4. Type of secratory cells and classification

Types of Secretory Cells and Classification of Glands

Secretory (glandular) epithelial cells perform various functions, including synthesizing, storing, and releasing proteins, lipids, or complexes of carbohydrates and proteins. Glands are specialized organs or tissues that contain epithelial cells responsible for producing and secreting substances. These glands develop from covering epithelia during fetal development through cell proliferation and invagination into the underlying connective tissue. They can be categorized into two main types:

1. Exocrine Glands:
   • These glands remain connected to the surface epithelium, forming tubular ducts lined with epithelial cells that deliver secreted material to its intended location.
   • Exocrine glands can be further classified as:
     • Simple: Non-branched ducts.
     • Compound: Ducts with two or more branches, which can have multiple tubular, acinar, or tubuloacinar secretory portions.
       • Tubular: Secretory portions may be short or long and coiled.
       • Acinar (Alveolar): Rounded and sac-like.

1. Endocrine Glands:
   • These glands lose their connection to the original epithelium and lack ducts. Instead, they release their secretions directly into the bloodstream.

1. • The products of exocrine secretion can be released via different mechanisms:
     • Merocrine Secretion: Most common for protein and glycoprotein secretion, involving exocytosis from membrane-bound vesicles or secretory granules.
     • Apocrine Secretion: The product accumulates at the apical end of the cell, and that portion of the cell is extruded, releasing the product (e.g., lipid droplets from mammary glands).
     • Holocrine Secretion: Cells accumulate large amounts of product, undergo terminal differentiation, and release the product along with cell debris into the gland’s lumen (e.g., sebaceous glands producing lipid-rich material in the skin).

1. Classification Based on the Nature of the Secretory Product:
   • Serous Glands: Synthesize proteins that are non-glycosylated (e.g., digestive enzymes). These cells have well-developed rough endoplasmic reticulum (RER) and Golgi complexes. The cytoplasm stains with basophilic dyes, and secretory vesicles are present in the apical cytoplasm.
     • Examples: Pancreatic cells, parotid salivary glands.

1. • Mucous Glands: Synthesize heavily glycosylated proteins called mucins. When released, mucins become hydrated, forming mucus. These granules stain poorly with eosin due to cell preparation, but oligosaccharides remain, allowing the use of periodic acid-Schiff (PAS) staining.

1. • • Goblet Cells: Stand-alone mucus-secreting cells within the intestinal epithelium. They have a goblet-shaped appearance, with a poorly stained apical cytoplasm and a densely stained basal nucleus.

1. • Seromucous Glands (Mixed Glands): Contain both serous and mucous cells. Most salivary glands exhibit this mixed composition.