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Hair
follicle outer root sheath
Each hair follicle has an Outer Root Sheath (ORS), which surrounds
the hair follicle like a sleeve all the way to the bulb, and is
distinct from other epidermal components of the hair follicle.
The outer root sheath is essentially a stratified epithelium
that is contiguous with the epidermis. Differentiation within
the outer root sheath proceeds along an axis that is parallel
to the surface of the skin.
The outer root sheath is thinnest at the level of the bulb and
thickest in the middle portion of the hair follicle, causing the
hair to be more or less eccentric in the follicle. Most of the
follicles have some degree of swelling of the outer sheath on
the side of bulge. The "bulge" region of the outer root
sheath is the site at which the arrector pili muscle is attached.
The pool of progenitor or stem cells for the outer root sheath,
inner root sheath, matrix cells and possibly other structures
of the hair follicle appear to reside
in this bulge area. Although this is a controversial issue, there
is evidence from studies conducted on mouse hair follicles and
vibrissae (hairs which grow around the nostrils or other parts
of the face in many mammals) to support the hypothesis that these
cells are the fundamental source for much of the hair follicle
structure..
The arrector pili muscle is connected to the epidermis at the
other end and 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. When the bulb is bent or curved,
the outer sheath is thicker on the convex side. At the level of
the sebaceous glands and above, the outer sheath is indistinguishable
from the surface epidermis.
Histologically, the outer root sheath is noticeable at the base
of the bulb as a thin rim of pale cells. Closer to the surface,
the cells are larger and columnar, with peripheral palisades.
Within the keratogenous zone, the outer root sheath is expanded
and stratified and the cells contain abundant glycogen and possess
clear cytoplasm. The store of glycogen in the outer root sheath
is probably utilized for a local supply of energy and materials
for bulbar cell activities.
The outer sheath extends all the way to the tip of the bulb,
around which it is composed of two layers of greatly flattened
cells. Just above the bulb the outer sheath attains three layers.
It becomes gradually pluristratified and attains its greatest
thickness a third of the way up the follicle. At this point nearly
all of its cells have vacuoles (fluid–filled bubbles), with
the exception of those in the axial border. The peripheral cells
are tall columnar and oriented perpendicular to the axis of the
follicle. In the upper third of the follicle, none of the cells
of the outer sheath are highly vacuolated and the peripheral cells
are cuboidal.
The cells of the outer root sheath contain clear vacuolated cytoplasm
because of the presence of large amounts of glycogen. The cells
at the periphery are more vacuolated than those in the axial border,
which show a relatively intact cytoplasm. Intercellular bridges,
nodes of Bizzozero (junctions where a cell's cytoplasmic processes
or spines attach to the processes of other cells), and tonofibrils
(a system of fibers found in the cytoplasm of epithelial cells)
are particularly well developed in the more axially located cells.
The cells that rest against Henle's layer (outermost layer of
the inner root sheath) are rich in tonofibrils; about halfway
up the follicle their cytoplasm becomes hyalinized and undergo
partial keratinization.
Keratinization
in the outer root sheath
In the upper suprabulbar
region, the outer root sheath cells possess less glycogen. The
outer root sheath does not keratinize
below the level of the isthmus, in contrast to the inner root
sheath. However, at the level of the isthmus where the inner root
sheath disintegrates, the outer root sheath keratinizes without
forming granules (trichilemmal keratinization), which is similar
to the keratinization of the hair cortex.
Trichilemmal keratinization converts the stratified epithelium
of the outer root sheath into a nuclear keratin without an intervening
keratohyalin layer. (Keratohyalin is a colorless translucent protein
present in the granules of the granular layer of the epidermis).
Trichilemmal keratinization is a distinct type of keratinization
in the hair follicle, not derived from the hair matrix. It occurs
wherever outer root sheath is not apposed to inner root sheath,
i.e. in anagen in the zone of sloughing just below the opening
of the sebaceous duct; in catagen in the trichilemmal sac surrounding
the lower end of the dying hair shaft where it forms the club
of the telogen hair.
At the level of the infundibulum, keratinization of the outer
root sheath changes to normal epidermal keratinization with formation
of the granular cell layer and outermost layer of the epidermis
(stratum corneum). The basal cell layer of the outer root sheath
contains inactive amelanotic (without melanin) melanocytes. However,
the surface epidermis that lines the infundibulum contains active
pigmented melanocytes. The inactive melanocytes in the basal cell
layer can become melanin-producing cells after skin injury. They
proliferate and migrate toward the regenerating upper portion
of the outer root sheath and the epidermis.
Outer
root sheath companion layer
The companion layer is the inner-most layer of the outer root
sheath, and is more prominent in some follicles (especially of
the beard) compared with others. This layer has unique properties
that clearly distinguish it from the rest of the outer root sheath.
It contains cells that are morphologically and biochemically different
from the inner root sheath and outer root sheath. The cells of
the companion layer are more flattened than the other cells of
the outer root sheath and do not contain glycogen. In addition,
a novel human type II cytokeratin, K6hf, is specifically expressed
in the companion layer of the hair follicle. It has been suggested
that the border of the companion layer and the outer root sheath
is the slippage plane between the inner root sheath and outer
root sheath.
Activity
in the outer root sheath
Some mitotic activity (cell division) is seen in the upper part
of the follicle, where the outer sheath blends with the surface
epidermis. This part is similar to the surface epidermis and forms
a keratinized surface layer which is constantly being shed. Lower
down in the follicle the outer sheath is a morphologically static
structure. Both necrotic cells (produced by un-programmed cell
death, as opposed to apoptosis, which is programmed cell death)
and mitotic figures are occasionally found in the outer sheath;
showing the occurrence of both cell death and cell division.
Active hair follicles can be divided into upper and lower halves,
where the lower half is mostly a transient structure that comes
and goes with the proliferation cycle of the hair follicle. During
catagen and subsequent telogen, the outer sheath in the upper
half of the follicle forms at least part of the "hair germ" and
the epidermal sac around the club hair. During wound healing,
interfollicular epidermis can be regenerated from the outer root
sheath of hair follicles.
Outer
root sheath references
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