178
UNIT TWO
Hair Follicles
A healthy person loses from twenty to 100 hairs a day as part
of the normal growth cycle of hair. A hair typically grows for
two to six years, rests for two to three months, then falls out.
A new hair grows in its place. At any time, 90% of hair is in
the growth phase.
Hair is present on all skin surfaces except the palms,
soles, lips, nipples, and parts of the external reproductive
organs; however, it is not always well developed. For exam-
ple, hair on the forehead is usually very ±
ne.
Each hair develops from a group of epidermal cells at
the base of a tubelike depression called a
hair follicle
(ha
¯r
fol
ı˘-kl). This follicle extends from the surface into the dermis
and contains the hair
root,
the portion of hair embedded in
the skin. The epidermal cells at its base are nourished from
dermal blood vessels in a projection of connective tissue (hair
papilla) at the deep end of the follicle. As these epidermal cells
divide and grow, older cells are pushed toward the surface.
The cells that move upward and away from the nutrient sup-
ply become keratinized and die. Their remains constitute the
structure of a developing
hair shaft
that extends away from
the skin surface. In other words, a hair is composed of dead
epidermal cells
(f gs. 6.7
and
6.8)
. Both hair and epidermal
cells develop from the same types of stem cells.
Usually a hair grows for a time and then rests while it
remains anchored in its follicle. Later, a new hair begins to
grow from the base of the follicle, and the old hair is pushed
outward and drops off. Sometimes, however, the hairs are
not replaced. When this occurs in the scalp, the result is
baldness, described in Clinical Application 6.2.
Genes determine hair color by directing the type and
amount of pigment that epidermal melanocytes produce.
Dark hair has more of the brownish-black
eumelanin,
while
blonde hair and red hair have more of the reddish-yellow
pheomelanin.
The white hair of a person with
albinism
lacks melanin altogether. A mixture of pigmented hairs and
unpigmented hairs usually appears gray.
Nail bed Nail plate
Lunula
FIGURE 6.6
Nails grow from epithelial cells that divide and become
as keratinized as the rest of the nail.
(b)
Hair
follicle
Hair root
Region of
cell division
Adipose
tissue
(a)
Hair shaft
Pore
Hair root
(keratinized
cells)
Arrector pili
muscle
Sebaceous
gland
Hair follicle
Region of
cell division
Hair papilla
Eccrine
sweat gland
Dermal
blood
vessels
FIGURE 6.7
Hair follicle. (
a
) A hair grows from the base of a hair
follicle when epidermal cells divide and older cells move outward and
become keratinized. (
b
) Light micrograph of a hair follicle (175×).
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