Every cell may be regarded as secretory because all of them can release products to the extracellular space by exocytosis. However, some of them have become specialized in this cellular process and thus they are referred as secretory cells. Secretory cells form glands, which are large structures arranged in tubules and acini, with secretion as their main function. During the embryonic development, glands are derived from covering epithelium, and they become exocrine or endocrine glands depending on where they release their products. Sometimes, secretory cells are found within covering epithelium. These are the intraepithelial glands, which can be unicellular or multicellular.
Exocrine glands release their products to internal cavities or to exterior surfaces of the body. Secretory cells release their products in different ways. For example, caliciform cells release directly to the epithelial surfaces, whereas multicellular exocrine glands have an excretory duct that connect the secretory cells and the free surface of the epithelium. Glands with excretory ducts show more complex morphology and are classified according to the topology of their excretory ducts and the organization of their secretory parts (see figure).
Secretory cells release their products by three different modes of secretion: a) merocrine, when products are released by typical exocytosis; b) Apocrine, when the apical domain of secretory cells are broken in pieces and released along with vesicles that contain the secretory products; c) Holocrine, when the cell membrane of the secretory cells becomes disorganized and the whole cytoplasmic content is released. (see figure)
Exocrine secretory cells release many types of molecules that carry out different functions. According to the chemical content of their secretions, glands are classified as mucous exocrine glands and serous exocrine glands. Mucous exocrine glands release glycosaminoglycans, proteoglycans, and glycoproteins, which cover the internal body surfaces, whereas the serous glands mainly release enzymes that aid in the digestion of food. However, there are other types, such as eccrine sweat glands that release mostly water and electrolytes, or as apocrine sweat glands that release water and hormones.
Endocrine glands do not have ducts and they release their products, such as hormones and proteins, to the extracellular space. From here, these products cross the blood vessel walls and are delivered to different parts of the body. There are many endocrine glands which are unicellular and, all together, comprise the diffuse endocrine system. These endocrine unicellular glands can be intraepithelial cells located inside the gastrointestinal or respiratory cell epithelium, or between the cells of the hypophysis. Those non intraepithelial endocrine cells are arranged as cords or glomeruli, surrounded by a dense network of capillaries. The molecules to be secreted are no exocytosed immediately after they are synthesized, but they are stored in vesicles until the release signal arrives. Sometimes these products can be extracellularly stored in follicles, which are compartments formed by secretory cells. This type of follicle can be found in the thyroid gland. In the adrenal gland, several types of endocrine cells that release different types of secretory products are located together.
In the pancreas, exocrine and endocrine glands coexist. The exocrine part releases secretory products to the gut, whereas the endocrine part forms the Langerhans islets.
Some cells release substances within the surrounding tissue but not into the bloodstream. Such products diffuse through the extracellular matrix and act on neighboring cells. This is called paracrine secretion and is present in epithelial cells, but also in other types of cells.