Hair Biology

To understand why we lose hair we first need to understand the biology of the hair and hair follicle.

Our skin acts as a barrier and is the primary organ through which we interact with the external world. Hairs (or pili; pilus in the singular) are characteristic of mammals. The hair consists of two parts – the follicle which is beneath the skin and hair shaft which is the part above the skin. The hair follicle is a narrow tubular depression in mammalian skin containing the root of a hair. It is lined with epidermal cells and extends down through the epidermis and dermis to its base in the subcutaneous tissue. The hair follicle is one of the important appendages of the skin. In fact the hair follicle with its sebaceous gland and the arrector pili muscle is called the pilosebaceous unit (Pilosebaceous unit – The structure consisting of hair, hair follicle, arrector pili muscle and sebaceous gland).

The shaft of a hair consists of three layers – the cuticle, the cortex of hard-keratin surrounding, and the soft-keratin medulla (this is absent in some hairs (vellus)) (fig.) Pigmented hairs contain melanin in the cortex and medulla, but pigment is absent from the surrounding sheaths. The color of hair depends mainly on the shade and the amount of pigment in the cortex and, to a lesser extent, on air spaces in the hair. In white hairs pigment is absent from the cortex, and the contained air is responsible for the whiteness; “gray hair” is generally a mixture of white and colored hairs.
The root of a hair is situated in an epidermal tube known as the hair follicle, sunken into either the dermis or the subcutaneous tissue. The follicle is dilated at its base to form the bulb (matrix).
In the obtuse angle between the root of a hair and the surface of the skin, a bundle of smooth muscle fibers, known as an arrector pili muscle, is usually found. It extends from the deep part of the hair follicle to the papillary layer of the dermis. On contraction it makes the hair erect. The arrectores pilorum are innervated by sympathetic fibers and contract in response to emotion or cold. This results in an unevenness of the surface called “goose pimples” or “goose skin.”