Every 39 hours the matrix cells in the hair follicle divide, giving rise to daughter cells. These are pushed upwards by the production of other cells and become keratinised in the upper part to produce the shaft. Thus it is in the matrix that the keratinocytes (the cells that produce keratin) multiply and begin to differentiate, forming the three large concentric regions of the external sheath, the internal sheath and the hair shaft. Each of these areas is characterised by keratinocytes containing different types of keratin. For example, the outer root sheath contains keratin 14, the inner root sheath contains a keratin with an amorphous disorganised keratin that supports the developing shaft and the hair shaft contains several specific series of keratin.
The intense mitotic activity of the keratinocytes in the matrix results in an average hair growth of 0.35 mm/day. The melanocytes are responsible for the colour of the hair. They synthesise melanin pigments in the form of little grains: the melanosomes. Melanocytes are derived from the melanoblasts which arise from the embryonic neural crest; they are recognisable by their prolongations — the dendrites — where the melanosomes are concentrated. The latter migrate towards the extremities of the dendrites using microtubules and microfilaments. At the end of the dendrites, the melanosomes are finally transferred to the neighbouring keratinocytes in the cortex of the fibre (2 to 5 keratinocytes are supplied with melanosomes by each melanocyte).