|
Hair
fiber
The hair fiber is the cylindrical, keratinized, often pigmented
filament that can be seen above the skin. It consists mainly
of dead cells that have turned into keratins and binding material,
together with small amounts of water. The hair shaft is composed
of three parts. From outermost to innermost part of a hair fiber;
the cuticle, the cortex, and the medulla (only present in large
terminal hairs,
i.e.
hairs greater than 60 microns in diameter).
The morphologic characteristics of hair vary from one species
of mammal to another. Within species as well, the hairs from one
region of the body are different from those from another region.
Furthermore, even within a single region, hairs of different length
and texture are characteristically found on the body.
Hairs also show extreme variations in color, diameter, and transverse
contour. Some hairs may be almost perfectly round while others
are so markedly flattened or oval that they resemble ribbons.
Twisting of such ribbon like hairs along the longitudinal axis
may give the impression that the hair varies widely in diameter,
although this is usually not the case, except in certain pathological
conditions.
Hair
fiber cuticle
The cuticle is a single row of cells, which may be identified
at the level of the lower bulb. Unlike the cells of the inner
sheath, the cuticle cells of the cortex do not show the presence
of trichohyalin. Trichohyaline is a structural protein that is
produced and retained in the cells of the inner root sheath and
medulla of the hair follicle. The cells of the cuticle can also
be differentiated from cells destined to become the cortex by
their lack of melanin (pigment). Melanin is responsible for the
color of hair and an individual’s hair color depends on
what kind of melanin the person has.
The cells of the cuticle can be recognized in the upper part
of the bulb as they sweep upward as a single row from the matrix.
About midway in the bulb, these cells are cuboidal in shape. When
stained with a basic dye, the microscopic appearance of a histological
section of these cells is found to be etched with numerous granules.
When the cells reach the upper region of the bulb, they become
columnar, with the long axis oriented radially. This orientation
is maintained for a short distance above the bulb, and then their
outer edges begin to be tipped upward. Since these cells are at
least twice as broad as they are high, they become imbricated
when the outer sides shift at the tips. Imbrication is a pattern
or design having regularly arranged, overlapping edges, as roof
tiles or fish scales.
As their orientation shifts from a horizontal plane to a vertical
one, the cells become flattened. This orientation is completed
below the midway mark of the follicle. In the upper half of the
follicle, these cells undergo hyalinization, a form of degeneration.
Their nuclei disappear, and the mature cuticle adheres to the
cells of the cortex.
The cells of the cuticle of the hair are interlocked with those
of the inner root sheath, 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.
A major function of cuticle on the hair shaft is to protect the
inner, softer tissue called the cortex, or else the cortex could
become frayed and fall apart.
The scale pattern of the cuticle in human hairs is routinely
imbricate. Cuticles are also classified as coronal when each cell
completely surrounds the hair. The free edge of cuticle cells
may be simple, dentate (with short ridges), or serrate (saw-edged).
Cuticle cells may be elongate, acuminate (gradually tapering to
a point), ovate (egg shaped), or flattened.
The cuticle cells have different staining properties than the
surrounding structures, which may signify chemical differences
in the keratins. The keratin of the cuticle of the cortex seems
to be different from that of the cuticle of the inner sheath,
and it might explain the split that occurs between the hair and
the inner sheath.
The cuticle is responsible for much of the mechanical strength
of the hair fiber. A healthy cuticle is more than just a protective
layer, as the cuticle also represents the structure that controls
the water content of the fiber. Much of the shine that makes healthy
hair so attractive is due to the cuticle.
Hair
fiber cortex
Cortical cells constitute the bulk of a hair, and it is the cortex
that gives a hair fiber its eventual shape, resilience, elasticity
and curl. The cortex as the main body of the hair is composed
of elongated and fusiform (spindle-shaped) cells. Within each
cortical cell are bundles of hair protein called fibrils, running
parallel to the fiber axis, and between the fibrils is a softer
material called the matrix.
The cortex may contain cortical fusi, pigment granules, and/or
large oval-to-round-shaped structures called ovoid bodies. Cortical
fusi in are irregular-shaped airspaces of varying sizes. They
are commonly found near the root of a mature human hair, although
they may be present throughout the length of the hair. In the
living portion of the hair root the fusi are filled with fluid;
as the hair grows and dries out, air replaces the fluid. Pigment
granules are small, dark, and solid structures that are granular
in appearance and considerably smaller than cortical fusi. They
vary in color, size, and distribution in a single hair.
Hair
fiber medulla
In some of the terminal hairs, the cortex has a central hollow
core, called the medulla. The medulla is easily identified as
a pale-staining, sometimes discontinuous line of cuboidal cells.
In some animals, the air within the medulla plays a role in the
regulation of body temperature. Humans always have an amorphous
medulla, if present.
In human hair, the medulla may be as much as one third of the
diameter of the hair fiber, and may be continuous, discontinuous,
or fragmental. In coarse hairs it is usually continuous or fragmental,
whereas in fine hairs it appears discontinuous or absent.
Some authors are of the opinion that even though some adult hairs
appear non-medullated when observed under the light microscope,
when viewed under polarized light, all hairs, with the exception
of the very fine ones, shows a fragmental or discontinuous medulla.
The medulla may be only one or two cells in diameter, but it is
present nonetheless.
The type of medulla present can vary even within the same hair.
The medulla is composed of large, loosely connected keratinized
cells. Large intra and intercellular air spaces in the medulla
determine to a large extent the sheen and color tones of the hair
by influencing the reflection of light. That is why hair color
looks a lot different in sunlight than it does in the shade.
Hair
fiber references
|
