Hair Growth Science

Human hair growth and shedding is not seasonal or cyclical as in other mammals. In fact it is quite random and at any given time indeterminate quantities of hair are in one of three stages known as anagen, catagen and telogen.

The anagen stage produces hair growth of approximately 1cm. every 28 days. Scalp hair remains in this actively growing state for two to six years. Growing hair beyond a certain length is difficult for some people because of their short active growth phase. Conversely, those able to grow very long hair have an inherently longer active phase of growth.

Eyelash, eyebrow, hair on the arms and legs are much shorter than scalp hair due a short active growth of about 30 – 45 days.

When new hair is formed during this anagen period the cells in the hair root quickly divide. The hair that at this point has stopped growing, is called a club hair, and it is pushed up through the follicle and replaced by a new hair.

The catagen phase lasts between two to three weeks involving about 3% of all hairs at any given time. The club hair is formed at this transitional point when growth stops, shrinking the outer root sheath, which in turn attaches to the root of the hair.

The resting phase of hair is known as telogen, accounting for 6% to 8% of all hairs. Plucking out a hair at this point will expose a solid, hard, dry white substance at the root base.

Hair on the scalp is in this phase for about 100 days with an even longer period for eyebrow, eyelash and hair on the arms and legs. Because the hair follicle is completely at rest at this point, this is when the club hair is formed.

The hair shaft is comprised of keratin, a three layered hard protein that is dead, so any exposed hair is not actually a living structure. The medulla is the inner layer, the second layer the cortex and the outer layer is the cuticle, a tightly formed structure of overlapping scale-like cells.

The bulk of the hair shaft is the cortex that functions in unison with the medulla containing pigment necessary for giving hair its color.

This shaft is visible above the scalp while the follicle resides in the skin. The hair follicle is a distinct tube- like part of the epidermis extending down into the dermis itself that contains several independently functioning layers. The papilla, containing capillaries, the small blood vessels necessary for cell nourishment, is located at the base of the follicle.

The bottom part of the papilla, called the bulb, contains cells that divide every 23 to 72 hours, faster than any other cell in the body and it is here that the living part of hair is found.

The inner and outer sheath surround and protect the follicle and form the growing hair shaft. The inner sheath follows the hair shaft ending below the opening of a sebaceous gland that produces oil and an apocrine gland that sometimes produces scent. Attached below the gland to fibrous layers around the outer sheath is a muscle known as the erector pili.

The contraction of this muscle causes hair to stand up resulting in the secretion of oil from the sebaceous glands. The function of the sebaceous gland is important in the production of sebum, necessary to condition the hair and skin.

However, during puberty, sebum production increases, which is one of the reasons why so many teenagers struggle with acne for years at a time. This process then lessens as we age, particularly in women.

It is apparent for most people that the density of scalp hair is considerably reduced with the aging process. This is because our scalp actually expand as we grow, sometimes causing hair loss.

Hair is known to play an important role in how we appear to each other, not only in gender identification but transmitting sensory information. The complexity of hair, however, goes far beyond what we see merely on the surface. It is notable that by week 22 about 5 million hair follicles will have already been developed in a fetus, which is quite extraordinary.

One hundred thousand of these follicles are found on the scalp. Because we do not generate any new follicles after this point, it is the largest number that a human will ever have in the course of a lifetime.