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Hair
follicle inner root sheath
The cells of the inner root sheath are interlocked with those
of the cuticle of the hair, firmly anchoring the hair in the follicle.
The inner root sheath must grow at the same rate as the hair,
or faster as it molds and guides the shaft in its passage outward.
The outer side of the Henle's layer of the inner root sheath must
slide over the axial border of the outer root sheath, which is
stationary. These two layers have very smooth surfaces at the
interphase, which facilitates the movement of the inner root sheath.
The cells of Henle's layer are keratinized immediately after they
rise from the matrix and slide easily against the partially keratinized
axial cells of the outer sheath.
Hair
follicle inner root sheath structure
The inner root sheath of the hair follicle is a layered structure
that extends from the base of the bulb to the isthmus. This structure
features three distinct layers of epithelial cells which are known
as Henle's layer, Huxley's layer, and the inner root sheath cuticle
(from outermost to innermost). A single type of progenitor cell,
the matrix epithelial cells gives rise to all the layers of the
inner root sheath. The cells move upward and laterally from the
matrix and become arranged into three concentric layers in the
upper bulb.
The cells in all three layers of the inner sheath are identifiable
by their large eosinophilic cytoplasmic inclusions called trichohyaline
granules. Trichohyaline is a structural protein that is produced
and retained in the cells of the inner root sheath and medulla
of the hair follicle. Keratin fibers are also produced in the
inner root sheath.
Henle's layer
Henle's layer is one cell layer thick and the cells of Henle's
layer acquire trichohyalin granules immediately after they have
arisen from the matrix. When they first appear, the granules are
very small and cuboids in shape. As the cells move upward, they
become elongated vertically and form large homogeneous globules
and parallel rods in the upper bulb. In this region the cells
undergo hyalinization, a form of degeneration. The nuclei of the
cells become indistinct and finally disappear. Unlike the cells
in other keratinizing tissues, those of the inner sheath do not
decrease appreciably in volume.
Huxley's layer
Huxley's layer is two to four cell layers thick and undergoes
cornification above Henle's layer at the region known as Adamson's
fringe. Cornification is the conversion of squamous epithelial
cells into a keratinized horny material. In the cells of Huxley's
layer, trichohyalin granules first appear at the top of the bulb,
at which point the cells of Henle’s layer are completely
hyalinized. Some cells without trichohyalin granules send lateral
cytoplasmic processes across Henle’s layer and penetrate
as far as the axial layer of the outer sheath. These specialized
Huxley cells have been termed as Flugelzellen, and represent living
bridges of cytoplasm across the dead Henle's layer. All nutrients
or energy sources from the outer sheath to the hair root come
across these bridges. About midway up in the follicle, the cells
of Huxley's layer are hyalinized.
Inner root sheath cuticle
The inner root sheath cuticle is one cell layer thick and the
cells of the inner sheath do not acquire trichohyalin granules
until they are about halfway up in the follicle. They are the
smallest cells in the follicle and can be recognized even in the
lower bulb. Above the bulb, the nuclei of the cuticle cells become
elongated vertically. They remain small and compressed up to nearly
halfway in the follicle, where they begin to show a few small
trichohyalin granules. At this level the cells are somewhat flattened,
and the proximal edges become slightly dislocated axially so that
they overlap the distal ends of the cells below them. These cells
are in opposition to the cells of cuticle of the hair shaft which
are oriented upwards, thereby anchoring the hair shaft in place.
Shortly after they acquire trichohyalin granules, the cuticle
cells become hyalinized and their nuclei fade away. Above the
middle of the follicle, the three layers of the inner sheath all
become fused and hyalinized. The fully cornified inner root sheath
anchors and directs the growth of the emerging hair shaft.
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Formation
of the inner root sheath
Matrix keratinocytes rapidly propagate at a rate of proliferation
that is one of the highest of any tissue in the body. As the cells
of the matrix divide and differentiate, they form columns of cells
that become the hair shaft and inner root sheath. The cells of
the future hair shaft are positioned at the apex of the dermal
papilla and subsequently form the medulla (in terminal hairs),
cortex and hair shaft cuticle.
Lateral to these are the cells which in due course become the
inner root sheath: the immediately adjacent single layer becomes
the cuticle of the inner root sheath, the next two to four cell
layers become Huxley's layer, and the next single layer becomes
Henle's layer. The actual line of keratinization varies with each
layer of the inner root sheath and hair shaft: Henle's layer is
the first to keratinize, and is then followed by the inner root
sheath cuticle, and then finally Huxley's layer and the hair shaft.
While in the first push the tip of the hair is sheathed by the
inner root sheath, the inner sheath is non existent in the pilosebaceous
canal. There is a suggestion that the disappearance could be caused
by chemical changes culminating in re-absorption or dissipation
of the inner sheath and that this could be brought about by an
enzyme (keratinase) possibly contained in the outer root sheath.
The hair shaft, being slightly more acidic and covered by an
epicuticle, consisting of lipids and carbohydrates, is protected
from enzymatic action. At the place where the inner sheath begins
to be dissipated are concentrated large amounts of AS esterase
which may also play a role in keratinolytic activity. At the level
of cornification of the inner root sheath, the organelles (compartments
within cells) and trichohyalin granules degenerate.
Inner
root sheath references
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