What substance is human hair made of? Hair grows, why eyebrows are not long?

How does hair grow? Hair growth is inseparable from hair follicles, and the existence of hair follicles is the premise to ensure hair growth and replacement. During the growing period, the hair follicle is active, the cells at the bottom of the hair bulb divide vigorously, and the separated cells move upward continuously. When the keratin of the soft capsule in the hair follicle is transformed into hard protein, the hair is pushed out of the skin and visible to the naked eye. When the hair grows near the end of the growth period, the cells of the hair bulb stop proliferating, the hair follicle begins to shrink and the hair stops growing. This is a retrogression period. At rest, all parts of the hair will age, degenerate and shrink, and the hair will fall off. At the same time, a growing hair ball is formed near the aging hair follicle, and a new hair is born.

2. Hair density

The density of hair varies with gender, age, individual and location. Adult males are estimated to have 5 million hair follicles, of which 6.5438+0 million are in the head and about 6.5438+0 million are in the scalp. Hair follicle density on forehead and cheeks is 4-6 times that of trunk and limbs. It is generally believed that the density of hair follicles is congenital, and it is impossible to add new hair follicles in adulthood.

The growth rate of hair is related to its position, and the head grows fastest, with a daily growth rate of 0.27-0.4MM, with an average of 1.9CM per month, and other parts grow about 0.2MM per day. Men's hair generally grows faster than women's. 15-30 years old grows fastest, and hair growth slows down in old age. It grows faster in summer.

The periodic changes of hair growth and rest are regulated by endocrine, and some people think it is related to ovarian hormones. In addition, nutrients can also affect hair growth, and vitamin B complex can affect hair growth and epidermal keratinization. When treating tinea capitis with vitamin A, it will lead to hair loss, and the reason is excessive vitamin A. ..

3. The growth period of the first hair

1) growth period

Also known as growth activity period, the growth period can last for 4-6 years or even longer, the hair is in an active proliferation state, the cells in the lower part of the hair bulb divide rapidly, and the cells in the upper part of the hair bulb differentiate into cortex and small fur; Dermal papilla is enlarged, cell division is accelerated, and the number is increased. The original inactive melanin grew dendrites and began to form melanin.

(2) Degeneration period

Also known as atrophy or degeneration, it lasts for 2-3 weeks. The positive proliferation of hair stops, forming rod-shaped hair, with eosinophilic homogenate at the lower end and bamboo sticks at the periphery. The inner hair root sheath disappeared, the outer hair root sheath gradually keratinized, the hair bulb was flat without depression, the hair papilla gradually narrowed and the number of cells decreased. Black cells lose their dendritic processes and are round and inactive.

(3) Resting period

Also known as hibernation or hibernation, it lasts about 3 months. At this stage, the hair follicles gradually shrink and re-form 1 growing hair ball near the aging hair follicles. Finally, the old hair falls off, but at the same time, new hair grows out and enters the growth period and repetition cycle. In the scalp, 9%- 14% of the hair is at rest, only 1% is at regression, and 90% of the eyebrows are at rest.

The proportion of hair in different stages of the growth cycle varies with different parts.

4. Physiological characteristics and mechanical energy of the first hair

Hair can be divided into papillae, hair follicle, hair root and hair shaft from bottom to top. The physiological characteristics and functions of hair mainly depend on nipples, hair follicles and sebaceous glands under the scalp epidermis.

Hair follicle is the part of hair root in dermis, which consists of inner hair root sheath, outer hair root sheath and hair ball. Inner hair root sheath is the sheath layer directly adjacent to hair in the later stage of hair development. The inner hair root sheath is a hard thick-walled keratinized tube, which determines the shape of the cross section when the hair grows. The lower part of the inner hair sheath has three layers: Huxley sheath, Henler sheath and the surface layer of the inner hair root sheath. Before hair keratinization, the inner hair root sheath grows with hair, and its source is the cells propagated at the bottom of hair follicle. Near the epidermis, the inner hair root sheath is separated from the epidermis and hair follicles.

Papillary process is the lowest end of hair follicle, which is connected with capillaries and nerve endings. At the bottom of the hair follicle, epidermal cells continue to divide and differentiate. These epidermal cells differentiate in different ways to form different components of hair (such as cortex, epidermis and medulla), and the outermost cells form the inner hair root sheath. At this stage, cells are soft and keratinized.

The function of sebaceous glands is to secrete sebum and squeeze it out through the sebum tube. When the hair passes through the sebum tube, it will take away the sebum squeezed out of the sebum tube. Sebum provides natural protection for hair and gives it luster and waterproof performance.

Hair muscle is a small muscle organ connected with epidermis, which depends on the external physiological environment. Hairy muscles can stretch or contract. The drop in temperature or the action of adrenal hormones can pull the hair follicle to a higher position and make the hair firm.

References:

(zz and Xinji Medical Beauty)