Modes of cellular secretion

Let’s now consider the three main mechanisms by which exocrine glands release their secretory products:

1. Merocrine (aka, eccrine)- secretory vesicles fuse with the apical plasma membrane to release their products into a duct (e.g. secretion of saliva by salivary glands). Take a look at this specimen of the submandibular gland, which uses the merocrine mode of secretion.
2. Apocrine- secretory products are released at the apical cell surface in small membrane bound vesicles that enter into a duct (e.g. secretion of lipid droplets in the mammary gland). Take a look at this section of lactating breast and the image below. Note the lipid droplets and protein-rich secretions within the lumens. Mammary glands also use the merocrine mode for secretion of milk proteins.
3. Holocrine- secretory products accumulate in the secretory cells, which then die and the entire cell and its secretory products are released into a short duct (e.g. secretion of an oily, lipid-rich product by sebaceous glands in the skin). Check out the sebaceous glands of the skin and the image below to see the holocrine mode of secretion. Lipids are extracted during tissue processing, resulting in pale staining within the secretory cells.

Now let's consider endocrine glands.