Modes of Secretion
Secretory cells of exocrine glands release their products into ducts in three different ways. The mode of secretion can be classified as merocrine, apocrine, or holocrine.
Cells that secrete products via the merocrine method form membrane-bound secretory vesicles internal to the cell. These are moved to the apical surface where the vesicles coalesce with the membrane on the apical surface to release the product. Most glands release their products in way.
In those glands that release product via the apocrine method, the apical portions of cells are pinched off and lost during the secretory process. This results in a secretory product that contains a variety of molecular components including those of the membrane. Mammary glands release their products in this manner.
The third type of secretory release, holocrine, involves death of the cell. The secretory cell is released and as it breaks apart, the contents of the cell become the secretory product. This mode of secretion results in the most complex secretory product. Some sweat glands located in the axillae, pubic areas, and around the areoli of the breasts release their products in this manner. Sebaceous glands also are of this type.
Regardless of gland type, structural complexity, or mode of secretion, epithelia are the secretory cells of all glands. Epithelia also form the ducts that connect the glands to the surface. Remember this as glandular structures found in tissues can be identified as clusters of tightly packed cells with very little intercellular space(an epithelial characteristic). When ducts are present and cut in longitudinal or cross-section, epithelial cells are also seen making up these structures. Simple cuboidal epithelia are the most typical type found in the body and ducts of exocrine glands.
Here is a view of simple cuboidal cells of a duct in cross-section and longitudinal section!