Hair is an appendage of the skin. Except for the palms, soles, sides and backs of the distal segments of the fingers (toes), lips and nipples, clitoris, labia minora and labia majora, glans penis and inner surface of the foreskin, hair grows on all other parts of the body.
According to the growth and texture of hair, it can be divided into three types: lanugo, vellus and terminal hair.
(1) Lanugo: grows in the fetus, is fine and soft hair without medulla and pigment, and generally falls off about 4 weeks before birth.
(2) Vellus: soft and without medulla, occasionally with pigment, usually not exceeding 2cm.
(3) Terminal hair: long and thick, with medulla and pigment, such as hair, eyelashes, eyebrows, pubic hair and armpit hair.
Hair consists of hair follicles and hair shafts. From an anatomical point of view, the hair follicle consists of the hair bulb, isthmus and infundibulum. The hair bulb extends from the base of the hair follicle to the attachment of the arrector pili muscle, including the stem and bulb. The part of the stem that contains special cells is called the bulge, where hair follicle stem cells exist and are the source of hair follicle germinal cells. The bulb contains the hair papilla and the hair matrix. The hair papilla is related to inducing and maintaining hair nutrition and growth. The hair matrix is the growth area of the hair follicle and hair, which contains melanocytes. The isthmus is from the attachment of the arrector pili muscle to the opening of the sebaceous gland. The infundibulum is from the opening of the sebaceous gland to the opening of the hair follicle. From the outside to the inside, the hair structure is divided into the connective tissue sheath, the outer root sheath, the inner root sheath and the hair shaft. The connective tissue sheath originates from the dermis. The outer root sheath turns over in the infundibulum and connects with the epidermis. At the end of the isthmus, the outer root sheath cells proliferate to form a bulge, which can serve as a reserve for regeneration after hair follicle damage. The inner root sheath provides a hard support for the growing hair shaft and separates from the hair shaft at the level of the isthmus. The sliding plane between the enclosed hair shaft and the residual hair follicle is in the paired layer of the outer root sheath. Hair is a special part of the epidermis that grows outward. It is composed of keratin cells, the main component of which is keratin, accounting for 85% to 95% of the total hair shaft. From the inside to the outside, it can be divided into three layers: inner, middle, and outer. The inner layer is located in the center of the hair and is called the medulla (but some hairs, such as vellus hair, have no medulla). The middle layer is called the cortex, which is the main part of the hair structure. Under an electron microscope, the cortical cells have tension filaments and interfibrillar matrix. The outer layer is called the keratin, also called the hair cuticle, which is composed of 6 to 10 layers of flat, long fish-scale cells arranged in an overlapping manner and is the protective layer of the hair shaft.
Although there is no essential difference in the composition of hair among different races, there are certain differences in the shape of hair, the most obvious of which is the hair. For example, Asians generally have black, straight hair, which is relatively uniform from the root to the tip, and the cross-sectional shape of the hair is round and thicker in diameter: Europeans have silver-gray, golden, brown-black and black hair, which are generally straight or slightly curved, with a rounder cross-sectional shape and thinner hair; while Africans generally have black, curly hair, and an oval cross-sectional shape; the physical form of the hair of mixed-race black and white people is basically between the hair forms of Europeans and Africans.
The color of hair is determined by the amount, distribution and type of melanin contained in the hair shaft, which can be black, brown, golden, red and white. There are two types of melanin in human hair: brown-black eumelanin and red pheomelanin. Most red-haired people have mutations in the melanin 1 receptor (MC1R) gene. The number of melanosomes and the degree of melaninization in blond hair are much lower. The cause of gray hair is the exhaustion of melanin stem cells and the reduction of the number of melanocytes. Melanin is the color base of hair and skin and is the target tissue for laser hair removal. Although melanocytes exist in all parts of the hair follicle, only those located in the upper part of the hair bulb and the upper part of the infundibulum at the hair follicle matrix can synthesize melanin granules. The melanin granules produced by the melanocytes in the upper part of the hair bulb will be transferred to the growing hair shaft. The function of the infundibulum melanocytes is similar to that of their corresponding epidermal melanocytes. The melanocytes in the outer root sheath of the hair bulb and the middle and lower parts of the hair follicle are typical non-pigmented cells, but they may be activated after being damaged.
