Glands in the Skin

Glands in the Skin

The skin contains two types of exocrine glands: sebaceous glands and sweat glands. The sebaceous (oil) glands are simple and branched alveolar glands cov-ering the body, except on the palms and soles. The sweat (sudoriferous) glands are found all over the body except for the lips, nipples, and certain parts of the external genitalia.

Sebaceous Glands

Sebaceous glands (oil glands) are made up of spe-cialized epidermal cells and are primarily located near hair follicles. These glands are largest on the face, neck, and upper chest. They are actually holocrine glands, secreting sebum , which is an oily mixture of fatty material and debris from cells. The central alve-oli cells accumulate lipids until they burst, and the combined lipids and cell fragments make up sebum. The sebum is secreted through small hair follicle ducts, helping to keep both hair and skin pliable and waterproof. The sebum is a mixture of cholesterol, triacylglycerides, proteins, and electrolytes. Sebum inhibits bacterial growth, protecting the keratin of the hair shafts. Sebum is forced out of hair follicles to the skin surface via arrector pili contractions. This lubricates the hair and skin, keeping the hair supple and slowing the loss of water from the skin during times of low environmental humidity. Sebum has a strong bactericidal action. Its secretion is stimulated by androgens, primarily. Hence, sebaceous glands are less active until a human reaches puberty and andro-gen production rises.

Sebaceous follicles are large sebaceous glands that surround hair follicles. Their ducts discharge sebum directly onto the epidermis (FIGURE 6-7). They are found on the face, chest, nipples, back, and exter-nal genitalia. During the final phases of fetal develop-ment, their secretions as well as epidermal cells that have been shed coat the skin surface to form a protec-tive layer. When the sebaceous glands become overac-tive, usually occurring on the scalp, an inflammation may develop around them. This is known asseborrheic dermatitis, which is a common cause of dandruff.

Sweat Glands

Sweat glands consist of a small tube originating as a coil in the deep dermis or superficial subcutaneous­ layers. The coiled portion is lined with sweat-­secreting epithelial cells. Sweat is carried out of the skin by tubes called pores that open at the skin surface. Sweat is made up of 99% water as well as salts, which are primarily­ sodium chloride, ascorbic acid, or vitamin­ C; antibodies;­ and waste products, including urea, ammonia, and uric acid, sweat tastes salty because of its ­electrolytes. Sweat also contains ­dermicidin, which is a peptide that kills microbes. Overall, sweat is a hypotonic filtrate of blood, passing through secre-tory cells via exocytosis. Its composition is based on diet, heredity, and partially certain drugs that are ingested. Sweat has a normal acidic pH of between 4 and 6. Sweating is regulated by the autonomic ner-vous system to prevent overheating. It begins on the forehead, spreading inferiorly to the rest of the body. When sweating is brought about by nervousness or fright (cold sweating), it starts on the palms, axillae, and soles before spreading throughout the body.

The skin contains two types of sweat glands (sudoriferous glands): merocrine sweat glands and apocrine sweat glands. Merocrine (eccrine) glands are the predominant type of sweat glands, responding to body temperature, and are present at birth. They excrete water and electrolytes and also provide protec-tion from hazards in the environment. Adult skin con-tains two to five million merocrine sweat glands. They are found on the forehead, neck, and back, although the palms and soles have the highest numbers. They are simple tubular glands with a coiled appearance. The secretory portion is found in the dermis, whereas the duct opens in a funnel-shaped pore at the surface of the skin. These pores are not the same as the “com-plexion pores,” which are the outlets of hair follicles.

Apocrine glands are sweat glands that become active at puberty and number about 2,000. They are found mostly in the armpits and groin, with the sweat excreted at these places developing a scent as they come into contact with skin bacteria (FIGURE 6-8). This is the basis of body odor. Modified sweat glands include the ceruminous glands of the external ear (which pro-duce earwax) and the mammary glands (which produce milk). Cerumen or earwax is believed to block entry of foreign materials or insects into the ear.

It should be noted that apocrine glands are still actually merocrine glands that produce their product in the same way as eccrine sweat glands. However, they are larger in size, are located in the dermis or hypo-dermis, and empty into hair follicles. Their secretions are not only similar to eccrine glands, but also include proteins and fatty substances. The color of these secretions­ may be white or yellow. The function of apocrine glands is controlled by androgens, activated by sympathetic nerve fibers during stress and pain. In women, they enlarge and recede along with the men-strual cycle. The secretory cells of apocrine glands are surrounded by myoepithelial cells that squeeze them to discharge accumulated sweat into the hair follicles.

1. Describe how hairs grow out of the skin.

2. Distinguish between eccrine and apocrine glands.

3. What is the function of the sebaceous glands?