Glands in the skin serve a variety of functions, ranging from thermoregulation to the production of breast milk. There are several different kinds of glands in the skin, including sweat glands, ceruminous glands, sebaceous glands, and mammary glands.
Sweat glands, also called sudoriferous glands, are tubular structures consisting of a coiled base and a long tube that leads to the surface of the skin. Sweat is produced by cells in the coiled base of the gland, which is located in the lower layers of the dermis or in the hypodermis. Sweat is excreted through a long tube that opens on the surface of the skin. There are two types of sweat glands⎯eccrine glands and apocrine glands.
An eccrine gland is a gland in the dermis that secretes sweat made mostly of water and plays an important role in thermoregulation. The sweat secreted by eccrine glands is clear and odorless. Eccrine sweat aids the body in thermoregulation through evaporative cooling on the surface of the skin. When body temperatures rise, hormonal signals activate eccrine glands to increase their output. In addition, eccrine sweat is relatively basic, which helps protect the body from pathogens. Finally, sweat from eccrine glands serves an excretory function; significant amounts of water and electrolytes, minerals such as sodium and potassium, exit the body through sweat.
An apocrine gland is a gland in the dermis that produces sweat containing lipids and proteins. Apocrine glands are located in places like the armpits and around the genitals. Although apocrine sweat is initially odorless, bacteria on the surface of the skin digest the lipid components. This produces a distinctive, pungent body odor. In humans, apocrine glands do not serve a thermoregulatory function. They are activated during times of stress, pain, or sexual arousal, and they are primarily involved in pheromone signaling, such as when chemicals are released to attract the opposite sex.
A sebaceous gland is a gland that produces oil to keep the skin supple and prevent the growth of microorganisms. Sebaceous glands are joined to hair follicles. They are found on all parts of the body except for the palms of the hands and soles of the feet. These glands are located in the dermis. They secrete an oily substance called sebum, which helps to lubricate and waterproof the skin and hair. Some sebaceous glands are located near hair follicles and secrete sebum along the shaft of the hair. Others secrete sebum directly onto the skin. Sebum combines with sweat to produce an oily sheen on the skin that aids thermoregulation. Together with sweat, sebum also forms the body's acid mantle, an acidic film on the skin that protects the body from pathogens.
The skin contains other more specialized glands as well. A ceruminous gland is a modified apocrine gland in the external ear whose secretions mix with sebum to form earwax. It protects, lubricates, and waterproofs the ear canal and eardrum, and it also kills bacteria and traps foreign particles.
A mammary gland, a specialized gland that secretes milk, is another variety of specialized skin gland. These glands respond to hormonal signals produced during pregnancy to make milk for the baby. They consist of clusters of hollow cavities called lobules, each of which drains into a duct with an opening in the nipple. The secreted milk contains proteins, fats, and carbohydrates that meet all of the nutritional needs of the offspring. It also includes antibodies that protect infants from infection in the first weeks and months of life.