The total number of hair follicles for an adult human is estimated
at 5 million with 1 million on the head of which 100,000 alone
the scalp. In humans, the only external regions of
skin devoid of hair follicles are the palms of the hands and soles
of the feet. The basic hair follicle structure remains essentially
the same throughout the range of mammalian species with modifications
for specialized functions. The hair follicle can be recognized
a separate entity within the skin with formation and maintenance
based on interaction between dermal and epidermal components.
Under the influence of the DP, epidermal cell differentiation
during anagen produces a keratinized hair fiber and associated products.
The source epidermal cells, called matrix cells, that lie in the
immediate vicinity of the dermal papilla are a living, actively
proliferating group of cells which differentiate and become keratinized
to form the hair cortex (Co) and surrounding hair cuticle (Hc) of
the hair shaft at the center of which is situated the medulla (M).
Cells Around the hair shaft comprise the inner root sheath (IRS)
which can be divided into three layers the cuticle (Cu), Huxley
layer (Hu) and Henle layer (He) based on structure, patterns of
keratinization and incorporation of a product called trichohyalin.
The IRS breaks down at the level of the sebaceous gland to leave
only the hair cortex and surrounding cuticle to protrude above the
the main differentiated layers in a mature anagen hair follicle.
The hair follicle penetrates the dermal layer of the skin
composed of fibroblast cells and collagen connective tissue
interspersed with blood vessels, sweat glands and sensory
nerves. The bulb region sits in the subcutaneous (adipose
fat) tissue layer.
|Image showing the
layers in a hair follicle from the outside (left) to the center
It is the dermal papilla (DP) which directs and dictates the embryonic
generation of a hair follicle and it also retains this instructive
ability throughout the life of the hair follicle. The DP presents
as a healthy "pear" shape in normal hair follicles. As the name
suggests, derived from the dermis mesenchyme the DP consists of
a highly active group of cells shown to be capable of inducing follicle
development from the epidermis and production of hair fiber (Oliver
1966a, Oliver 1966b, Oliver 1967).
The DP consists of a small group of fibroblast cells derived from
the mesoderm. The cells are held close to the base of the epidermal
derived cells that produce the hair fiber and root sheaths but there
is a thin layer, called the basement membrane (or basement lamina,
or glassy membrane) that separates the DP cells from the hair fiber/sheath
cells. In other words, the basement membrane provides a physical
dividing line between cells descendant from embryonic ectoderm (epidermis)
and embryonic mesoderm (dermis). This physical barrier has a role
to play in our immunological protection. Holding the DP cells in
place is a capsule that surrounds the DP cells in a cup and extends
up the sides of the hair follicle to the epidermis. The whole follicle
structure sits on a pad of fibrous tissue called the Arao-Perkins
body. Nerve fibers and blood vessels penetrate through small gaps
in the base of the hair capsule and invade into the DP area.
The bigger the DP, the more cells it has, then the thicker the
hair fiber that the hair follicle produces. The DP cells are very
active with lots of cytoplasm when the hair follicle is producing
a hair fiber although the DP cells do not multiply and proliferate
unlike the hair producing cells above the DP. When a hair follicle
is not producing a fiber the DP cells lose much of their cytoplasm
and become inactive.
Oliver et al revealed that the removal of the DP stops
hair growth but that the lower third of the dermal sheath is capable
of supplying new cells for regeneration of a new DP by infiltrating
and transforming at the site of the original DP with subsequent
hair follicle regrowth (Oliver 1966a, Oliver 1966b, Oliver 1966c,
Jahoda 1992). With removal of more than the lower third of a hair
follicle, reformation of a DP is unable to occur and the hair follicle
is effectively permanently destroyed. The DP cells retain their
embryonic functional abilities and are able to induce new hair fiber
growth in mature, adult skin when implanted into previously deactivated
hair follicles and in close association with ORS epidermal cells
(Oliver 1967, Horne 1986).
DP cells can also interact with adult epidermis to induce the
development of new hair follicles (Jahoda 1990). In the established
hair follicle the DP cells act in conjunction with epidermal cells
via mechanisms similar to those in embryogenesis to permit hair
follicle cycling through hair production and resting phases. DP
cells are almost unique in maintaining their embryogenic regenerative
properties in adults making them potentially attractive for investigation
with a view to gaining an insight on organ/limb regeneration and
the bulb region of a hair follicle.
