Hair

Hair Anatomy 

Hair follicles cover the entire skin surface except the palms, soles, glans penis, and labia minora.  There are approximately 5 million hair follicles on the body with 100,000 on the scalp.  The scalp has the greatest density of hair follicles with roughly 300 to 500 hairs per square centimeter.

The hair follicle may be divided anatomically into four parts:

1. The bulb consisting of the dermal papilla and matrix
2. The suprabulbar area from the matrix to the insertion of the arrector pili muscle
3. The isthmus that extends from the insertion of the arrector pili muscle to the sebaceous gland
4. The infundibulum that extends from the sebaceous gland to the follicular orifice.

The lower portion of the hair follicle consists of five major portions:

1. the dermal papilla
2. the matrix
3. the hair shaft, consisting from inward to outward the medulla, cortex, and cuticle
4. the inner root sheath (IRS) consisting of the inner root sheath cuticle, Huxley’s layer, and Henle’s layer
5. the outer root sheath (ORS).

The base of the follicle is invaginated by the dermal papilla, which has a capillary loop that passes through the papilla.  Signal transduction and communication between the dermal papilla and the matrix cells influence how long and how thick the hair shaft will grow.  The melanocytes within the matrix also produce the pigment in the hair shaft.

The hair medulla is only partially keratinized and therefore appears amorphous and may not always be present.  The hair cortex cells lose their nuclei during their upward growth and do not contain any keratohyaline or trichohyaline granules unlike the ORS and IRS, respectively.  The keratin of the cortex is hard in contrast to the IRS or epidermis, which are soft.  The cuticle is firmly anchored to the IRS cuticle.

The cuticle of the IRS consists of a single layer of flattened overlapping cells that point downward and interlock tightly with the upward angled cells of the hair shaft cuticle.  Huxley’s layer is composed of two cell layers, whereas the outer Henle’s layer is only one cell thick.  Just before the isthmus, the IRS becomes fully keratinized but disintegrates at the level of the isthmus. Although the IRS is not present in the emerging hair shaft, the IRS serves as a strong scaffold in the lower portion of the hair follicle.

The ORS extends from the matrix all the way to the sebaceous gland, where it becomes the surface epidermis.  The ORS is thinnest in the hair bulb and progressively becomes thicker with the thickest portion at the level of the isthmus.

The portion where the arrector pili muscle inserts is known as the bulge area and is thought to be the location for the stem cells that can help regenerate the hair follicle.

Hair Physiology;The appearance and function of the hair shaft can be affected by porosity, elasticity, and texture.

Hair Porosity

Porosity is defined as the hair shaft’s ability to absorb moisture.  Without cuticular damage, the hair shaft is relatively impermeable to water and other substances.    With changes caused by permanent waves, coloring, temperature or pH changes, the cuticle can be rendered more vulnerable permitting penetration and damage to the cortex.  With repeated injury, the cuticle can be rendered permanently damaged allowing moisture to escape and making the hairy dry, brittle, and exhibit split ends.

  

Hair Elasticity

Elasticity measures the tensile strength of the hair with a normal hair able to stretch to about 1 and 1/3 its original length with healthy hair able to support a 100-g weight without breaking.  Elasticity is dependent upon a healthy cortex, and a hair shaft that has poor elasticity can break easily with grooming and with product application.

Hair Texture

Texture is affected by two determining factors:  the thickness of the hair shaft and the feel of the hair itself.  Wiry hair possesses its characteristics by virtue of the tight adhesion of the cuticle to the cortex and the flat arrangement of the cuticular scales.  Wiry hairs may be more resistant to chemical alteration.

Hair Permeability

Permeability is affected by the degree of porosity, elasticity, and texture of the hair shaft with the porosity and texture having the major influences on permeability.  Porosity clearly plays a greater role than texture.  However, fine hair will absorb more applied product than coarse hair given the same degree of porosity.  Coarse hair with great porosity will still have greater permeability than fine hair with low porosity.

Hair Embryology Explained

In utero, follicles begin formation on the scalp and then move downward to the remainder of the body.  Lanugo hairs are the first to develop, which are characterized by their fine, short, and soft nature.  These hairs disappear by the 32nd to 36th week of gestation but can persist in up to one third of newborns.  Most hairs however change over to vellus hairs with 90% of the scalp hairs becoming larger and thicker terminal hairs.  There are no further development of new follicles after birth.

FORMATION OF THE EPITHELIAL PLACODE AND PRIMARY HAIR GERM

In the human fetus, at about 10 weeks gestation, the hair follicle develops from a small collection of cells called the epithelial placode.   Cells of the epithelial placode then begin to profilerate to form the primary hair germ, which in turn will develop into the epithelial portion of the hair follicle.  The formation of hair follicles is predicated on multiple interactions between the epithelial cells and the mesechyme (primitive dermis).  The first dermal signal arises in the mesenchyme that stimulates the overlying epithelium to generate regularly spaced placodes.  The epithelial placode then causes an aggregation of cells in the underlying mesenchyme to form the dermal condensate, which ultimately becomes the dermal papilla.  The dermal condensate then effects growth of the epithelial placode to create the mature follicle.

THE HAIR FOLLICLE PEG

In the next stage, the primary hair germ elongates into a follicle peg.  The mesenchymal cells that flank the peg develop into a fibrous sheath with the tip of the peg developing into the dermal papilla.  Melanocytes responsible for the hair shaft pigment begin to migrate from the neural crest down to the region above the somite then through the dermis to the epidermis.

THE HAIR BULBOUS PEG

The bulbous peg arises from the elongation of the follicle peg.  The epithelial cells within the bulbous base portion of the bulbous peg become the matrix cells, which will in turn give rise to the hair shaft and inner root sheath.  The outer root sheath forms two bulges on the posterior portion of the hair follicle:  the superficial bulge develops into the sebaceous gland, whereas the deeper bulge becomes the mature bulge area into which the arrector pili muscle inserts