HAIR
A hair fiber
is composed of a tightly fused meshwork of cells filled with the hard protein called
keratin. Hair growth is similar to the growth of the epidermal layer of the
skin. Deeplying cells in the hair root produce keratinized cells
that move upward through hair follicles (sacs within which each hair fiber
grows). Melanocytes (see Figure 16-2) are located at the root of the hair
follicle, and they donate the melanin pigment to the cells of the hair fiber. Of
the 5 million hairs on the body, about 100,000 are on the head. They grow about
½ inch (1.3 cm) per month. Cutting the hair has no effect on its rate of
growth.
NAILS
Nails are
hard keratin plates covering the dorsal surface of the last bone of each toe
and finger. They are composed of keratinocytes that are cemented together
tightly and can extend indefinitely unless cut or broken. A nail grows in
thickness and length as a result of division of cells in the region of the nail
matrix, which is at the base (proximal portion) of the nail plate. Fingernails
grow about 1 mm per week, which means that they can regrow in 3 to 5 months.
Toenails grow more slowly than fingernails; it takes 12 to 18 months for
toenails to be replaced completely. The lunula is a semilunar (half-moon–shaped)
whitish region at the base of the nail plate. It generally can be seen in the
thumbnail of most people and is evident to varying degrees in other
fingernails. Air mixed in with keratin and cells rich in nuclei give the lunula
its whitish color. The cuticle, a narrow band of epidermis (layer of keratin),
is at the base and sides of the nail plate. The paronychium is the soft tissue
surrounding the nail border. Figure illustrates the anatomic structure of
a nail.
Nail growth
and appearance commonly alter during systemic disease. For example, grooves in
nails may occur with high fevers and serious illness, and spoon nails
(flattening of the nail plate) develop in iron deficiency anemia. Onycholysis (onych/o
= nail) is the loosening of the nail plate with separation from the nail bed. It may occur with infection of the nail and is often seen in
psoriasis.
GLANDS
Sebaceous Glands
Sebaceous
glands are located in the dermal layer of the skin over the entire body, with
the exception of the palms (hands), soles (feet), and lips. They secrete an
oily substance called sebum. Sebum, containing lipids, lubricates the skin and
minimizes water loss. Sebaceous glands are closely associated with hair
follicles, and their ducts open into the hair follicle through which the sebum
is released. Figure shows the relationship of the sebaceous gland to the hair
follicle. The sebaceous glands are influenced by sex hormones, which cause them
to hypertrophy at puberty and atrophy in old age. Increased production of sebum
during puberty contributes to blackhead (comedo) formation and acne in some people.
Sweat Glands
Sweat glands
(the most common type are eccrine sweat glands) are tiny, coiled glands found
on almost all body surfaces (about 2 million in the body). They are most
numerous in the palm of the hand (3000 glands per square inch) and in the sole
of the foot. As illustrated in Figure, the coiled eccrine sweat gland
originates deep in the dermis and straightens out to extend up through the
epidermis. The tiny opening on the surface is a pore. Sweat, or
perspiration, is almost pure water, with dissolved materials such as salt
making up less than 1% of the total composition. It is colorless and odorless.
The odor produced when sweat accumulates on the skin is caused by the action of
bacteria on the sweat. Sweat cools the body as it evaporates into the air.
Perspiration is controlled by the sympathetic nervous system, whose nerve
fibers are activated by the heart regulatory center in the hypothalamic region
of the brain, which stimulates sweating.
A special
variety of sweat gland, active only from puberty onward and larger than the ordinary
kind, is concentrated in a few areas of the body near the reproductive organs
and in the armpits. These glands (apocrine sweat glands) secrete an odorless
sweat, containing substances easily broken down by bacteria on the skin. The
bacterial waste products produce a characteristic human body odor. The
milk-producing mammary gland is another type of apocrine gland; it secretes
milk after the birth of a child.
