How Does Hair Grow

Hair is a key characteristic of mammals, but what is it made of?

Your hair is primarily made up of keratin, which is a form of protein. It grows from follicles situated in the dermis – a specific layer of the skin found below the epidermis but above the subcutaneous tissue -and is exclusive to mammals.

The human body is nearly entirely covered in hair, although some of it is so fine you cannot see it unless you look very closely. This fine hair is called vellus hair and the thicker, pigmented hair that is more visible is called either terminal orandrogenic hair, dependent on location. Ratios of each will depend on the individual, heavily influenced by their sex.

The biology of both types of hair is quite similar, with each containing a cuticle, cortex and medula, which together form the shaft, bulb and follicle. The major difference between vellus and androgenic or terminal hair is the pigmentation, which is determined by melanin situated in the cortex, and thickness of the hair.

There are two types of melanin -eumelanin and pheomelanin – and the levels of these types determine the colour of the hair. Although colour is primarily determined by genetics, grey hair results from a disappearance of melanin from the hair.

Hair can grow up to 5.5 metres long in some individuals that see high levels of hair growth in the active phase, but most hair reaches a set length and will then rest before shedding. The length of the phases differ depending on the type of hair, with head hair having a long growth period of around a year and body hair having a much shorter period of growth, but an extended resting period before shedding.

Whether hair is curly or straight is determined by the shape of the hair fibre – if it is circular it will be straight, but if it is oval it will be wavy or curly. The degree of curliness is determined by how stretched the oval shape is.

Functions of hair and fur in mammals are wide ranging, with thermoregulation being among the most important, alongside sensory function. As humans have evolved, we’ve seen a massive reduction in the thickness of hair covering our bodies, as we’ve found other ways to keep warm. The hair on our heads is retained to aid thermoregulation and in other areas, the functions of which are debated.

Facts about hair growth

Your hair grows in a cycle of growth and rest phases:

Growth phase (anagen)

This is the phase when hair grows out from the follicle. The length of this phase can vary quite dramatically, dependent on where the hair is growing and what type of hair it is. Vellus hair grows for a short period of time, whereas the terminal hair situated on the head can grow for up to a year and can grow up to 5.5 metres long.

Regeneration phase (catagen)

In this phase, the hair detaches from the follicle and the blood supply and is pushed up through the skin out of the follicle. The follicle renews itself in readiness for another new hair to grow once the present hair has shed from the skin.

Resting phase (telogen)

The hair rests for between one and four weeks before a new hair pushes the present hair out. This will happen all across the body and with all types of hair.