Anagen phase – The proliferation of the hair shaft starts with the cells of the dermal papilla instigating the anagen phase by signalling to the stem cells in the hair bulge. The stem cells move down to the germinal matrix and differentiate into keratin cells. The hair follicle derives nourishment from the dermal layer which feeds the keratinocytes. The keratinocytes grow larger and continue to divide whilst the keratin cells are biologically viable. The position of the keratin cells defines what part of the hair fibre it will become.
The growing hair shaft is pushed up by the increasing population of new keratin cells from the basal layer into the upper area of the root bulb. The keratin cells continue the journey through the hair follicle, still growing and acquiring pigment from the melanin produced by melanocytes. The keratin cells then enter the fibrillation zone and the framework of the cell skeleton starts to develop.
Once cells enter the lower keratinisation zone, substantial disulphide cross linking starts creates a protein matrix.
Hydrogen bonding of amino acids further stabilises the structure and then more disulphide linking begins to create a rigid structure.
As the keratin cells reach the upper stabilisation zones, the cells begin to harden and the keratin cells produce more and more keratin protein . At this point the water content of the cell drops to around 20% and disulphide bonds continue to form. As the cells unite, the absence of water leaves air-filled gaps in the hair shaft. If the keratin cells have not produced an adequate amount of keratinous protein the gaps will be large and the hair will be fragile and coarse. These gaps are known as fusi, they are larger at the base of the the hair and the air dissipates near the tip of the hair. This eventually leads to split ends.
The cell membrane complex binds to the matrix structure acting as an intracellular cement and provides suppleness and flexibility to the hair fibre. The cell membrane complex is also responsible for the hydrophobicity of the hair fibre.
The hair follicle unit with a hair fibre growing in the hair canal through the zone of protein synthesis, the zone of differentiation, and the zone of keratinisation where the keratin cells start to fill with keratin. In the region of permanent hair fibre all keratin cells will have lost all their cytoplasm and be full linked by disulphide bonds. The epidermal layer provides zinc and sulphur for the differentiating (growing) keratin cells. The bulge area is home to the stem cells where the keratin cells originate. The dermal papilla is the anchorage site of the hair follicle.