mucous: Viscous and slimy, mucinogen stain palely may appear empty. E.g. goblet cells, salivary glands, surface cells of stomach
  • Serous: Watery protein secretions, cytoplasm stain intensely basophilic E.g. digestive enzymes by pancreas
  • Lipid: Waxy lipid secretion. E.g. sebum secreted by sebaceous glands
• Histology of different secretory cells

  • Serous: darkly stained, contains zymogen granules, round nuclei towards the base
  • Mucous: lightly stained, appears empty, filled with mucoid material, flattened nuclei towards the base
  • Lipid: cluster of lightly stained cells, some cells may appear hollow and vacuolized, centrally placed nuclei
• Criteria for classifying covering epithelium: number of layers, shape of cell found in surface layer, specialization
• Criteria for classifying glandular epithelium: structure of duct, shape of secretory portion
• Sebaceous gland is a simple branched alveolar gland, with holocrine mode of secretion and lipid nature of secretion, located in thin skin