The hair fiber is the core part of any hair follicle. Epidermal
derived cells close to the DP remain undifferentiated cells, called
matrix cells, that focus on multiplying and proliferating to produce
more cells. Those cells made in the center of the hair follicle
are destined to become part of the hair fiber and are called cortex
(cortical) cells. As the cells multiply the constant stream of production
pushes the cells upwards towards the skin surface. As they move
up the hair follicle they begin to differentiate into particular
cell types. The cortex cells change from a round into a flattened
appearance. They are squeezed together into layers (lamella). If
the hair follicle contains melanocyte cells then melanin pigment
is incorporated into the cortex cells. These cortex cells become
keratinized and harden. As they do so it becomes impossible for
the cells to function properly and the cells die. The keratinized
cells are then pushed away from the hair bulb region and upwards
as new cells come in behind. The cortex cells are now part of the
dead keratinized fiber.
Some large hair follicles have a central strand of cells that
are loosely organized and not packed together. This tube in the
very center of the hair fiber is called the medulla.
Around the outside of hair fiber we see a cuticle. The cuticle
is made up of more keratinized cells but they arrange themselves
in a slightly different way to cortex cells. As the cuticle cells
are produced, they lay over the cortex cells and flatten into an
overlapping roof tile fashion. Cuticle cells become progressively
flatter as they get older. As with cortex cells, when they keratinize
the cell can no longer function properly and dies.
The Outer Root Sheath (ORS) is distinct from other epidermal components
of the hair follicle being continuous with the epidermis. The "bulge"
region in the ORS is the site at which the arrector pili muscle
is attached. The arrector pili muscle is connected to the epidermis
at the other end. This is the muscle that makes hair stand erect
and produces goose bumps in your skin when you are cold. The contraction
of the muscle pulls on both the hair to make it erect and pulls
on the skin making a bumpy surface.
The bulge region is believed to be the storage area for hair follicle
stem cells. Hair follicles go through a cycle of growth and rest
(below). With each renewed attempt to produce hair fiber, the hair
follicle must obtain a source of cells to form the matrix cell population
that make hair fibers. The source of these cells is believed by
some dermatologists to be the bulge region. Other dermatologists
suggest that stem cells are not present in the bulge region at all
and that new matrix cells are obtained from the root sheath.
Also extending from the ORS is the sebaceous gland. It consists
of a few cells focused on production of oils (lipids). These cells
are large with their cytoplasm filled with vacuoles containing lipid.
The cells are often divided into several lobes of the sebaceous
gland connected together by a sebaceous duct. The duct has a single
opening into the tube where the hair fiber sits.
The ORS surrounds the hair fiber and inner root sheath until deep
into the dermis. Just above the bulb region containing the dermal
papilla the ORS tapers and ends so the ORS does not entirely cover
the hair fiber and inner root sheath. The ORS consists of several
layers of cells that can be identified with unique ultrastructural
The inner root sheath (IRS) is produced by matrix cells sitting
above the hair follicle. While those matrix cells in the center
of a hair follicle proliferate and produce the hair fiber and cuticle,
the matrix cells towards the periphery of a hair follicle proliferate
and produce the IRS. As with cells making up the hair fiber, the
cells destined to be IRS gradually become differentiated and keratinized
as they are pushed away from the bulb region. As keratinization
occurs, the cells die and form the IRS surrounding and protecting
the development of the hair fiber. The IRS can be subdivided into
several layers. Adjacent to the hair fiber we see a single cell
thick IRS cuticle layer that closely interdigitates with the hair
fiber cuticle layer. The next IRS layer is called the Huxley layer
that may consist of up to four cell layers. Outside of this there
a single cell layer called the IRS Henle layer. The Henle layer
runs adjacent to the ORS layer.
The products of the sebaceous gland are believed to help break
down the IRS. The IRS does not extend out of the hair follicle.
Only the hair fiber itself protrudes above the skin surface. The
IRS disintegrates at the level of the sebaceous duct opening. The
debris from the IRS break down mixes with the sebaceous gland oils
and the result is sebum. Sebum is an oily solid that is expelled
from the hair follicle and normally scraped or washed away in our
general skin care habits. Sometimes overproduction of sebum can
build up in the hair follicle and harden into plugs of material
called comedones (blackheads). Sebum is a nutrient rich material
and an ideal habitat for bacteria. Colonies of bacteria, particularly
one called Propionibacterium acnes may proliferate in hair
follicles using sebum as a nutrient supply. The bacteria feeding
on the sebum changes its composition that makes it irritating. The
immune system responds to the antigens in the modified sebum and
the bacteria resulting in inflammation and acne.