Excretory Duct

Excretory duct, a component of exocrine glands, serving to secrete substances onto the surface of the body or into internal cavities. In simple glands, tubular or vesicular, the E. d. represents a more or less short segment, or neck, and is lined with a single layer of epithelium (glands of the stomach, straight tubules in the kidney), and sometimes with a double layer (sweat glands). In complex tubular and grape-like glands, the E. d. represents a system of tubules, beginning from the glandular sections and merging into larger ducts, forming in whole a tree-like branching (as in the salivary glands) or plexiform (pancreas), when small ducts from all sides flow into a central duct. The walls of such ducts are formed by a connective tissue membrane and lined with epithelium, which in large sections is multilayered, most often cylindrical, and in small sections transitions to a single-layered cylindrical and cuboidal. The terminal branches may be lined with flattened epithelium, which in the pancreas penetrates even into the glandular cells, forming so-called centro-acinar cells. In some cases (salivary glands) sections with high cylindrical epithelium having a rod-like striation at the base (secretory sections) may be found in the E. d. Glands with internal secretion are devoid of E. d. In unicellular glands of invertebrates and in some glandular cells of vertebrates (salivary, gastric glands, liver), there are intracellular excretory ducts or capillaries (see also Glands, Cell).