Hair and its Growth Cycle

 

 

Claire Stevens LTTS  FTTS

The Hair and its growth patterns are genetically determined in both males and females. Hormones are also responsible for characteristic changes.

The human foetus possesses a covering of fine soft usually pigment free hair. This covering is usually shed in-utero during the seventh or eighth month of gestation.

Children can possess fine short non-pigmented vellus hairs on all body surfaces except palms soles eyelids, backs of distal phalanges, glans penis, lips.

At puberty, terminal hair appears in the secondary sexual areas, commencing at the pubic region where it is pigmented and becomes coarser. These programmed changes occur due to androgenic stimulation in both males and females. Pubic hair growth  is followed by axillary growth approx. 24 months later. Males thereafter produce hair on the face, back and limbs. Many men at some time after puberty suffer scalp follicular regression with a temporal hairline recession and the familiar bald patch at the posterior vertex. These often coalesce to produce MPA (male pattern baldness - androgenetic alopecia). This type of baldness is genetically inspired and occurs in the presence of the converted androgen di-hydro-testosterone. 'Greying' of hair is due to the gradual loss of the melanocytes' pigment - melanin. In white hairs the melanocytes are absent. This is largely determined by heredity factors, hyperthyroidism and some autoimmune diseases.

Hair is an epidermal appendage that lies within the dermis. Each hair emerges from a tubular invagination called a follicle. The follicle resembles a narrow pocket within the skin, as if a tiny finger had pushed the epidermis down into the Dermis and the underlying subcutaneous tissue. The lower extreme is penetrated by the Dermal Papilla an upward protrusion of connective tissue which produces microscopic cells of several kinds from which the hair is formed and developed by cellular elongation and keratinisation.

Hair is closely associated with sweat gland and sebaceous gland activity. Each hair-producing follicle with its sebaceous gland is known as a pilo-sebaceous unit. The arrector pili muscle joins the wall of the follicle to the epidermis and is responsible for the erection of hairs and goose flesh during cold weather or emotional stresses. The hair shaft is currently believed to be a dead structure composed of cells which undergo keratinisation after leaving the dermal papilla. As all follicles are established before birth no new ones are created thereafter. All characteristics are genetically determined.

The hairshaft - a keratinised structure composed of an outer cuticle (an imbricated multy layer of tile-like protective keratinised cells) the cortex where cells are held firmly together, and an inner medulla where the cells are larger more loosely connected and partially separated by air spaces. The hair is approximately cylindrical in cross-section.

The chemical composition of hair:
Carbon 45%, Oxygen 27%, Nitrogen15%, Hydrogen 6%, Sulphur 5%.

The 'hard' keratin found in hair is a resilient, insoluble (in water) protein containing the following amino acids:
Cysteine, Cystine, Serine, Glutamic Acid, Threonine, Glycine, Leucine, Valine, Arginine, Aspartic Acid, Alanine, Proline, Isoleucine, Tyrosine, Phenylalanine, Histidine, Methionine.

Cystine is an abundant amino acid providing its strength.

HAIR CYCLE
Normal hair follicles undergo periods of growth (Anagen) followed by regression (Catagen), resting (Telogen) and evacuation (Exogen) followed by regenesis ('new' Anagen)..
No hair therefore grows continuously. This hair cycle, which dictates the ultimate hair length attainable by an individual, is explained under the following headings:-

Anagen: The period of follicle regeneration (folliculo-genesis) with active hair growth. (scalp hairs grow for 2-7 years).
Hairs in the anagen phase may grow faster during the early years. Average growth rate is 1 - 1.25cm per 28 days.

Catagen: the preliminary stage of the retrogressive catagen phase. During this changes may occasionally be seen above the skin surface with the naked eye: e.g. loss of pigment and the narrowing of the hairshaft accompanied by a narrowing and eventual loss of the medulla. During this short period of change (approximately 2 weeks) the follicle rests the dermal papilla stops production of new cells, the dentrites and melanocytes contract and melanin production ceases. The follicle and epithelial sheath contract and the hairshaft is ejected.

Telogen: the resting phase of the follicle which lasts for approximately four months. The follicle remains quiescent in its shortened state and awakens to regenerate with the onset of the new Anagen phase.

Exogen: The process of hair shedding. Whereas hairs may be shed at any stage of the cycle, the majority of shedding occurs during the 'new' Anagen phase.

The follicle is a tubular epidermal structure; its wall forms an inner and outer sheath. The inner sheath grows up with the hairshaft and has two layers:

the outer or Henle layer
the inner or Huxley layer, there is also a thin cuticle which provides the cells which interlock with the hair shaft.
The outer sheath consists of elasticated connective tissue surrounding the follicle wall.

Transverse sections of a hair-shafts show that it is composed of a thin outer layer - the cuticle, and an inner cortex. Certain hairs have a third structure - the central medulla. The medulla is usually absent from the inner tip and root portions of hairs. In general only the thicker hairs possess a medulla and usually its thickness is relative to that of its host hair. Its structure is a latticework of keratin.
These concentric parts of the hair, the cuticle, cortex, medulla as well as the two layers of epithelial sheath and its cuticle, originate as separate streams of cells from the dermal papilla.

Hair Density, Numbers of Hairs, Rate of growth
The average human scalp measures approximately 120 sq. inches (770 sq.cm).
The hair covering varies numerically according to hair colour and ethnicity.

Caucasian hair type: The figures quoted are based on research published by Drs. Erasmus Wilson, Withof & Stelwagon in the nineteenth century and are therefore in inches.
Blonde 146000 hairs, diameter measuring 1/1500th - 1/500th in.
Black 110000 hairs, diameter measuring 1/400th - 1/250th in.
Brunettes 100000 hairs, diameter infinitely variable
Titian (Red) 86000 hairs, diameter infinitely variable

Afro hair type: Colour is predominately black. Hairshafts are multi helical with a wool-like character. Healthy individuals possess between 50000-110000 scalp hairs.

Mongolian (Asian) hair type: Possessed by the greatest numbers of people worldwide. Hairshafts are predominately straight. Colour is predominately black. Regional variations exist, but healthy individuals possess an estimated 80000-140000 scalp hairs.

Rate of growth
Normal Caucasian hair growth rate is 1 - 1.25 cm per 28 days. Researchers have shown that this rate of growth may reduce beyond the length of approx. 27cm. Afro-Caribbean hair growth rate is approximately half that of Caucasian, and due to the fragility of their multi-helical structure, rarely attain great length. Mongolian hair grows rapidly exceeding the average for Caucasians and may attain great length.

In the majority of humans, terminal hair is straight and is roughly circular in cross section - such hair-shafts having emerged from host follicles with similar characteristics.

If however, due to genetic factors or skin injury the follicle is of a spiral or zigzag structure, the hair-shaft will emerge in conformity and exhibit appropriate helices. This is demonstrated in some Afro-Caribbean type hair, and also occasionally as a result of pressure employed during multiple hair follicle transplantation surgery.
Naturally flattened, imperfect, oval or kidney shape; the hair-shaft will emerge exhibiting waves/curls of appropriately varying degree. Numerous factors incidental to natural healthy life can influence hair formation. Other aberrations may result from ill health, drug therapy and ageing etc.


© C Stevens LTTS FTTS  Contact the author

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