Exocrine glands are those that release their secretions through ducts. Based on their mode of secretion, they can be classified into merocrine, apocrine, and holocrine.
Merocrine Secretion
Merocrine secretion is the most common type of exocrine secretion. The secretions are enclosed in vesicles and moved to the cell's apical surface, where the contents are released by exocytosis. For example, mucous, a watery secretion rich in the glycoprotein mucin, is a merocrine secretion. The eccrine glands that produce and secrete sweat are another example of merocrine secretions.
Apocrine Secretion
In apocrine secretion, the accumulated secretory contents pinch out along with the apical portion of the cell and are released to the outside of the cell. For example, fatty secretions produced by the sweat glands of the axillary and genital areas are apocrine secretions. Both merocrine and apocrine glands continue to produce and secrete their contents with little damage caused to the cell because the nucleus and Golgi regions remain intact after secretion, enabling the cell membrane to reform after secretion.
Some glands use a combination of these two secretory methods to release their products. For example, the lactating mammary glands release lipid droplets into milk by apocrine secretion, while the protein casein is released into milk by merocrine secretion.
Holocrine Secretion
In contrast, in holocrine secretion, the cell accumulates its secretory products and releases them only when it bursts, thus leading to the rupturing and destruction of the entire cell. Cells from the surrounding tissue then differentiate into new gland cells to replace cells lost during secretion. The sebaceous glands that produce the oils on the skin and hair are holocrine glands.
This text is partially adapted from Openstax, Anatomy and Physiology 2e, Section 4.2: Epithelial Tissues
