[Hair physiology]

The first hair follicles appear on the body toward the end of the 8th week of intrauterine life. The appearance on the scalp is more tardive 7th month. The hair is a keratin-containing appendage that grows from cavities called follicles. Hair follicles extend from the surface of the skin through the stratum corneum and the epidermis into the dermis. Keratin contains almost all aminoacids but particularly rich in cystein. Hair fibers grow in three distinct stages. The first one, the anagen stage, is the growing phase with a duration of 3 to 5 years. The catagen stage, the transition stage, lasts only a few weeks; it precedes the telogen stage, resting or shedding phase, lasts for 2 months.