8.10
Exocrine Glands: Methods of Secretion
Exocrine glands are those that release their secretions through ducts. Based on their mode of secretion, they can be classified into merocrine, apocri…
Exocrine glands are classified as merocrine, apocrine, or holocrine based on how they secrete their glandular products.
Cells of merocrine glands, such as salivary gland cells, release their products in secretory vesicles via exocytosis while the cells remain intact and prepare for additional secretions.
Apocrine gland cells, such as those in the axillary sweat glands, accumulate their secretory products right below the surface. When the apex of the cell pinches off, the secretory granules are released together with a small amount of cytoplasm.
The cells then regrow and repair their apices before further product release.
In holocrine glands, such as sebaceous glands of the skin, superficial epithelial cells become packed with secretory vesicles. When the cell bursts, the product is released.
The continuous division of underlying layers of stem cells replaces the ruptured cells.
Q1: What are the three main types of exocrine gland secretion?
Exocrine glands are classified into three types based on secretion method: merocrine, apocrine, and holocrine. Merocrine glands release products via exocytosis while cells remain intact. Apocrine glands release secretory products along with a portion of cytoplasm. Holocrine glands accumulate products until the entire cell bursts, releasing everything at once.
Q2: How do merocrine glands release their secretions?
Merocrine glands, such as salivary glands and eccrine sweat glands, release secretory products enclosed in vesicles through exocytosis at the cell's apical surface. The cell membrane remains intact after secretion, allowing the cell to prepare for additional product release without significant damage.
Q3: What happens to cells during apocrine secretion?
During apocrine secretion, the apex of the cell pinches off, releasing secretory granules along with a small amount of cytoplasm. The cell then regrows and repairs its apex before releasing additional products. Examples include axillary sweat glands and mammary glands, which release fatty secretions this way.
Q4: Why is holocrine secretion destructive to gland cells?
In holocrine secretion, cells accumulate secretory products until the entire cell bursts, destroying it completely. Sebaceous glands of the skin use this method to release oils. Underlying stem cells continuously divide to replace ruptured cells, maintaining gland function despite the loss of individual secretory cells.
Q5: How do merocrine and apocrine secretion differ in cell damage?
Both merocrine and apocrine glands cause minimal cell damage because the nucleus and Golgi apparatus remain intact after secretion, allowing the cell membrane to reform. In contrast, holocrine secretion destroys the entire cell. This difference enables merocrine and apocrine glands to continue producing and secreting products repeatedly.
Q6: Can a single gland use multiple secretion methods?
Yes, some glands use combination secretion methods. Lactating mammary glands release lipid droplets into milk through apocrine secretion while simultaneously releasing the protein casein through merocrine secretion, demonstrating that a single gland can employ different secretory pathways for different products.
Q7: What role do stem cells play in holocrine gland function?
Underlying layers of stem cells in holocrine glands continuously divide to replace cells that rupture during secretion. This regeneration maintains gland structure and function despite the destructive nature of holocrine secretion, ensuring sebaceous glands and similar structures can sustain long-term oil production.