Hair is developed in the bottom of follicles, which are made of an introjection of the epidermis in the derma having a quite cylindrical shape that ends with a hollow called Bulb. A minute muscle, the arrector pili, is attached to each hair follicle, where the duct of the sebaceous gland comes to. The bulb is the most important part of the hair follicle, as it contains hair matrix cells originating all hair structures and layers. As matrix cells multiply, they pull the older cells upwards.

As these cells go up the hair follicle, they differentiate and undergo several structural and morphological modifications; among the changes they undergo there is the process of keratinisation, that is hardening, leading to the formation of the hair protein. The evolution of this growing process finally ends with the formation of the hair stem (the visible part of the hair), which is now completely keratinised (rigid).