Epidermal appendages are specialized structures derived from
epidermis. Hair follicles arise by follicular keratinocyte growth
into the dermis during the eighth to ninth week of fetal life. Hairs
are classified into three types: lanugo, vellus, and terminal. Lanugo
hairs are normally present only in fetal life, and most
are shed in utero during the first portion of the last trimester.
Widespread lanugo at birth is therefore an indicator of significant
prematurity. Terminal hairs are the coarse, longer
hairs found predominantly on scalp, eyebrows, and, after puberty,
on axillae and pubis. Vellus hairs are the finer,
shorter, less pigmented hairs found on other portions of hair-bearing
skin. Hair follicles undergo a cycle of several years of growth
(anagen phase), followed by a brief involution phase (catagen phase),
and a 3-month shedding (telogen phase). Normally, hair follicle
growth is asynchronous, and the duration of the anagen phase, and
hence hair length, is genetically determined. Stressful life events,
however, such as illness or giving birth, may abruptly shunt follicles
into the resting stage, producing 2 to 4 months later a marked shedding
of hair (telogen effluvium). Hair growth is androgen-dependent to
a varying extent, depending on body site, regulated by the conversion within
the hair follicle of testosterone to its active metabolite, 5α-dehydrotestosterone
via the enzyme 5α-reductase. Inhibitors of this
enzyme are commercially available to reduce scalp hair loss in androgenetic
alopecia.