Classification by release method (morphological types)
This classification is based on the mechanism of secretion release and the degree of destruction of the secretory cell during this process.
- Merocrine secretion
This is the most common type. The main feature is that the cell completely preserves its integrity. Mechanism: The mature secretion inside the cytoplasm is surrounded by a membrane, forming special vesicles. These secretory granules gradually move to the edge of the cell, fuse with the plasma membrane, and their contents are released outside. This process is called exocytosis. Examples: Glands of the gastrointestinal tract, salivary glands, and 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, detachment (shedding) of specifically the apical (upper) part of the cell containing the secretion occurs, while the basal part remains in place and subsequently restores the lost volume. Examples: Mammary and sweat glands.
- Holocrine secretion
Characterized by the complete destruction of the secretory cell. Mechanism: The secretion continuously accumulates inside the cell, ultimately leading to its complete destruction. The destroyed cell itself and its contents together become the released secretion. Examples: Sebaceous glands.
Classification by elimination direction
Depending on where exactly 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 can enter directly into the bloodstream, lymph, or intercellular (tissue) fluid. Glands operating on this principle have no excretory ducts.
- External secretion (exocrine). The secretion is eliminated 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 are anatomically connected to the external environment (e.g., the lumen of the digestive tract).
Briefly: answers to topical questions
Which glands are classified as mixed secretion glands?
Mixed secretion glands include the pancreas and gonads.
- Pancreas (pancreas) — has exocrine (pars exocrina pancreatis) and endocrine (pars endocrina pancreatis) parts.
- Gonads — have both exocrine and endocrine parts.
Which cell organelles participate in the formation of secretory vesicles?
The rough endoplasmic reticulum and the Golgi apparatus participate in preparing the secretion and directing it into secretory granules.
- Rough endoplasmic reticulum — ensures the synthesis of protein components of the secretion, as well as the synthesis, transport, and initial sorting of proteins.
- Golgi apparatus — participates in the process of secretion release, synthesizes mucopolysaccharides, ensures glycan modification and sorting of mature cargo in the trans-Golgi network, including into secretory granules.
To memorize the morphological types by the degree of destruction, use the acronym MAG: Merocrine (Minimum destruction — cell is intact), Apocrine (Amputation of the apex — partial destruction), Holocrine (Death of the cell — complete destruction).
Class and exam questions
With which type of secretion does exocytosis occur?
Exocytosis is characteristic of merocrine secretion. Secretory vesicles fuse with the plasma membrane, releasing their contents to the exterior without damaging the cell itself.
How does apocrine secretion differ from holocrine secretion?
During apocrine secretion, only the apical (upper) part of the cell is shed, and the cell can recover. During holocrine secretion, the cell is completely destroyed, dies, and turns into secretion itself.
Where is the secretion released during exocrine secretion?
Exocrine secretion is released onto the surface of the skin, mucous membranes, or into the lumen of the digestive tract organs (i.e., into an environment communicating with the outside).
What else to study on this topic
- Mechanisms of secretory vesicle formation
- The process of vesicle fusion with the plasma membrane (exocytosis)
- Features of cell regeneration following apocrine secretion
- Differences in the composition of intercellular fluid, lymph, and blood during endocrine secretion
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