They are classified into simple and compound types. 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.