Why does natural hair grow curly or straight? There are a number of theories:
It is thought that the shape of the hair follicle may influence hair type. If the follicle is twisted then the hair will be curled, if it is straight then the hair will grow straight. Newly formed hair at the base of the follicle is soft, pliable and supplied with nutrients, however as the cells are gradually pushed toward the surface they start to harden and die. This process is called keratinisation.
When the hair grows out of the follicle opening it is already very firm and hard. The form is influenced by the inner root sheath below the sebaceous glands.
Whilst the above theories may have some influence on curl, they do not account for changes as a result of extreme diet, pregnancy, drugs or illness. It is possible in these instances for normally curly hair to straighten, either temporarily or permanently. Additionally, some straight hair has been found to have an oval profile.
Cell growth, called mitosis, at the base of the root bulb occurs in a roughly circular format. If cell division is uniform around this circumference then the hair grows at a uniform rate out on to the scalp, eventually hardening into a straight form. However, if the rate of mitosis varies around the circumference then the hair will become kinked, resulting in a curly or wavy shape. This theory is supported by the previously mentioned changes in curl due to diet, illness, using drugs or a pregnancy. Each of these scenarios can result in hormone, vitamin and mineral enrichment or deficiency, leading to a change in the nutrient supply around the circumference of the root bulb, which ultimately affects hair growth.
The structure of curly hair is different to that of straight hair, resulting in two different forms of cortex. The orthocortex lies on the outside of a curl and is less dense than the paracortex, which is more compressed and is found on the inside of a curl (see image Cortex Density). In reality the difference between orthocortex and paracortex can only be seen in Afro or Permed Caucasian hair through an electron microscope.
Hair can be round, oval or flat in profile (see image Hair Follicle Cross-sections) as well as fine, medium or coarse (see image Differences in Hair Profile). These attributes affect the way hair responds to chemical treatments. For example, fine hair as well as oval or flat hair is more difficult to curl and tends to result in variable curls, whilst round, normal to thick hair is easy to mould, readily adopting the form of chosen styler.
Natural hair with tight curls can also be difficult to process, as the denser paracortex is more resistant to treatment. Different strengths of perm and relaxer are available to suit all hair types, although hair in poor condition should not be treated at all. Additionally it is important constantly to monitor hair that is undergoing treatment, as the results will vary from individual to individual.
