Skin Glands
The glands of the skin are of two major types: the oil, or sebaceous, glands and the sweat, or sudoriparous, glands. The oil glands are scattered over the skin surface except on the palms and soles. They are situated in the dermis, mostly in direct association with hairs so that their ducts open into the hair follicles. Oil glands are from 0.004 inch in diameter and consist of several pear-shaped lobules of specialized epithelium surrounded by a fibrous capsule.
The product of these glands, called sebum, is formed by fatty degeneration of the epithelial cells within the lobules and is collected in a common gland cavity, from which it is discharged into the upper portions of the hair follicles. Meibomian glands are sebaceous glands not associated with hairs. They are found on the male and female genital organs, about the lips and mucous membranes of the cheeks, and around the anus and areolae of the nipples.
Special large sebaceous-type glands on the inner surface of the prepuce, called Tyson’s glands, produce smegma. Other large sebaceous glands, called the glands of Zeiss, are found along the free borders of the eyelids associated with the eyelashes.
Sweat glands are of two major types: eccrine and apocrine. The eccrine glands are present in large numbers over the entire skin, extending deep into the corium and sometimes even deeper. Each eccrine gland consists of a single, long, epithelium-lined, blind tubule. The lower secretory portion is coiled into a ball-shaped conglomerate, while the upper ductal portion spirals up through the epidermis to the surface sweat pore.
Besides the secretory and ductal epithelium, the sweat glands have an outer layer of contractile myoepithelial cells, which are in turn surrounded by a thin, ground-substance basement membrane and by nerves and blood vessels. The diameter of the lumen or cavity of the eccrine sweat tubule varies from 5 to 20 microns (1 micron = approximately 1/25,000 inch).
The apocrine glands are found only over a few areas, such as the armpits, perineal and pubic regions, about the nipples, and occasionally on the abdomen and chest. The ducts of the apocrine glands usually open into follicles rather than independently onto the skin surface. They have about ten times the diameter of the eccrine ducts. The number of glands of both types in the skin has been estimated at between 2 and 15 million, with a total combined tubular length of about 8 miles.
The ceruminal glands of the ear canals, which secrete cerumen or earwax, and Moll’s glands of the eyelids can be considered as specialized types of apocrine glands.