Comedones: The plural of comedo, the primary sign of acne, consisting of a dilated (widened) hair follicle filled with keratin squamae (skin debris), bacteria, and sebum (oil). Comedones may be closed or open.
An open comedo has a wide opening to the skin and is capped with a blackened mass of skin debris. It is commonly known as a blackhead.
Acne occurs when sebaceous glands of the skin begin to secrete oil during puberty. These glands are stimulated by male hormones that are produced in the adrenal glands of both boys and girls. The oil lubricates and protects the skin. Under certain circumstances, cells that are close to the openings of the sebaceous glands block the openings. This causes a buildup of oil underneath the skin. Bacteria, which live in everyone's skin but generally mind their own business, feast on this oil, multiply, and cause the surrounding tissues to become inflamed after the follicle ruptures.
If the inflammation is right near the surface, a pustule is formed; if it is deeper, a papule (pimple); deeper still and it becomes a cyst. If the poreis completely closed to the surface, the result is a whitehead. If the pore is wide open, oil becomes oxidized (that is, acted on by oxygen in the air), the oil changes from white to black, and the result is a blackhead. This is how the two types of comedones develop.
Comedo is the Latin word for glutton. The ancients believed that the contents of a comedo were the remains of a gluttonous worm. Note that the preferred plural form of comedo is comedones.