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In humans, development of hair follicles first begins on the head, particularly on the eyebrows, lower, and upper lip when an embryo is about 60 days old. From these first few hair follicles the development of the skin and hair follicles gradually expands to cover the entire embryo save for the palms of the hands and soles of the feet. Skin areas further away from the head, especially those on the limb extremities, take up to 110 days to develop embryonic hair follicles and it may be 160 days before all the basic hair structures are complete.
In embryogenesis the establishment of a structure called the dermal
papilla (DP) is vital to the development of all hair follicles. A DP
marks the site for future development of a hair follicle. The DP is a
group of specialized dermal fibroblast cells that begin to clump
together just below the outer skin layer - the epidermis.
Above
the DP, an epidermal plug, or peg, of cells develops and proliferates,
growing into the dermis towards the DP. The peg of cells first grows
straight down into the dermis but soon after develops a slight angle in
its penetration. The angle of growth always points away from the top of
the scalp wherever the hair follicles are on the body. As the epidermal
plug comes into contact with the DP, the growth is still downward. The
epidermal plug almost seems to "push" the DP down into the dermis as it
grows to its full size. The dermal papilla "communicates" with the
epidermal cell peg and encourages the cells to proliferate and mature.
This
gradual differentiation of the hair plug first begins with the
development of three distinct buds of cells, one above the other, on
the same side of the down growth. The cell bud closest to the epidermis
may develop into an apocrine sweat gland. This only happens in a
minority of hair follicles, primarily those on the scalp, genitals and
anus. The cell bud in the middle will gradually develop into the oil
producing sebaceous gland while the cell bud below it forms what is
called the "bulge". The bulge is the area of the hair follicle to which
a small muscle will attach. The "arrector pili" muscle develops
separately from the hair follicle. It grows up towards the epidermis
and down towards the hair follicle bulge region. Eventually it will
attach to both the epidermis at one end and the hair follicle at the
other.
The DP develops into a more identifiable structure of
rounded cells containing organelles vital for product synthesis. The DP
cells encourage the epidermal cells to arrange themselves into
concentric layers above the DP. The layers eventually differentiate
into the hair fiber and the inner and outer root sheaths encasing the
fiber. These layers begin to keratinize higher up the hair follicle
while the cells close to the DP remain undifferentiated and continue to
multiply. The cells push up and away from the DP pushing other cells in
front of them. As the cells mature they are incorporated into the
layers of the hair follicle, become keratinized, and die. Hair can grow
and extend out of the skin before the embryo is born. This first hair
is called lanugo hair. By birth the child may have quite a good crop of
hair, some hair follicles may even have gone through their first hair
cycle.
Diagram
showing accumulation of dermal papilla cells below undifferentiated
epidermis which is then stimulated to grow down into the dermis as a
hair "peg". Interaction between the hair peg and the dermal papilla
cells promotes differentiation into a mature hair follicle.
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