Hair is a slender filament of keratinized cells that grows from an oblique tube in the skin called a hair follicle. The base of the follicle is a tiny pit that penetrates into the dermis, but most of the cells are located in the epidermis. When the follicle produces new hair, these cells become keratinized and undergo pigmentation.
Humans have many types of hair, which vary in length, color, and texture. The earliest hairs, called lanugo, grow during the third or fourth month of embryonic development and are shed before birth. They form a layer over the entire body except the palms of the hands and soles of the feet, where they form a vellus hair layer (fine, downy, unpigmented).
During the next few months of development, a new layer of vellus and terminal hairs develop. The vellus hairs are very fine and unpigmented, and they cover most of the surface of the body except for the scalp, where they form the eyebrows and eyelashes. They also develop in the armpits, genitals, and male facial regions.
Some hairs, called vellus and terminal hairs, are replaced in cycles that last from four to six months. This cycle is triggered by circulating hormones, which alter the way the hairs respond to stress and promote their growth. The terminal hairs are heavier, more heavily pigmented, and sometimes curl.
Most people have between 100,000 and 150,000 hairs, whereas the average for mammals is around 200,000. Each of these hairs is formed by divisions of cells at the base of a hair follicle, and they are bonded together with extracellular proteins.
The follicle is enclosed in a capsule made of epidermis, and the hairs are produced by a mechanism called anagen. This is an asynchronous process, which means that the hairs in different areas of the body develop in waves rather than in a single synchronized pattern.
Each hair is nourished by a small amount of secreted sebum or oil, which keeps it moisturized and supple. If your scalp is oily, you should use a special shampoo to wash the excess sebum away; this will keep your strands soft and healthy.
Your hair density determines how much hair you have on your head, and it can be measured by grabbing a large section of hair from the scalp and seeing how much you can see through it. The thickness of your hair is a good indicator of your density, too.
If you have a thin head of hair, you’ll need more product to keep it looking great, as your strands are likely to look limp and tangled. But if you have medium or thick hair, you can get away with fewer products and still have the same amount of volume.
Several factors contribute to hair health, including diet and environmental conditions. A diet high in antioxidants and fatty acids can protect your hair from damage, while frequent use of a conditioning shampoo can help restore the shine. Having your hair regularly trimmed and colored can improve its health and appearance, too.