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Hair
follicle bulb
The bulb is the deep, bulbous portion of the follicle that surrounds
the dermal papilla. While the dermal papilla is responsible for
directing and dictating the embryonic generation of a hair follicle,
the bulb is the structure of actively growing cells, which eventually
produce the long fine cylinder of a hair.
The bulb of the hair follicle can be divided into two regions:
a lower region of undifferentiated cells, and an upper region
in which the cells become differentiated to form the inner sheath
and the hair. A line across the widest part of the papilla separates
the two regions at the critical level, and it is known as the
line of Auber. Below the critical level lies the matrix, or germination
center of the follicle, where every cell is mitotically active
and the
dermal papilla. From the matrix, cells move to the upper part
of the bulb, where they increase in volume and become elongated
vertically. Some of the cells in the upper bulb still show some
mitotic activity, but these are too few to account for much of
the growth of the hair.
The upper bulb can he divided into four parts.
- Immediately above the critical level, in the wide portion
of the bulb is the pre-elongation region, where the cells align
themselves vertically and become slightly larger.
- Above this
region, where the diameter of the bulb is narrower and the cells
become conspicuously elongated, is the cellular
elongation region.
- Immediately above, in the cortical pre-keratinization
region, distinct, fine fibers or fibrils stainable with basic
dyes can
be seen. The pre-keratogenous zone refers to the post-mitotic
and differentiating cells located above the critical line of
Auber, and extending to the region where the inner root sheath and hair
shaft cornify and lose their keratohyalin granules. Keratohyalin
is a colorless translucent protein present in the granules of
the granular layer of the epidermis.
- Further up is the keratogenous
zone where the cells become hyalinized, and the keratin of
the hair is stabilized. Distinct fibrils
can be seen only with certain staining techniques and under polarized
light. Depending upon the length of the follicle, the keratogenous
zone terminates at approximately one-third of the way between
the tip of the papilla and the surface of the skin. The mature
hair above the keratogenous zone shows no distinct cellular
elements, and has a narrower diameter. 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.
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| Hair follicle bulb region |
Hair
follicle bulb function
Matrix keratinocytes rapidly proliferate to generate the hair
shaft. This rate of proliferation is one of the highest of any
tissue in the body. Matrix cells are relatively undifferentiated,
with oval nuclei and scant cytoplasm. 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
it will form the medulla (in terminal hairs), cortex, and hair
shaft cuticle. At the side of these are the cells which will become
the inner root sheath: the immediately adjacent single layer will
become the cuticle of the inner root sheath, the next two to four
cell layers will become Huxley's layer, and the next single layer
will become Henle's layer.
When hair is formed by a rapid division of cells in the bulb,
it absorbs melanin, which is produced by pigment cells. Melanin
in the follicles is found interspersed in a remarkably rigid pattern
in the follicle. Melanin is relatively absent in the matrix. Consequently,
the distinction of the upper from the lower bulb at the critical
level is clearly distinguishable. The outer and inner sheaths
are free of melanin. Traces of pigment may occasionally spill
into the inner sheath, and very small dendritic cells are found
there on rare occasions, but the dividing line is very sharp between
the cells of the hair proper, which are pigmented, and those of
the sheaths, which are not.
Most of the tall columnar cells lining the dermal papilla are
melanin producing dendritic melanocytes. They are arranged as
an inner core intermingled with the epithelial cells surrounding
the upper part of the dermal papilla. The dendrites (threadlike
extensions of the cytoplasm of a neuron) that radiate from these
cells are insinuated between the undifferentiated cells of the
cortex and the medulla. These melanocytes are larger, but otherwise
similar to those found in the epidermis. As the cells of the hair
cortex move up from the matrix, they acquire pigment granules.
The exact mechanism of transfer of or induction of pigment granules
is yet unknown. As they move up, they are cut off from their supply
of nourishment and start to form a hard protein called keratin
(keratinization). As this process occurs, the hair cells die.
The dead cells and keratin form the shaft of the hair.
Hair
follicle anatomy references
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