The Hair

THE HAIR

Hair grows out of the epidermis as a keratinized filament. The origin of the hair is the hair

follicle, which is located in the dermis. The hair shaft is that portion of the hair not within the

follicle. Most of the shaft is visible above the surface of the skin. The root of the hair is within

the follicle, ending in the hair bulb. The hair matrix involves those basal cells that continually

divide in order to make the hair grow. There is a connective tissue-derived hair papilla that is

enclosed within the hair bulb and that contains both nerve endings and capillaries in the dermal

layer.

The basal cells that make up the hair shaft divide and push cells outward in the hair root and up

into the hair shaft. This causes the hair to grow. There is an inner medulla and an outer cortex,

which is made from compressed, keratinized cells. The cortex is covered by the cuticle, which is

a hard-packed exterior of dead keratinized cells. Figure 27 shows the basic anatomy of a hair

bulb in the hair follicle:

The texture of the hair (whether it is curly or straight) is determined by the appearance and

structure of the cortex and the medulla. As new cells are placed into the hair bulb, the shaft is

pushed upward, causing hair to grow. Keratin is added to these cells so that they are fully

keratinized by the time they reach the surface of the skin. This makes the visible hair

completely dead and made only of keratin. When hair is cut or shaved, only the visible portion

is removed and the hair follicle remains intact and continually grows hair. Only with electrolysis

and pulling the hair out can the bulb of the hair be destroyed.

In the hair follicle, there are three concentric layers of cells. The internal root sheath cells

surround the root of the hair that is growing. These cells extend to the level of the shaft of the

hair and come from the basal cells of the hair matrix. The external root sheath is an extension

of the epidermis and encloses the hair root. It is made from basal cells near the base of the hair

root. The glassy matrix is the outer connective tissue sheath that covers the hair root,

connecting it to the dermal tissue.

Hair provides protection, sensory input, communication, and thermoregulation of the body.

Hair in the ears, nose, and around the eyes (the eyelashes) defends the body through trapping

of dust particles, microbes, and allergens. The eyebrow hair protects the eyes by trapping

sweat into them. The hair follicle is highly sensitized with sensory receptors so the hair is

important to sensation. Hair is particularly sensitive to air movement disturbances and the

invasion of insects near the skin.

Each hair is connected to the arrector pili muscle—a smooth muscle that contracts by input

from the sympathetic nervous system, causing the hair to “stand erect.” This adds to

thermoregulation by trapping hair that can be warmed by the body. Goose bumps are the

erection of hair in response to cold exposure and are seen in other mammals as well.

There are three phases to hair growth. The anagen phase is when there is rapid division of hair

at the root so that the hair can grow out of the skin. This phase lasts the longest at 2-7 years. It

is followed by the catagen phase, which is the shortest phase of hair growth, lasting just 2-3

weeks. It is a transition phase that marks the end of active growth. The telogen phase is last,

lasting just 2-4 months, during which no new growth occurs. Then another anagen phase