Human Histology: Secretory Epithelia

• Unit 2 focuses on epithelial tissue in Human Histology.
• The unit aims to differentiate types of secretory epithelia based on structure and products they secrete.
• The course includes epithelial cells, domains, covering/lining epithelia, secretory epithelia and glands, transport across epithelia, and renewal of epithelial cells.

Secretory Epithelia

• Epithelial cells that produce and secrete macromolecules are present in epithelia or specialized organs (glands).
• Secretory cells synthesize, store, and release macromolecules.
• Secretory cells secrete water and electrolytes.
• Scattered secretory cells (unicellular glands), like goblet cells, reside within simple cuboidal, columnar, and pseudostratified epithelium.
• It is beneficial for the respiratory tract epithelium to have goblet cells.
• Glands form from covering epithelia.

Exocrine Glands

• Exocrine glands possess a general structure comprising of stroma, capsule, parenchyma, ducts, lobules, and secretory acini.
• Simple glands have unbranched ducts.
• Compound glands have ducts from several secretory units converge into larger ducts.
• Simple tubular glands feature elongated secretory portion and a short or absent duct, as seen in the colon's mucous glands.
• Branched tubular glands have several long secretory parts joining to drain into one duct, such as glands in the uterus.
• Coiled tubular glands have a long, coiled secretory portion, exemplified by sweat glands.
• Acinar (or Alveolar) glands have a rounded, sac-like secretory portion, like the small glands in the urethra.
• Branched acinar glands present multiple sac-like secretory parts entering the same duct, similar to sebaceous glands.
• Compound tubular glands have elongated coiled secretory units, with ducts converging to form larger ducts (e.g., submucosal mucous glands of Brunner).
• Compound acinar (alveolar) have multiple sac-like secretory units with small ducts merging at a larger duct (e.g., exocrine pancreas).
• Compound tubuloacinar glands contain both tubular and acinar secretory units, with their ducts converging into larger ducts (e.g., salivary glands).
• Exocrine glands contain a surface epithelium, secretory cells, a duct, and secretory portions (simple tubular).

Exocrine Gland Secretion

• Merocrine glands release contents via exocytosis while the cell remains unharmed.
• Holocrine glands disintegrate cells, with their contents becoming the secretion.
• Apocrine glands pinch off of apical portion of secretory cell.
• Acne vulgaris arises from blocked ducts in sebaceous glands due to excessive sebum and keratin production during puberty.

Exocrine Glands Secretions are categorized as

• Mucous
• Serous
• Seromucous or mixed.

Exocrine Glands: Myoepithelial Cells

• Exocrine glands contain myoepithelial cells.

Endocrine Glands

• Endocrine glands do not have myoepithelial cells.
• Endocrine glands are specialized for steroid or protein hormone synthesis.
• Proteins are released by exocytosis.
• Steroids are released via diffusion through the cell membrane.

Transport Across Epithelia

• Ions and water are either absorbed or secreted.
• Transcytosis involves receptor-mediated transport or receptor-independent transport.

Renewal of Epithelial Cells

• Most epithelial cells undergo continuous renewal.
• Epithelial cells in complex glands (e.g., liver) belong to a stable cell population with limited mitotic activity, dividing only when stimulated.
• Mitotic activity of self-maintaining adult stem cells produces replacement cells.

This lesson explores secretory epithelia, epithelial cells specialized for producing and secreting macromolecules, and their presence in glands. It covers unicellular glands like goblet cells and exocrine glands, including their structure and components.