Glandular Epithelium

Glandular epithelia, composed of epithelial tissue, are specialized structures that play a role in the production and release of enzymes, hormones, sweat, oil, and mucus in organisms. The secretion and release of these substances are prompted by either external or internal stimuli. Products of glandular epithelia are released either into ducts leading to the surface of the epithelium or into the blood. The 2 types of glands are exocrine and endocrine glands. The classification is based on the number and location of secreting cells and the type of secretions, among other factors.

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Overview

Epithelial tissue

  • 1 of the 4 main kinds of tissues in the body (along with muscular, nervous, and connective tissues)
  • Lines the surfaces of most of the external and internal structures of the body
  • General structure: multiple layers of epithelial cells organized over a basement membrane (composed primarily of collagen) that separates it from the underlying connective tissue
  • Can be subdivided into:
    • Secretory (e.g., glands)
    • Nonsecretory (e.g., skin)

Types of glands

  • Exocrine glands:
    • Synthesize and secrete their products onto a surface:
      • Directly: unicellular glands
      • Via a tube (duct): multicellular glands
    • Examples:
      • Sweat glands
      • Sebaceous glands
      • Salivary glands
      • Mucous glands in the intestinal and respiratory tracts
      • Pancreas (digestive enzymes)
    • Unicellular (mucous cells and goblet cells):
      • Secrete by exocytosis
      • Present in the intestinal and respiratory tracts
    • Multicellular:
      • Secreted by ducts
      • Composed of 2 basic parts:
        • Epithelium-derived duct
        • Secretory unit (acinus)
      • Connective tissue surrounds the acinus and supplies it with blood and nerve fibers.
  • Endocrine glands:
    • Ductless
    • Synthesize and secrete their products into the extracellular space
    • Secretory products reach target cells via the blood.
    • Examples:
      • Pancreas (insulin, glucagon)
      • Thyroid
      • Adrenal glands
      • Ovaries
      • Testes
      • Pituitary gland
    • Hormones produced in epithelial cells are released into the interstitial fluid.
  • Paracrine glands: Secretions affect adjacent epithelial cells.

Related videos

Exocrine Glands

Structural classification

  • Simple glands: unbranched ducts
  • Compound glands: branched ducts
  • Based on the secretory unit shape:
    • Alveolar (secretory cells forming small sacs)
    • Tubular (secretory cells forming tubes)
    • Tubuloalveolar (a mix of alveolar and tubular)

Ways of secretion

  • Merocrine secretion:
    • Most glands are merocrine.
    • Secretory cells are not altered by the process of secretion.
    • Cells produce and store the products at the apex of the cell → stimulation → granules move to the apical surface and release products by exocytosis
    • Examples: pancreas, salivary and sweat glands
  • Apocrine secretion: Secretion product is stored at the apical surface → component of apical surface breaks off → secretion product + part of the cytoplasm released
    • The release of lipid droplets by mammary glands is an example of apocrine secretion.
    • Secretions of the mammary glands are merocrine when proteins are released in milk.
  • Holocrine secretion:
    • Cells synthesize the product and accumulate it in the cytoplasm → undergoes apoptosis → cell is lost in the luminal space
    • Example: sebaceous glands of the skin

Clinical Relevance

  • Diseases of the salivary glands: Sialadenosis, sialadenitis, sialolithiasis, and neoplasms are among the disease processes that affect salivary glands. The common symptom among these conditions is swelling. Salivary glands can swell repeatedly either bilaterally (sialadenosis) or unilaterally (sialadenitis). The parotid gland is commonly affected by infection (mumps). Some glands may even develop stones in the salivary ducts (sialolithiasis).
  • Goiter and thyroid nodule: a swollen thyroid gland due to hyposecretion or hypersecretion of the thyroid hormone or due to inflammation. A goiter can be composed of a single nodule (solitary nodular goiter) or > 1 nodule/swelling/lump in different parts of the gland (multinodular goiter).
  • Hyperparathyroidism: pathologically elevated parathyroid hormone (PTH) levels. Depending on the pathogenesis, hyperparathyroidism can present as 3 forms: primary, secondary, and tertiary hyperparathyroidism. The most frequent cause includes adenomas of the parathyroid gland.
  • Sjogren syndrome: a systemic chronic inflammatory disease characterized by infiltrative lymphocytic inflammation of the exocrine organs.

References

  1. Kierszenbaum, A. L, & Tres, L. L. (2019). Histology and Cell Biology: An Introduction to Pathology (5th ed.). Elsevier.

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