The glandular epithelium is an epithelial tissue that specializes in the production and secretion of substances, including various types of proteins, enzymes, lipids, polysaccharides, and hormones. Glands are composed of secretion specialized epithelial cells derived from epithelial sheets. The secreted substances are of various types: enzymes, substances with a protective function (e.g. secretion of mucous cells), toxic substances to be eliminated, hormones, etc.
In particular, glands are formed as a result of the proliferation of epithelial cells that penetrate into the underlying connective tissue to form cellular structures with a secretory function. If the gland maintains some relationship with the free surface by means of secretory parts (adenomeres) or an excretory duct, thereby pouring the product of secretion onto the surface of the epithelium of origin, we have an exocrine gland. If, on the other hand, the duct disappears and the secretion is released into the bloodstream, we have an endocrine gland (the product of these glands is generally called a hormone).
Not all glands are of epithelial origin, there are some secreting cells of different nature as the interstitial cells of the gonads, the cells of the theca of the ovarian follicle, neurons that secrete hormones (neuroendocrine cells).
Glandular secretion may be continuous or discontinuous. Continuous secretion is when the secretion is expelled from the cell as it is produced. Examples include endocrine cells that secrete steroid hormones and mucous cells. In discontinuous or rhythmic secretion, the secretion product accumulates in the cell in the form of granules and is later released in response to hormonal, nervous, or chemical stimuli. The cells of the pancreas are a typical example.