Some glands destroy their own cells to release oil
A gland is a cell or organ that makes and releases substances an animal needs—into blood or onto a surface. Built mostly of epithelium with connective scaffolding, glands begin as ingrowths from an epithelial sheet and then branch into tubes or sac-like alveoli.
Every gland starts as an ingrowth from an epithelial sheet. Sometimes it is a tube from the outset; sometimes a solid column of cells hollows out later. As it grows the column may split and sprout, producing a compound gland. Many stop after a few branches, but the salivary glands and pancreas keep dividing into large structures. Branches normally stay separate, except in the liver, where they join into a network. The secretory cells sit at the tips, while the connecting stretches become ducts lined with plainer epithelium, all held in connective tissue and a capsule. Glands that stay tube-shaped are called tubular; those whose secretory ends swell are alveolar or saccular.
Endocrine glands have no duct. They release hormones through a basal lamina into the blood, and that layer can be served by more than a million tiny vessels. The pineal, thymus, pituitary, thyroid and paired adrenal glands belong here, helping hold the body steady. Exocrine glands empty through a duct onto the skin or into the gut and other inner surfaces. Their products can be watery and protein-rich like sweat, sticky and carbohydrate-laden like goblet-cell mucus, or oily like sebum.
Exocrine cells release cargo in three ways. Merocrine or eccrine cells use exocytosis and survive intact, as in the main human sweat glands, tear glands and salivary glands. Apocrine cells shed part of their own body, as in the mammary gland, though so-called apocrine sweat glands may not truly work that way. Holocrine cells disintegrate completely, which is how sebaceous glands such as the meibomian glands of the eyelid work. Any disease of a gland is called adenosis.
Research keeps adding detail. A Polish study of 18 great spotted woodpeckers found their Harderian and lacrimal eye glands to be compound tubuloacinar structures with distinct secretions. In children, infrared imaging of 81 patients showed markedly greater meibomian gland loss in blepharo-keratoconjunctivitis than in allergic conjunctivitis or healthy eyes.
Source: Gland
