Hair grows, sheds and regrows continuously throughout the course of a person’s life. In general, the number of naturally shed hairs is 50-100 per day, with a corresponding number of new hairs sprouting, maintaining a dynamic balance overall. However, this metabolic process can be influenced by many factors.
I. Hair growth rate.
1.Hair development
The original base of hair appears on the scalp around the 10th week of embryonic development, and by the 22nd week the hair follicles are fully developed and all grow out. When infants are 2-3 months old, the first hairs at the back of the neck will fall out naturally, many people mistakenly believe that they fall off by friction. In infancy, hair grows better at the front of the head first, and then gradually develops backwards. There are about 100,000 hairs on the adult scalp. Generally speaking, the density of hair follicles is congenitally generated, and the number of new follicles cannot be added in adulthood.
2.Factors affecting hair growth
Hair is the fastest growing hair, daily growth of 0.27-0.4mm, about 10-14cm a year. each hair can grow 2-7 years. However, hair cannot grow indefinitely, otherwise, by the age of 10, hair is more than 1m long. Generally hair grows up to 50-60cm, then it will fall off and regrow new hair. The growth limitation of hair is mainly influenced by the hair growth cycle. In addition, the growth rate of hair is also related to hair follicles, gender, age, location, season, day and night, nutrition and health condition.
Second, hair growth cycle
Hair growth and replacement are not continuous, but the cycle has been established in the 4th and 5th month of embryonic life. From the growth period, migration period to the resting period, a cycle is formed, just like the process of birth, aging, illness and death in a person’s life. Not all the hair on the head is active and alive, a large part is alive and a small part is dead but still “dying” on the scalp or in the hair follicle. The following are the different stages of hair growth.
1. The anagen phase
The anagen phase lasts from 2 to 7 years, or even longer, with an average of 4 years, which is the normal lifespan of each hair. During the anagen phase, the hair is actively proliferating and the cell division of the lower part of the hair bulb in the hair follicle is accelerated; the hair papilla increases in size, cell division is accelerated and the number increases. About 85%-90% of the hair is in this phase.
2.Degenerative stage
Also known as the atrophy or degeneration phase, it lasts for 2-4 weeks. The active proliferation of hair stops, the structure of the hair bulb is flattened, the hair papillae gradually shrink, and the number of cells decreases. The melanocytes lose their dendrites, are round and inactive, and stop producing melanin. The inner hair root sheath gradually disappears. 1% of hairs are in this stage.
3.Resting phase
Also known as resting phase or resting phase, it lasts for about 3-4 months. In this stage, the base of the hair follicle gradually atrophies and further atrophies in 2-4 months, and finally the hair falls out. The hair that remains on the comb or falls out after each combing is part of the resting phase hair. An anagen hair bulb is re-formed near the already senescent follicle and a new, healthy hair will then begin to grow in that location, repeating the entire cycle. Between 9% and 14% of the hair in the scalp is in this phase.
III. Pigment metabolism of hair
The pigment metabolism of each hair differs little, and the process considers 3 main aspects, such as the occurrence of melanocytes, the mechanism of pigment metabolism and regulation.
IV. Regulating factors of hair growth
It has been confirmed that there are 4 major categories of regulatory factors that have direct effects on hair follicles, mostly some growth factors, namely: epidermal growth factor, fibroblast growth factor, transforming growth factor and other regulatory factors such as vascular endothelial growth factor, hepatocyte growth factor, interleukins, etc. These growth factors exert their pro-growth effects by binding to specific receptors located in the hair follicle.
V. Aesthetic criteria of hair
There are objective criteria to determine whether the hair is healthy and beautiful, based on the following aspects.
1, hair without disease, can not have pathological alopecia, hair clean, neat, no head scale, dandruff.
2, the hair is glossy and soft, not oily nor dry, silky visible luster, elastic.
3, moderate thickness, smooth surface, no split ends, no knots, no defects.
4, moderate sparse and dense, hair root distribution evenly.
5, uniform color, hair dry and hair ends do not appear two colors, no white, yellow, brown and other colors mixed.
6, no change due to external influences.