Hair Biology

Hair can vary in shape, length, diameter, texture, and color. The cross-section of the hair may be circular, triangular, irregular, or flattened, influencing the curl of the hair. The texture of hair can be coarse as it is in whiskers or fine as it is in younger children.

Hair color Hair types are classified on the basis of structure/size, color, ethnicity, anatomical regions etc. The human hair follicle is capable of producing different types of hair under different circumstances the type of hair produced in any particular follicle can change with age or under the influence of hormones. In humans, a prenatal coat of fine, soft, unmedullated and usually unpigmented hair, known as lanugo (looks like fur), is normally shed in utero in the eighth to ninth month of gestation.

Postnatal hair may be divided at the extreme into two kinds: vellus, which is soft, unmedullated, occasionally pigmented and seldom more than 2 cm long (frequently referred to as “peach fuzz,”); and terminal hair, which is longer, coarser and often medullated and pigmented. This type of natural hair, associated with women and young children, is fairly unnoticeable. However, there is a range of intermediate kinds. Terminal hair is the type of natural hair to which most people refer in their everyday lives.Before puberty, terminal hair is normally limited to the scalp, eyebrows and eyelashes. After puberty, secondary sexual ‘terminal’ hair is developed from vellus hair in response to androgens.

Ethnic variations of hair

While no two heads of hair are the same, there are three main classifications of hair type based on ethnic/geographical/racial basis into caucasoid, negroid, and mongoloid. These terms are now obsolete as they are scientifically inaccurate and geopolitically incorrect. Better and more appropriate alternatives are – African or Equatorial-African, European or Caucasoid or Indo-European (IE), and Asian. In spite of these variations the basic composition remains the same.

The Eurpoean/Caucasoid hair is generally straight or wavy with small pigment granules which are evenly distributed. It is oval or round on cross-section, of moderate diameter with minimal variation. Color may be blond, red, brown, or black

The Asian hair is straight and has dense distribution of pigment granules. The cross-section is round with a fairly large diameter. The shaft tends to straight and coarse.

The African hair on the other hand is kinky, curly or coiled with variable sized pigment granules which are densly distributed. On cross-section the African hair is flattened or sometimes ribbon like with significant variation in diameter.

What are the different parts of the hair follicle?

The follicles are slanted in epidermis and an oblique muscle the arrector pili runs from the middle of the follicle wall to the dermo epidermal junction, additionally one or more sebaceous glands open into the follicle.The follicle may be divided into vertical components: Infundibulum, Isthumus, Suprabulbar region and Bulb.

The hair as we see it is a compact structure made up of dead cells. Individual hairs are composed chiefly of the horny scleroprotein known as keratin and contain neither blood vessels nor nerves. They usually contain pigment (except in the case of albinos). The shaft of the hair consists of modified epithelial cells arranged in columns (cortex) surrounding a central medulla (or core) and covered with thin, flat scales (cuticle).

The hair follicle bulb is like a minute factory which manufactures cells continuously. As new cells are formed the older ones are pushed into the follicle towards the surface. The hair follicle is an anatomical structure which evolved to produce and extrude (push out) a hair shaft.The cells on reaching a particular point undergo cell death and are converted into a scleroprotein called keratin, which is then compacted to form the hair. This continuous activity is responsible for the hair growth.

During anagen the follicle actively grows hair.

During catagen the follicle is almost entirely degraded.

During telogen the follicle rests prior to re-initiation of an anagen phase and the growth of a new hair shaft. On any given day, human beings lose about 50-100 scalp hairs due to exogen, these are the hairs we find in our comb, brush and shower drain. Shedding in excess of this may be due to an increase in the follicles of scalp hair in the telogen stage and should be addressed to contain hair loss.

At any given time approximately 90% of our scalp hair are in the growing phase and about 10% in the resting phase, if the scalp is healthy and not affected by any condition that causes hair loss.
A fourth phase known as the Kenogen or the lag phase has been described. This is the period after the hair has been shed and before the new hair formation begins. It is this phase that is prolonged in patients of Pattern Hair Loss.

The normal growth rate of scalp hair is 0.3 – 0.4 mm per day or roughly 1cm per month. Hair growth has a cyclical pattern that can be affected by a number of genetic, disease, medication or other factors to cause hair loss.