Glandular Epithelium (Exocrine) and Gland Classification
Glandular epithelia form glands, organs specialised to secrete substances. Exocrine glands release their products through ducts to an organ's surface or the body's exterior.
Endocrine glands, in contrast, lack ducts and secrete hormones directly into the bloodstream.
Glands arise from covering epithelia during fetal development, with exocrine glands maintaining a duct connection to the surface. These glands are supported by a stroma of connective tissue, which also encapsulates the gland and separates it into lobules.
Exocrine glands can be classified based on the structure of their secretory portions and ducts:
- Simple glands have unbranched ducts.
- Compound glands have branched ducts.
- Tubular glands have a tubular shape.
- Alveolar (or acinar) glands form small, sac-like structures.
- Compound glands can exhibit combinations of these features, such as branching ducts with multiple tubular, acinar, or tubuloacinar secretory portions.
Exocrine glands also vary in their mechanism of secretion:
Merocrine secretion involves exocytosis of proteins or glycoproteins from membrane-bound vesicles, representing the most common method.
Holocrine secretion entails the complete disintegration of mature cells to release the accumulated product and cellular debris. Sebaceous glands are a prime example.
Apocrine secretion involves the extrusion of the apical cell portion, containing the product along with some cytoplasm and plasma membrane. This mechanism is observed in mammary glands.
The nature of the secretory product further classifies merocrine glands as either serous or mucous:
Serous cells, like those in the pancreas and parotid gland, synthesise mostly non-glycosylated proteins and stain intensely due to their abundant rough endoplasmic reticulum and secretory granules.
Mucous cells, such as goblet cells, produce heavily glycosylated proteins (mucins) that form mucus upon release. These cells stain poorly with routine stains but can be visualised using the periodic acid-Schiff (PAS) method.
Mixed seromucous glands contain both serous acini and mucous tubules, producing a mixture of enzymes and mucus.
Many exocrine glands also contain myoepithelial cells within the basal lamina. These contractile cells aid in propelling secretory products into the duct system.