Hair growth is cyclical and can be divided into growth phase, regression phase and resting phase. The growth of hair follicles in different parts is not synchronized and has its own cycle, the length of which is related to the different growth cycle time. The length of the growth phase determines the length of the hair shaft. The regression phase is relatively constant, but the resting phase varies greatly in different parts. For example, the growth phase of hair can reach 3 years, the growth phase is generally 3 weeks, and the resting phase is about 3 months. The growth phase of eyebrows and eyelashes is only about 2 months, and the growth phase of armpit hair and leg hair is about 4 months.
The histochemical changes of hair in different cycles are different. The growth and coloring of hair only occur during the growth phase. Hair growth originates from the secondary hair buds in the subepidermal bulge. As the growth phase continues, the hair bulb moves deep into the dermis. The depth varies in different parts, and the deepest can reach the fat layer. The transition from the growth phase to the regression phase is a very important step in the hair removal process, which is regulated by changes in the expression of multiple growth factors (such as β, transforming growth factor and fibroblast growth factor 5). During the regression phase, the hair matrix degenerates, cell division stops, capillaries decrease, the hair papilla atrophies, and the melanocytes of the hair bulb stop producing and transporting melanin. This phase can last for several weeks. Controllable hair regression is caused by the massive death of the epithelial cells of the lower hair follicles. During the resting phase, melanin synthesis stops, and a part of the hair follicle melanocytes apoptosis, resulting in the loss of pigment in the hair shaft proximal to the hair rod, forming a pigment-free hair rod in the resting phase. At the same time, the hair follicle separates from the hair papilla, and the hair falls off. The resting phase has the characteristics of relative rest during the proliferation phase. The mechanism of hair follicles returning to active growth from the resting phase is not completely clear. It may be that when the resting phase is nearing the end, the dermal papilla shrinks and moves upward to the bulge, and the degenerating epithelial column is transformed into a secondary hair bud. With the reconstruction of the undifferentiated epithelial cells of the hair papilla, the hair begins the growth phase, forming new hair matrix and hair bulb, and the melanocytes in the hair bulb also resume normal function. This process is related to the interaction between the secondary hair bud, bulge, dermal papilla and different signal molecules.
There is a potential correlation between the timing of laser hair removal treatment and these cycles. Understanding them can not only help us determine the best cycle for laser treatment, but also determine the effect of laser permanent hair removal. It is currently believed that hair in the early growth phase is more sensitive to the destructive effect of laser. Because hair growth is active during this period, hair matrix cells divide rapidly, and melanin is the most. In addition, because the hair follicles are small and the growth site is shallow, the laser can penetrate to a sufficient depth. The hair follicles in the middle and late growth phases are the largest and deepest. The hair follicles that have just left the resting phase have shallow hair bulbs, but there is no pigment at the proximal end of the hair shaft and cannot absorb the laser energy well. These will affect the number and time of laser hair removal treatments. On the other hand, the depth of the hair bulb does not always have a negative impact on the effect of hair removal. Because fat has better thermal insulation than collagen, the heat damage to the hair bulb located deep in the fat tissue will be better confined to the hair follicle after receiving heat. The hair bulb stops producing melanin during the regression and resting phases, and the laser has no obvious effect. Laser treatment can only work after the melanocytes resume function when the hair enters the growth phase. Since the hair growth cycle is not synchronized, the relationship between the hair growth cycle and the effect of laser hair removal is more complicated. People have tried to focus the treatment target on the hair in the growth phase, hoping to improve the effect of hair removal, but ultimately failed. Many results in this field are contradictory and require more in-depth research. As can be seen from the above, laser hair removal requires multiple treatments because hair in different parts has different growth cycles. If the proportion of hair in the growth phase of a certain part is small, the number of treatments will be relatively more; conversely, the number of treatments can be reduced. At the same time, there are differences in the intervals between laser treatments. The treatment cycle of general laser hair removal is 4 to 8 weeks apart, and the response time of different parts to laser treatment is different. For example, the lip hair has a relatively short resting period, so the treatment interval can be as short as 1 month; the trunk and limbs hair have a relatively long resting period, so the treatment interval is preferably about 2 months. In addition, the depth of the hair is related to the choice of laser. If the depth is deep, a longer wavelength laser must be used for treatment; otherwise, a shorter wavelength laser can be used for treatment.
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