(c) Bulb. An enlarged, onion-shaped structure called a bulb is at the
base of each hair follicle. The bulb contains the papilla of the hair, an indentation filled
with loose connective tissue. The papilla of the hair contains many blood vessels and
provides nourishment for the growing hair. There is a region in the bulb called the
matrix. When older hairs are shed, matrix cells produce new hairs by cell division. The
replacement occurs within the same follicle that lost a hair.
Figure 1-3. Principal parts of a hair.
(d) Arrector pili. The arrector pili are bundles of smooth muscle that
extends from the dermis of the skin to the side of the hair follicle. Normally, hair is at an
angle to the surface of the skin. Under stress, from fright or cold, for example, the
arrector pilorum muscles contract and pull the hairs into a vertical position. The result is
"goosebumps" or "gooseflesh" because the muscle contraction has made the skin
around the hair shaft rise a little.
(4) Color. A substance called melanin is responsible for hair color.
Melanocytes in the matrix of the bulb of the hair follicle form melanin. Hair pigment
exists in three colors: yellow, brown, and black. Yellow comes from pheomelanin,
brown pigment from brown melanin, and black pigment from black melanin.
color. Graying hair is the result of the loss of hair pigment. The cause is thought to be
a progressive inability of the melanocytes to make tyrosinase, the enzyme necessary for
the synthesis of melanin. Air in the medullary shaft causes white hair.