Taking a closer look at hair, nails, and glands

Now hair this

Your body has millions of hair follicles, about the same number as the chimpanzee, humanity’s closest evolutionary relative. (Although we don’t know who counted all of them!) Like chimps, humans have hairless palms, soles, lips, and nipples. Unlike a chimpanzee, most of your hair is lightweight and fine. The hair on your head is coarser and longer to help hold in body heat. Puberty brings about a surge of sex hormones that stimulate hair growth in the axillary (armpit) and pelvic regions and, in males, on the face and neck. Women with hormonal imbalances can develop facial hair, too. Turn to Chapter 8 for more about hormones.

A hair arises in a hair follicle, a small tube made up of epidermal cells that extend down into the dermis to take advantage of its rich blood supply. Like the epidermis, cells at the bottom of the hair follicle, called the papilla, continually divide to produce new cells that are added to the end of the hair and push the older cells up through the layers of the epidermis. On their way up and out, the hair cells become keratinized. By the time you see it, the hair is mostly completely flattened, dead cells full of keratin. The curvature of the follicle causes the shape of the exposed hair, ranging from tight curls to stick straight.

Hair goes through cycles of growth and dormancy. When the cells of the papilla begin to divide again, the hair falls out. This is why the length of your hair maxes out. The hairs on your head live about three to four years before you shed them, and eyelashes live about three to four months before falling out. People don’t go bald overnight. Baldness (called alopecia) occurs when follicles go dormant do not reactivate.

 

It will make your hair stand on end

Each hair follicle has a tiny arrector pili muscle. When these smooth muscles contract, the previously bent hair shafts become erect. This movement causes the epidermis to be pushed to the side, and a goose bump appears. When you’re cold or frightened, the sudden tightening of the arrector pili muscle causes the hair to rise up, and as a result, air is trapped between the hair and the skin. When you’re cold, the trapped air acts as insulation — the same way a coat keeps you warm. When you’re frightened, the hairs standing on end serve to make you look scary to whatever is scaring you, a form of signaling used by many mammals.

 

Nailing nails

Your fingernails and toenails lie on a nail bed (not to be confused with a bed of nails). At the back of the nail bed is the nail root. Just like skin and hair, nails start growing near the blood supply that lies under the nail bed, and the cells move outward at the rate of about 1 millimeter per week. As they move out over the nail bed, they become keratinized. (See Figure 4-2.)

At the bottom of your nails is a white, half-moon-shaped area called the lunula. (Lun- is the Latin root for moon, as in lunar.) The lunula is white because this is the area of cell growth. In the nail body, the nail appears pink because the blood vessels lie underneath the nail bed. But many more cells fill in the area of growth. This layer is thicker, and you see white instead of pink.

 

Nothing’s bland about glands

Glands in the skin make and secrete substances that are transported to your body’s outer surface. The contraction of tiny muscles in the gland accomplishes this secretion. The two main types of skin glands are sudoriferous glands (sweat glands) and sebaceous glands (oil glands).

 

Sudoriferous glands

Your body contains two types of sudoriferous glands. Eccrine sweat glands are distributed all over the skin. These glands open to the skin’s surface, and when you’re hot, they release sweat to reduce body temperature by a process of evaporative cooling. When the sweat, which is mostly water, evaporates, it takes the heat with it.

Apocrine sweat glands start to develop during puberty and connect to the hair follicles of the armpits and groin. Apocrine sweat contains a milky white substance and 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 one woman can influence the menstrual cycle of other women who live with her.) Apocrine glands become active when you’re anxious and stressed, as well as when you’re sexually stimulated. Bacteria on the skin that digest the milky white substance produce unpleasantly odiferous byproducts.

The milk-secreting glands in mammary tissue are thought to have evolved from apocrine sweat glands.

 

Sebaceous glands

Sebaceous glands secrete an oily substance called sebum into hair roots. Besides wreaking havoc with teenage facial pores, sebum has physiological functions. It helps maintain your hair in a healthy state, which is important in regulating body temperature. It flows out along the hair shaft, coating the hair and the epidermis, forming a protective, waterproof layer. Sebum prevents water loss to the outside. Sebum also helps to protect you from infection by making the skin surface an inhospitable place for some bacteria.

In the watery environment of the amniotic sac, the human fetus produces a thick layer of sebum, called the vernix caseosa. Ear wax (cerumen) is a type of sebum produced by specialized cells in the ear canal.

 

Vitamin D

Sunlight has a nasty way of damaging skin, but you need a regular dose of sunlight to keep your bones healthy. Bones need vitamin D to develop healthy new bone cells. Insufficient vitamin D can lead to a condition called rickets, which results in soft, curved bones that can’t support the body’s weight.

Skin cells contain a molecule that’s converted to vitamin D when struck by ultraviolet rays (UV) in sunlight. The vitamin D leaves the skin and goes through the bloodstream to the liver and kidneys, where vitamin D is converted to the hormone calcitriol. Calcitriol then circulates through the entire body and regulates the physiology of calcium and phosphorus, important minerals for the development and maintenance of healthy bones. (Scientists are discovering that vitamin D plays important roles in keeping your body healthy in other ways, too, such as mood regulation. To find out more, pick up "Vitamin D For Dummies" by Dr. Alan Rubin [Wiley].)

The use of sunscreen and sunblock products can prevent the UV radiation from reaching the skin cells and initiating vitamin D synthesis.

Just a few minutes of sunshine a day is all that’s needed for your skin to make adequate amounts of vitamin D to keep your bones healthy. Anything beyond that amount of exposure is risky.