Hair growth and hair loss

blonde woman with hair on the wind

We are all born with all our hair follicles in place and under normal circumstances do not produce any more. A proportion of these follicles are programmed to grow relatively long, thick, pigmented hair, for example, scalp, eyebrow and eyelash hair. These are called primary terminal hairs. Other follicles grow short, fine, non-pigmented or only slightly pigmented hair, primary vellus hair which covers most of the body. This is often not visible to the naked eye.

Hair cycle

We all lose hairs every day throughout life – between about 40 and 120 hairs from the scalp alone. This is replaced; for example, the average head has 100,000 hairs. Supposing one loses 100 hairs per day, complete baldness would occur in every one of us within 1,000 days without regrowth! We maintain a status quo of hair density by what is called the hair cycle, a method by which nature replaces hair. At any one time, approximately 90% of head hair is growing and advancing in length by approximately 1 centimetre per month. Each hair grows for a period lasting from about two to six years; during this time the hair attains its maximum diameter and length. When the growth phase ends, the hair follicle begins a two to six month period of rest and the hair is shed. Only about 10% of hair is in the resting phase at any one time and when it sheds it does so randomly. Eventually, the hair follicle enters a new growth period and a new hair shaft appears.

Normally, as the scalp goes through these cycles, the follicles and the hairs they produce remain constant in size. However, hair follicles throughout the body can change the size and produce different types of hair. During and after puberty, governed by hereditary and hormonal influences, for example, follicles on some parts of the body increase in size and produce longer and thicker terminal hair instead of the fine vellus hairs previously produced. This is often called secondary sexual hair.

Hair loss

95% of the hair loss seen in both men and women is caused by a progressive condition called androgenetic alopecia or common hereditary hair loss. In men, this is known as male pattern baldness and may progress to the familiar horseshoe-shaped fringe of hair. The varying degrees of male baldness is known as Hamilton patterns. In women, it manifests itself as a diffuse thinning of the hair over the crown (vertex) of the head (Ludwig pattern).

It is estimated that androgenetic alopecia affects the majority of men and women to some degree. Hair loss can begin as early as the teens, and by age 35 almost 40% of men an women show some degree of hair loss; in men, this is usually obvious but in women, it is typically subtle and not noticed without a close examination. In androgenetic alopecia, a combination of heredity, hormones and age causes a progressive shrinking, or miniaturisation, of certain scalp follicles. This causes a shortening of the hair’s growth cycle. Over time, as the active growth phase becomes shorter, the resting phase becomes longer. Eventually, there is very little growth at all.

Because the hair length and thickness are determined by how long the hair is allowed to grow before entering the next resting and shredding phase, the hair loss process in both men and women is thus a gradual conversion of terminal hair follicles to value-like hair follicles, secondary vellus hairs. Some hairs only partly miniaturise and are called intermediate hairs. A net result is an increasing number of short, thin hairs that may be barely visible above the scalp surface. Despite the sometimes dramatic change in follicle size with androgenetic alopecia, the follicle is not altered in the structure; and the number of follicles does not change, apart from in old age when some follicular loss (deletion) occurs.

At the time of writing only two medicines are licensed for use in minimising the progression of androgenetic alopecia and to some degree improving it – oral Finasteride (Propecia, MSD Ltd) and topical Minoxidil (Regaine, Pharmacia and Upjohn Ltd). Neither of these is strict ‘cures’ in that any improvement or ‘arrest’ of hair loss gained from them will be lost within a few months of stopping the treatment.

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