Glandular epithelium is composed of cells that are specialized to produce and secrete substances. Such cells occur most commonly within columnar and cuboidal epithelia, and one or more of these cells constitute a gland. Glands that secrete their products directly into tissue fluids or the blood without ducts are called endocrine glands. Those that secrete their products into ducts that open onto internal or external open surfaces are called exocrine glands.
Exocrine glands are classified according to whether the duct system is unbranched (simple) or branched (compound), and according to the distribution of secretory versus ductile epithelium. If the duct and secretory portions are of uniform diameter, the gland is tubular. If the secretory cells form a dilated sac at the end of the duct, it is acinar, and if the secretory cells extend uniformly from the duct then form a sac, it is called tubuloacinar.