Classification by Release Mechanism (Morphological Types)
This classification is based on the mechanism of secretion discharge and the degree of destruction of the secretory cell during the process.
- Merocrine Secretion
This is the most common type. The key feature is that the cell completely preserves its integrity. Mechanism: The mature secretion inside the cytoplasm is enclosed by a membrane, forming specialized vesicles (secretory granules). These granules gradually migrate to the cell periphery, fuse with the plasma membrane, and release their contents to the exterior. This process is called exocytosis. Examples: Glands of the gastrointestinal tract, salivary glands, and certain endocrine glands.
- Apocrine Secretion
This type involves partial destruction of the cellular structure. Mechanism: The mature secretion accumulates in the upper third of the cell. Then, precisely the apical (upper) portion of the cell containing the secretion pinches off, while the basal part remains intact and subsequently restores its lost volume. Examples: Mammary glands and certain sweat glands.
- Holocrine Secretion
Characterized by the complete death of the secretory cell. Mechanism: Secretion continuously accumulates inside the cell, ultimately leading to its complete destruction. The destroyed cell and its contents together constitute the discharged secretion. Examples: Sebaceous glands.
Classification by Direction of Discharge
Depending on where the produced secretion goes, two fundamental directions of secretion are distinguished:
- Internal secretion (endocrine). The secretion is released exclusively into the internal environment of the body. Substances enter the bloodstream, lymph, or interstitial (tissue) fluid directly. Glands operating by this principle lack excretory ducts.
- External secretion (exocrine). The secretion is discharged into the external environment. This implies release onto the body surface (e.g., skin or mucous membranes) or into the lumen of internal organs and cavities that anatomically communicate with the external environment (e.g., the lumen of the digestive tract).