Hair falls out… and grows again
Once the follicle is in place and its morphogenesis is complete, it goes through growth and resting cycles that change its structure. The average length of these hair cycles is genetically programmed and lasts for two to four years in men and from four to six years in women.
The four phases of a hair
- The anagen phase: for a period of about three years, the hair shaft is produced continually and all of its structures and compartments are active. The average rate of growth during this period is 0.35 millimetres per day.
- The catagen phase: the mitotic activity of the matrix cells and the differentiation programme of the inner root sheath stop. This short phase (about three weeks) is characterised by regression and the beginning of involution of the hair follicle as the epithelial column declines, particularly by programmed cell death, also known as apoptosis.
- The telogen phase is a resting phase, which lasts for about three months. During this phase, the hair remains attached to the follicle by a structure in appearance similar to a golf club. The hair bulb becomes superficial and continues to rise until the hair is shed, thus terminating the telogen phase. More than 50 hairs are lost like this each day. Luckily, humans have non-synchronised hair follicles and hair renewal cycles are independent. If this were not the case, seasonal moults would lead to sudden hair loss as is the case with animal fur.
- The exogen phase: at the end of the telogen phase, a new hair bud forms in contact with the dermal papilla which has remained below. A new follicle is generated and a new anagen phase begins in order to produce a new hair shaft. While the old hair at the end of the telogen phase can coexist with the new hair, a latent phase of two to five months is generally seen: this is called the exogen phase. To begin a new anagen phase, a permanent component of the follicle structure must be able to detect the biological messages emitted by dermal papilla and thus initiate a new cycle. The existence of these renewal cycles is dependent upon this.