The length of the hair shaft is dependent on the duration of the hair cycle phases (9). However, the length and duration of the cycle phases is related to various things including the age, gender, hormonal influence, body region, genetic, nutrition, individual health status, environmental factors, neurogenic stimulation, composition of extracellular matrix and drug therapy (4). For instance, the anagen phase in mouse hair follicles as well as human eyebrow hair follicles last for only 2 to 4 weeks. Whereas, in human scalp hair follicle, the anagen lasts for 2 to 6 years (4, 10). On the other hand, the hair follicles stay in catagen for 2 to 3 weeks and the duration of telogen is 3 months on average.
In addition, human hair growth is not synchronised. This means that in different follicles, various phases of cycle can occur simultaneously. Also, in humans around 80% of the scalp hair follicles are in anagen state, where as in poodles it can be up to 98% (4).