Glandular Epithelia

Glandular Epithelia
Glandular epithelia are formed by cells specialized to secrete. The molecules to be secreted are generally stored in the cells in small membrane-bound vesicles called secretory granules.
Glandular epithelial cells may synthesize, store, and secrete proteins (ex, in the pancreas), lipids (ex, adrenal, sebaceous glands), or complexes of carbohydrates and proteins (ex, salivary glands). Mammary glands secrete all three substances. The cells of some glands have low synthetic activity (ex, sweat glands) and secrete mostly water and electrolytes transferred into the gland from the blood.
The epithelia that form glands can be classified according to various criteria. Unicellular glands consist of large isolated secretory cells and multicellular glands have clusters of cells. The classic unicellular gland is the goblet cell in the lining of the small intestine or respiratory tract.
Exocrine glands retain their connection with the surface epithelium, the connection taking the form of tubular ducts lined with epithelial cells through which the secretions pass to the surface. Endocrine glands have lost their connection to the surface from which they originated during development. These glands are therefore ductless and their secretions are picked up and transported to their sites of action by the bloodstream rather than by a duct system. Multicellular glands, whether exocrine or endocrine, also have connective tissue in a surrounding capsule and in septa that divide the gland into lobules. These lobules then subdivide, and in this way the connective tissue separates and binds the glandular components together.
Exocrine glands have a secretory portion, which contains the cells specialized for secretion, and ducts, which transport the secretion out of the gland. The morphology of these components allows the glands to be classified and summarized as follows:
1- Ducts can be simple (unbranched) or compound (with two or more branches).
2-Secretory portions can be tubular (either short or long and coiled) or acinar (round or globular).
3-Either type of secretory portion may be branched.
4-Compound glands can have tubular, acinar, or tubuloacinar secretory portions.
Exocrine glands are also classified functionally according to the way the secretory products leave the cell.
1-Merocrine secretion (sometimes called eccrine) involves typical exocytosis of proteins or glycoproteins. This is the most common mode of secretion.
2-Holocrin esecretion involves the cell filling with secretory product and then the whole cell being disrupted and shed. This is best seen in the sebaceous glands of skin.
3-In an intermediate type, apocrine secretion, the secretory product is typically a large lipid droplet and is discharged together with some of the apical cytoplasm and plasmalemma. Exocrine glands with merocrine secretion can be further categorized as either serous or mucous according to the nature of the proteins or glycoproteins secreted and the resulting staining properties of the secretory cells