Grow from special epithelial cells at the base of the nail in the region called the nail root (Fig. 5.4). These cells become keratinized as they grow out over the nail bed. The visible portion of the nail is called the nail body. The cuticle is a fold of skin that hides the nail root. Ordinarily, nails grow only about 1 millimeter per week. The pink color of nails is due to the vascularized dermal tissue beneath the nail. The whitish color of the half-moonshaped base, or lunula, results from the thicker germinal layer in this area.
Glands
The glands in the skin are groups of cells specialized to produce and secrete a substance into ducts.
Sweat Glands
Sweat Glands
Sweat glands, or sudoriferous glands, are present in all regions of the skin. There can be as many as 90 glands persquare centimeter on the leg, 400 glands per square centimeter on the palms and soles, and an even greater number on the fingertips. A sweat gland is tubular. The tubule is coiled, particularly at its origin within the dermis. These glands become active when a person is under stress. Two types of sweat glands are shown in Fig. 1.4. Apocrine glands open into hair follicles in the anal region, groin, and armpits. These glands begin to secrete at puberty, and a component of their secretion may act as a sex attractant. Eccrine glands open onto the surface of the skin. They become active when a person is hot, helping to lower body temperature as sweat evaporates. The sweat (perspiration) produced by these glands is mostly water, but it also contains salts and some urea, a waste substance. Therefore, sweat is a form of excretion. Ears contain modified sweat glands, called ceruminous glands, which produce cerumen, or earwax
Most sebaceousglands are associated with a hair follicle. These glands secrete an oily substance called sebum that flows into the follicle and then out onto the skin surface. This secretion lubricates the hair and skin, and helps waterproof them. Particularly on the face and back, the sebaceous glands may fail to discharge sebum, and the secretions collect, forming whiteheads or blackheads. If pus-inducing bacteria are also present, a boil or pimple may result. Acne vulgaris, the most common form of acne, is an inflammation of the sebaceous glands that most often occurs during adolescence. Hormonal changes during puberty cause the sebaceous glands to become more active at this time.
Eccrinel sweat glands are widely distributed over the body especially on the forehead, back, palms, and, soles. These glands are formed before birth and function in evaporative cooling (Fig. 1.4a and 1.4b).
Apocrine sweat glands are much larger than the eccrine glands. They are found in the axillary and pubic regions where they secrete into hair follicles. Apocrine glands are not functional until puberty and their odoriferous secretion is thought to act as a sexual attractant.
Mammary Glands
Apocrine sweat glands are much larger than the eccrine glands. They are found in the axillary and pubic regions where they secrete into hair follicles. Apocrine glands are not functional until puberty and their odoriferous secretion is thought to act as a sexual attractant.
Mammary Glands
The mammary glands are located within the breasts. A female breast contains 15 to 25 lobes, which are divided into lobules (Fig. 1.5). Each lobule contains many alveoli. When milk is secreted, the milk enters a duct that leads to the nipple. Cells within the alveoli produce milk only after childbirth in response to complex hormonal changes occurring at that time.
