THE HUMAN HAIR is a protein filament. It grows from follicles found in the epidermis. It is also considered to be a derivative of the epidermis having two distinct structures; the hair follicle, and the hair shaft. It is naturally perplexing however, how the human hair grows, being that it is considered to be non-living.
STRUCTURES OF THE HUMAN HAIR
HAIR FOLLICLE
This is the only living portion of the hair and therefore is the only essential growth structure of the hair. It resides in the epidermis of the skin. Hair growth actually begins here, specifically the bottom section of the follicle.
The only germinal cells are the matrix cells that are restricted to the lower portion of the hair follicle.
As the number of cells in the follicle that gets replicated increases, the hair subsequently gets pushed out of the skin until it reaches the surface of the skin. As the hair emerges to the skin, the matrix cells process the keratin which would then go towards the “process of keratinization”.
The complex interaction of the hair follicle with neuropeptides, immune cells, and hormones actually replicates the cells as well as the protrusion of the hair to the surface of the skin, resulting in actual physical hair growth.
The hair follicle influences not just the growth of hair but also the physical appearance of the hair. It should be noted, however, that to ensure the protection and precise shape of the hair, the follicle is lined by inner and outer root sheaths.
INNER ROOT SHEATH (IRS)
The inner root sheath is a layer that envelopes the shaft of the hair and also forms the needed channel for the growing hair. It is pertinent to note that the upward growth and differentiation of the matrix cells actually lead to the formation of the inner root sheath.
The IRS (1) provides a coating and support to the hair shaft until it gets to the Isthmus level where the disintegration of the IRS is observed to occur.
The inner root sheath comprises three distinct layers including the cuticle layer, Henle’s layer, and Huxley’s layer.
Cuticle layer: This is the innermost layer of the IRS having cells that interlock with cells of the hair cuticle right beneath the skin. However, at the skin surface, separation occurs to actually encourage elongation of the hair shaft.
The link through which the hair shaft gets connected to the follicle is very tight.
Henle’s layer: This is the outermost layer of the IRS and it is the layer onto which the first keratinization appears.
Huxley’s layer: Adamson’s fringe is the region located above the Henle’s layer where the keratinization of the Huxley’s layer takes place.
OUTER ROOT SHEATH (ORS)
The outer root sheath is an elongation of the epidermis at the infundibulum up until the hair bulb where the cell gets diversified through the hair follicle. The ORS (2) houses certain cells that are responsible for specific functions of the hair follicle including acting as a sensory organ and standing in for the skin as an immunologic sentinel.
These cells include melanocytes, Langerhans cells, and Merkel cells. Other functioning compartments of the outer root sheath include:
The bulge, which stands in as hair stem cell reservoir
The sebaceous gland which serves as hair lubricant
FEATURES OF HAIR FOLLICLE
- The following are the observable features characteristic to the hair follicle;
- The skin surface serves as a route for the emergence of the hair follicle through an opening called Astrium.
- The shaft occurs as a result of the narrowing of the upper third.
- The Infandibulum is the featured portion that occurs between the Astrium and the shaft.
- The bulge is the portion of the hair follicle that is sited between the shaft and the lower part of the erector pili muscles.
- The hair root is the part observed to be underneath the bulge.
HAIR SHAFT
This is the portion of the human hair that does not conform to growth because it only protrudes from the skin, technically from the follicle. The hair shaft can also be seen as the keratinized extension of the human hair which emerges from the follicle to, and beyond the surface of the skin.
The hair shaft consists of a cortex, cuticle cells as well as medulla.
The cuticle is more prone to damage in cases where the hair mistreated. This is because of where it is positioned.
The cortex is positioned at the intermediate level and it is the thickest part and houses thicker cells that are spindle-shaped.
The medulla, positioned on the inside has its layers demarcated into columns that are segmented by air pockets which are filled by citrulline, an amino acid. The thickness of the hair is determined by the number of medulla cell columns.
PHYSICAL APPEARANCE OF THE HUMAN HAIR
Hair color is determined by pigment cells producing melanin in the hair follicle, specifically the melanocytes which are localized in the matrix stem cell for the purpose of hair pigment production. The hair color, therefore, is dependent on the amount and type of major pigment that gets phagocytosed.
This is because the matrix cells actually phagocytose either melanin or pheomelanin from the dendritic elongations of melanocytes, during the differentiation phase.
The melanin, however, loses its dexterity and efficacy as a person ages, as a result, grey or white hair is observed to replace formerly black hair. Subsequently, when the pigment cells die off, the hair would turn totally grey.
CHEMICAL COMPOSITION OF THE HAIR
The chemical composition of the human hair is comprised of keratin, lipid, pigment, and minerals.
Keratin is known to be the protein found in the cortex and it is composed of 18 amino acids including cysteine, serine, glutamic acid, threonine, arginine, glycine, leucine, and isoleucine which are most abundant.
The lipid component of the hair’s chemical composition is made up of triacylglycerols, phospholipids, waxes, cholesterol, squalene, and free fatty acids.
The pigment consists of the melanin which is a colored substance.
Chemical elements found in the hair include carbon, oxygen, hydrogen, Sulphur, and nitrogen.
Minerals of the hair structure are an essential component of the protein-enzymatic systems and they include iron, magnesium, zinc, copper, and lead.
The above-mentioned hair components occur in different proportions in different hair types of different people in different geographical locations.
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