In the embryo, the skin begins as a single layer of epidermal
stem cells. Soon after, as mesenchymal cells populate the skin
to form the underlying collagenous dermis, morphogenesis of
the hair follicle begins (Schmidt-Ullrich and Paus, 2005
). Specialized
dermal cells organize in small clusters directly beneath the
epidermal layer, stimulating the overlying epithelial stem cells
to grow downward and produce a hair follicle. The follicle is
contiguous with the epithelium; both are separated from the
dermis by a basement membrane rich in extracellular matrix and
growth factors synthesized and deposited largely but not solely
by epithelial cells. As the follicle grows down, it assumes
the shape of a rod several cell diameters wide. The inner layers
begin to differentiate into concentric cylinders to form the
central hair shaft (HS) and the surrounding channel, the inner
root sheath (IRS). An inductive mesenchymal cluster called the
dermal papilla (DP) becomes a permanent part of the follicle
base (Jahoda et al., 1984
; Kishimoto et al., 2000
). It travels
with the epithelial downgrowth and becomes enveloped by the
hair bulb. The follicle becomes fully mature as its bulb nears
the bottom of the dermis. At this point (in mouse back skin
around postnatal day 6 or P6), the proliferative cells (matrix)
at the follicle base continue to divide, producing progeny cells
that terminally differentiate to form the growing hair that
exits the skin surface.
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