A gland is an organ or tissue that produces and releases substances for a specific purpose. These specialized structures are formed primarily from epithelial tissue and perform diverse functions, such as regulating metabolism or aiding digestion. Scientists categorize glands using several classification methods based on the destination of the secreted product, the physical organization of the cells, and the specific biological process used for release.
Classification Based on Secretion Destination
The most fundamental classification method determines where the substance produced is released, separating glands into two major functional groups: exocrine and endocrine.
Exocrine glands use a ductal system to transport secretions to a specific location, typically an epithelial surface, body cavity, or lumen. Their products, such as mucus, sweat, saliva, and digestive enzymes, are delivered outside of the bloodstream. Common examples include salivary glands, eccrine sweat glands on the skin, and sebaceous glands. The exocrine part of the pancreas releases digestive juices into the small intestine through a duct to assist with food breakdown.
Endocrine glands are ductless. Their secretions, known as hormones, are released directly into the surrounding interstitial fluid and then diffuse into the bloodstream. Once in the blood, these hormones travel throughout the body to reach distant target cells that possess the appropriate receptors. The thyroid gland, which controls metabolism, and the pituitary gland are prime examples.
Classification Based on Structural Complexity
Glands are also categorized based on their physical arrangement and complexity. The simplest form is the unicellular gland, consisting of a single, isolated secretory cell scattered within an epithelial sheet. The most common example is the goblet cell, found lining the respiratory and intestinal tracts, whose function is to produce and secrete mucus.
The majority of glands are multicellular, composed of many cells working together. Multicellular glands are divided based on the branching pattern of their excretory duct. A simple gland has a single, unbranched duct leading to the secretory unit (e.g., intestinal glands). A compound gland features a duct that branches repeatedly before connecting to the secretory units (e.g., large salivary glands).
The final structural classification focuses on the shape of the secretory end pieces. If the secretory portion is tube-shaped, the gland is tubular (e.g., sweat glands). If the secretory unit is flask-shaped or spherical, it is called alveolar or acinar (e.g., exocrine pancreas and sebaceous glands). Glands with both shapes are classified as tubuloacinar, such as the submandibular salivary gland.
Classification Based on Secretory Mechanism
The third classification system is based on the biological mechanism cells use to discharge their secretory product. This method primarily applies to exocrine glands and defines three distinct processes: merocrine, apocrine, and holocrine secretion.
Merocrine Secretion
Merocrine secretion is the most common method, involving the cell releasing its product through exocytosis. Secretory vesicles fuse with the plasma membrane to expel the contents without causing any damage to the cell. This continuous, gentle process is used by most sweat glands, salivary glands, and the acinar cells of the pancreas. The cell remains intact and can continue production immediately.
Apocrine Secretion
Apocrine secretion involves the accumulation of the product in the apical portion of the cell. The cell then pinches off this apical portion, enclosed in a small piece of membrane and cytoplasm, to release the substance. This results in a small loss of cellular material, but the main part of the cell, including the nucleus, remains viable. This mechanism is characteristic of mammary glands when secreting milk lipids and specialized sweat glands.
Holocrine Secretion
Holocrine secretion is the most destructive method, where the entire secretory cell accumulates the product and then disintegrates completely to release the substance. The whole cell, along with its contents, becomes the actual secretion. Because the cell is destroyed, new cells must continuously differentiate from underlying layers to replace the lost ones. This mechanism is utilized by sebaceous glands, which secrete the oily substance sebum.