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Hair
follicle infundibulum
For our understanding of the anatomy of the hair follicle, it
can be divided into 3 regions: the lower segment (bulb and suprabulb),
the middle segment (isthmus), and the upper segment. The upper
segment extends from the entrance of the sebaceous gland duct
to the follicular orifice and is called the infundibulum.
Therefore the infundibulum is that part of the hair follicle
that extends from the epidermis to the opening of the sebaceous
gland duct. The epithelium of the infundibulum is continuous with
the epidermis, thus its cells can regenerate the epidermis and
replenish it after wounding or injury.
This uppermost part of the follicle is the first structure in
the developing skin to contain keratohyalin granules and keratin.
It shares a similar mode of keratinization with the interfollicular
epidermis, with a granular layer underlying cornified squamous
cells in a basket-weave pattern. Although the epithelium is similar
in appearance to the epidermis, its cells appear to have a higher
proliferative capacity and they can regenerate the after its removal.
The lumen of the infundibulum normally contains the hair shaft,
keratin material and sebum. Sometimes the lumen may also contain
secretions from apocrine glands (Apocrine glands are outgrowths
of the superior portions of pilosebaceous units), lipophilic yeast
(yeast having an affinity for lipids), bacteria, and parasitic
mites called Demodex.
Cysts with infundibular differentiation are called epidermal
inclusion cysts or infundibular cysts. These are simple cysts
lined by stratified squamous epithelium with all layers present.
Lesions are solitary or multiple with cornified cells loosely
packed in the cyst lumen.
Hair
follicle isthmus
The isthmus is the shortened segment of the hair follicle, extending
from the attachment of the erector pili muscle (bulge region)
into the entrance of the sebaceous gland duct. It is a border
zone peculiarly devoid of specific features.
The vitreous membrane does not cover the isthmus. After this
segment has been hollowed out by the advancing hair the posterior
wall consists of two or three rows of flattened cells, the nuclei
of which exhibit a characteristic change of direction. Those of
the lower part slant from the inside up and out and the cells
resemble those of the bulge.
The nuclei of the upper part of the isthmus slant downward and
those in between are quite flat and parallel to the axis of the
follicle. The cells are more or less devoid of glycogen and contrast
with those of the bulge and lower follicle. The transition is
not quite so abrupt along the anterior wall, but the cells also
are less high and contain much less glycogen. The nuclei of the
central cells of the isthmus undergo pyknosis (a thickening, especially
degeneration of a cell in which the nucleus shrinks in size and
the chromatin condenses to a solid, structure less mass) before
the tip of the root sheath reaches the area.
The outer root sheath of the isthmus, in contrast to the epithelium
of the infundibulum, has no granular layer and its cells have
pale cytoplasm, an indication of an increased amount of glycogen.
The isthmus has a distinctive corrugated, compact, eosinophilic
keratinization called trichilemmal keratinization, a sharp contrast
to the basket-weave keratinization of the indundibulum.
Unlike the epidermis, the outer root sheath remains non-keratinized
throughout its entire lower portion where it abuts the inner root
sheath and only begins to keratinize in the isthmus, immediately
above the bulge area. At or above the insertion of the arrector
pili muscle, the fully keratinized inner root sheath disintegrates.
Above this point, there is trichilemmal keratinization of the
outer root sheath.
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 that is 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. Cysts, which are thought to
arise from this area have similar differentiation and are called
trichilemmal cysts, pilar cysts, or isthmus catagen cysts. Isthmus
cystic lesions are lined by a stratified squamous epithelium without
a granular layer.
The inferior most portion of the isthmus contains the bulge area
of the hair follicle. The bulge is composed of a biochemically
distinct population of keratinocytes that possesses the characteristics
of epithelial stem cells. Keratinocytes in the bulge area are
relatively undifferentiated structurally. They are normally slow
cycling, but can be stimulated to proliferate transiently. Moreover,
they are multipotent, giving rise to several different cell types
including epidermal keratinocytes, sebaceous gland cells, and
at least seven different types of epithelial cells in the lower
follicle. Although the bulge area is cylindrical and often difficult
to identify in adult anagen hair follicles, it becomes quite prominent
during the telogen phase of the hair cycling process. The insertion
of the arrector pili muscle generally approximates the location
of the isthmus.
Hair
follicle suprabulbar region
The suprabulbar area of the follicle is the region below the isthmus
and above the bulb. This region is comprised of three layers
from outermost to innermost: outer root sheath, inner root sheath,
and hair shaft.
The inner root sheath typically 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).
However, in the suprabulbar region, these three layers completely
keratinize and become indistinguishable. The cells at the periphery
of the outer root sheath are columnar and walled. The outer root
sheath cells progressively contain less glycogen in the superior
portion.
Hair follicle infundibulum, isthmus, and suprabulbar region references
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