Exocrine & Endocrine Glands

• Glands are cells or aggregations of cells with the function of secretion.

Exocrine Glands

• Release secretory products via a duct system onto a body surface in contact with the external world, such as skin or the gastrointestinal tract.
• Maintain a connection with the body surface.
• Classified by cell number, shape, and branching pattern of secretory portions and ducts.

Endocrine Glands

• Release secretory products (typically hormones) into spaces between secretory cells (extracellular space), entering the bloodstream.
• Lose their connection to the body surface.

Development of Glands

• Both endocrine and exocrine glands derive from epithelia.
• Epithelia form a down-growth into the underlying connective tissue.

Unicellular Glands

• Consist of a single secretory cell.
• An example in mammals is goblet cells.
• Goblet cells occur in the epithelium of many mucous membranes.
• Goblet cells secrete the glycoprotein mucin.
• Mucin is converted into mucus by water uptake.

Multicellular Glands

• The simplest form is a secretory epithelial sheet, consisting entirely of secretory cells.
• For example, the stomach lining has a surface epithelium whose mucous secretion protects the stomach wall from acidic contents.
• Secretion may be discharged directly from the secretory portion onto the epithelium or reach the epithelium via a duct system.
• Myoepithelial cells are contractile, consisting of actin and myosin filaments.

Glandular Morphology

• Secretory portions have various shapes; secretory cells may form:
  • Tubes in tubular glands
  • Acini in acinar glands
  • Alveoli in alveolar glands

Combinations in Glandular Structure

• Combinations of structures exist, for instance, the pancreas, a tubulo-acinar gland, where each section has a specialized function.
• Acinar cells produce precursors of digestive enzymes.
• Tubular cells secrete alkaline bicarbonate solution to neutralize acidic stomach contents released into the duodenum.

Simple vs. Compound Multicellular Glands

• Simple multicellular glands have an unbranched excretory duct.
• Compound glands have a branched excretory duct.

Tubular Glands

• Straight tubular glands extend from the colon surface into underlying connective tissue.
  • They are largest in the colon because of its smooth inner surface.
• Visible in longitudinal or transverse sections

Secretory Mechanisms

• Secretory cells release products via three mechanisms:
  • Merocrine
  • Apocrine
  • Holocrine

Merocrine Gland

• Corresponds to exocytosis, where vesicles open onto the cell surface.
• The secretory product is discharged without further loss of cell substance.
• Major salivary glands use this mechanism.

Apocrine Gland

• Part of the apical cytoplasm is lost along with the secretory product.
• The plasma membrane is restored by fusion of broken edges, cell accumulates the product anew.
• Apocrine sweat glands and mammary glands use this mechanism.

Holocrine Gland

• Involves the breakdown and discharge of the entire secretory cell which is only seen in the sebaceous glands of the skin.

Explore exocrine glands, which secrete via ducts onto body surfaces, and endocrine glands, which release hormones into the bloodstream. Both originate from epithelia and include unicellular types like goblet cells.