Hair follicle cycle

The hair cycling procedure parallels morphogenesis in that it involves multiple signalling events incorporating developmental pathways - the same reoccur during the different hair cycle stages. The hair cycle includes a complex remodelling and regenerating of the complete non-permanent portion of the hair follicle.

Anagen
The anagen phase, experiences the active growth of hair and deposit of materials in the hair shaft by cells found in the hair follicle. There are certain metabolically active and dividing cells above and around the dermal papilla of the follicle - they grow upward during this phase to form the hair shaft. The anagen phase is chracterized by the growth of the hair, but more importantly, it is characterized by a highly increased proliferative rate of all hair follicle cells in all epithelial compartments with the highest activity observed in matrix cells.

Catagen 
The anagen phase is followed by catagen phase which is short stage in which the hair follicle regresses and dismantles the hair growing part of the hair follicle. This phase can be divided into eight subphases starting with late anagen and ending in early telogen. The hair follicles undergo regression during this phase and this phenomenon is characterized by a cessation of protein and pigment production, involution of the hair follicle and a fundamental re
structuring of the extracellular matrix. Also, massive apotosis in the bulbar, transient, portion of the hair follicle contributes to regression of the hair follicle and the formation of a fibrous streamer in the skin.

Telogen
The regression continues with the telogen phase - now the hair follicle has regressed to about half of its previous size and does not extend beyond the upper dermis. Morphologically, a “finger” of epithelial cells overlies a cluster of dermal fibroblasts; the remains of the dermal papilla. In this phase the dermal papilla ceases to be enveloped by surrounding epithelial cells. It now sits as a small ball of cells in close association with the epithelial cell finger. The epithelial cells of the lower telogen follicle are characterised by the absence of DNA or RNA synthesis.

Kenogen
Scientists observed a novel phenomenon in hair cycling when they used phototrichograms to study hair growth - the empty hair follicles were noticed even after shedding of the hair fiber. Thus the term “kenogen” was coined to describe the interval of the hair cycle in which the hair follicle remains empty after the telogen hair has been extruded and before a new anagen hair reappears.