The Mammalian Hair, A Fascinating Tissue
The hair typically occurs
in the context of a pilosebaceous
unit, which consist of the hair-producing tissue itself,
one or several sebaceous glands that funnel their secretions
into the hair canal, an arrector pili muscle that inserts
in a specific location below the sebaceous gland, and a
pocket of specialized mesenchymal cells known as the dermal
papilla at the base of the unit, in a region known as the
hair bulb. This papilla
is the source of essential instructions during both development
and homeostatis in the adult. Several types of mammalian
hairs can be recognized based on criteria such as size,
curl (shape), color, anatomical location, sensitivity to
androgenic hormones, features of the growth cycle, and more.
Histologically, the cylinder-shaped
hair is comprised of three major epithelial
compartments organized in concentric layers. From outside
in, these are the outer root sheath, inner root sheath,
and hair shaft (see figure above). The outer root sheath
is a stratified epithelium that resembles to and is contiguous
with the epidermis.
It has its own pool of progenitor cells located within the
outermost (basal) layer that contacts the basal lamina.
Within the outer root sheath, differentiation proceeds along
an axis that is parallel to the surface of the skin. A local
swelling of the outer root sheath occurs at the point of
insertion of the arrector pili muscle. This region is known
as the bulge,
and is believed to contain the pool of stem cells for all
the epithelia found in hairy skin tissue. This pool of epithelial
cells plays a key regulatory role during the adult hair
cycle. The inner root sheath features three distinct layers
of epithelial cells known as (from outside in) Henle's,
Huxley's, and a cuticle. The hair shaft itself is also comprised
of three layers, known as the hair cuticle, the hair cortex,
and the medulla (again, from outside in). A single type
of progenitor cell, the matrix
epithelial cells gives rise to all three layers of the
inner root sheath and of the hair shaft, a remarkable occurrence
given that they each are the result of a distinct program
of terminal differentiation. Presumably, the position of
a matrix cell relative to the dermal papilla plays a role
in its choice of a specific differentiation program. The
growing hair is therefore a complex mosaic involving multiple
programs of terminal differentiation, each characterized
by unique biochemical markers.
In mature skin tissue, the
hair undergoes a cycle with phases of active growth interspersed
with phases of rest. Three main stages are recognized: anagen,
during which the follicular unit is maximally elongated,
features a prominent bulb structure, and actively produces
new hair; catagen, a transient phase during which the germinative
(lower) segment of the follicle is destroyed by a massive
wave of apoptosis (with the notable exception of the dermal
papilla), thereby drastically shortening the follicle; and
telogen, a resting phase during which only the permanent
part of the follicle persists. Resting hairs extend down
to a region just below the bulge, the postulated reservoir
of stem cells. The dermal
papilla remains physically proximal to the epithelium
at the base of the follicle throughout the hair cycle. The
size of the papilla, which is largest in full anagen and
becomes small and compact in telogen, is the primary determinant
of the width of the hair being produced. The length of the
hair, on the other hand, is primarily determined by the
relative length of the anagen and telogen phases. Cycle-related
variations also occur in the vasculature, innervation, and
in the composition of the extracellular matrix that surround
the hair tissue. In wild animals, seasoning molting represents
a manifestation of synchrony in the progression of large
groups of hair follicles through the cycle, and is under
endocrine control. In the skin of laboratory rodents as
in human, only the first and second hair cycles are believed
to be truly synchronized. The existence of this fascinating
growth cycle gives the hair a unique status in mammalian
biology, and many of the signaling pathways that play fundamental
roles in development and morphogenesis are also at play
in adult skin.
Mammals feature several types
of hair that are organized according to intricate patterns
of density, spacing and orientation relative to the main
axes of the body, limbs, etc. In rodents, for instance,
the highly specialized vibrissae follicles are restricted
to the anterior segment of the head whereas foot pad skin
feature no hair follicle. In addition, most hair lie at
a characteristic angle relative to the skin surface. These
characteristics are reflected in the organization of every
one of the components making up a pilosebaceous unit, and
are established at an early stage during embryonic development.
