The skin contains two different groups of sweat glands
: apocrine sweat glands
and merocrine sweat glands
. Both gland types contain myoepithelial cells
(from Latin myo-
, "muscle"), specialized epithelial cells located between the gland cells and the underlying basal lamina. Myoepithelial cell contractions squeeze the gland and discharge the accumulated secretions. The secretory activities of the gland cells and the contractions of myoepithelial cells are controlled by both the autonomic nervous system and by the circulating hormones.
Additionally, ceruminous glands, which produce ear wax, and mammary glands, which produce milk, are frequently considered to be modified sweat glands.
Apocrine sweat glands
Sweat glands that release their secretions into hair follicles in the axillae (armpits), around the nipples (areolae
), and in the groin are termed apocrine sweat glands
. The name apocrine
was originally chosen because it was thought that the gland cells used an apocrine method of secretion. Although it is now known that their secretory products are produced through merocrine secretion, the name has not changed. Apocrine sweat glands are coiled tubular glands that produce a viscous, cloudy, and potentially odorous secretion. They begin secreting at puberty; the sweat produced may be acted upon by bacteria, causing a noticeable odor. Apocrine gland secretions may also contain pheromones
, chemicals that communicate information to other individuals by altering their hormonal balance. Some research has indicated that the apocrine secretions of mature women can alter the menstrual timing of other women (this is called the McClintock effect
), though the research methods used have been criticized. The significance of human pheromones, and the role of apocrine secretions in males, remains unknown.
Merocrine sweat glands
A type of sweat gland that is far more numerous and widely distributed than apocrine sweat glands is the merocrine sweat glands
, also known as eccrine sweat glands
. The adult integument contains around 3 million merocrine glands. They are smaller than apocrine sweat glands, and they do not extend as far into the dermis. Palms and soles have the highest numbers; estimates are that the palm of the hand has about 500 glands per square centimeter (3000 glands per square inch). Merocrine sweat glands are coiled tubular glands that discharge their secretions directly onto the surface of the skin.
The clear secretion produced by merocrine glands is termed sweat, or sensible perspiration. Sweat is mostly water (99 percent), but it does contain some electrolytes (chiefly sodium chloride), metabolites, and waste products. The presence of sodium chloride gives sweat a salty taste.
Merocrine glands have three primary functions:
- Thermoregulation. Sweat cools the surface of the skin and reduces body temperature. This cooling is the primary function of sensible perspiration, and the degree of secretory activity is regulated by neural and hormonal mechanisms. When all of the merocrine sweat glands are working at maximum, the rate of perspiration may exceed a gallon per hour, and dangerous fluid and electrolyte losses can occur. For this reason athletes in endurance sports must pause frequently to drink fluids.
- Excretion. Merocrine sweat gland secretion can also provide a significant excretory route for water and electrolytes, as well as for a number of prescription and nonprescription drugs.
- Protection. Merocrine sweat gland secretion provides protection from environmental hazards by diluting harmful chemicals and discouraging growth of microorganisms.........
Control of glandular secretions
Sebaceous glands and apocrine sweat glands can be turned on or off by the autonomic nervous system, but no regional control is possible, meaning that when one sebaceous gland is activated, so are all the other sebaceous glands in the body. Merocrine sweatglands are much more precisely controlled, and the amount of secretion and the area of the body involved can be varied independently. For example, when one is nervously awaiting an exam, one's palms may begin to sweat.