[Abstract]: Hair transplantation first began in the 19th century and has been continuously developed and perfected to the level of microscopic single hair transplantation. Hair transplantation is now widely used in the clinical treatment of various hair loss, with drugs to achieve better results. Meanwhile, basic research on hair follicle growth and the increasing maturity of surgical instruments and methods have yielded fruitful results in recent years. Further breakthroughs in research areas such as hair follicle cloning and allogeneic hair transplantation will also be achieved soon. [Keywords]: hair; transplantation; progress Hair is not only an important aesthetic component of the human body, but also has certain physiological functions. Hair loss, especially the common androgenetic alopecia, has become a common disease in China, causing psychological stress and inconvenience to patients’ lives. Over the past half century, with the mature development of hair transplantation technology, more than 220,000 patients around the world receive hair transplantation surgery every year, and more and more patients with hair loss have received safe and satisfactory treatment, enabling hair transplantation technology to be widely used in clinical practice. I. History of hair transplantation The earliest account of hair transplantation can be traced back to the 19th century, and as early as 1800, Baronio had successfully conducted experiments on animal autologous hair transplantation. in 1933, Okuda, a Japanese dermatologist, invented the perforated scalp composite tissue sheet method to perform hair transplantation for the first time for patients with hair loss. in 1943 and 1953, Tamura and Fujita, respectively, performed hair transplantation on patients with hair loss. In 1959, Norman Orentreich adopted a standard 4mm punch to obtain hair for autologous hair transplantation and published a follow-up report with the famous “donor zone advantage” theory. “theory. Since then, the Okuda-Orentreich technique has become a classic surgical method for autologous hair transplantation and is considered the surgical technique for permanent hair transplantation. After 1980s, with the success of micro hair grafting, hair transplantation technique developed rapidly. after 1990s, the method based on follicular unit transplantation, together with multi-blade scalpel or CO2 laser instead of punching, formed a perfect systematic surgical plan to achieve treatment results with near natural appearance. In recent years, along with the innovation of surgical instruments and the skill of the surgeons, single hair follicles have been applied to localized hair loss and the reconstruction of body hair such as eyebrows and eyelashes, obtaining near-perfect postoperative results. The microscopic hair transplantation technique has been widely recognized clinically. The basic principle of hair transplantation for hair loss is based on the “donor zone advantage” theory of hair proposed by Orentreich. The content mainly refers to the fact that the occipital scalp is not regulated by androgens and can maintain its own characteristics after transplantation for life without being affected by the hormonal regulation of the recipient area. In addition, the post-operative effect of hair transplantation is based on the following aesthetic principles: 1, the normal hair density is much greater than the density discernible to the naked eye, if less than the normal number of hairs and reasonable distribution, but still can achieve a natural appearance. 2, the forehead hairline implantation adjustment, for hair loss patients to improve the frontal view has an important role. 3, for patients with hair loss in a large area of the recipient area, the unbalance of the partial side Transplantation, post-operative high-density transplantation area hair to the low-density area of the cover, can make up for the lack of relative shortage of the amount of hair in the donor area. However, hair transplantation surgery is not immediate. After surgery, hair follicles generally enter the regression and resting phase, and hair trunks are usually shed gradually after surgery. 6-12 months later, the transplanted hairs gradually enter the anagen phase and the hair trunks grow abundantly to see the best results. Pre-operative evaluation Pre-operative evaluation and communication with the patient is a key step to the success of the surgery. For patients with different conditions and different degrees of hair loss, screening of the suitable population should be carried out, and preoperative physical examination should also be conducted to check the thickness, color, texture and morphological characteristics of the patient’s hair, as well as the density of hair in the donor area and the extent of the hair loss area. The procedure will be designed and planned by combining the above elements. For example, for patients with thick hair shaft, dark color, hard texture, curly morphology, high density of the donor area and small extent of the recipient area, satisfactory results can be achieved by reducing the area and number of hairs taken from the occipital area in response to the above factors. Drug and surgical synergy treatment should also be fully communicated with the patient before surgery. In this way, it can be ensured that the hair survival is maintained based on the maximum amount of implants. Finasteride (finasteride), a drug that regulates androgen metabolism, and Minoxidil (minoxidil), a drug that promotes hair growth, are clinically proven to delay hair loss, and both are approved by the FDA for male pattern baldness, while Minoxidil can be used for female pattern baldness, and are now widely used clinically. Post-operative complications and treatment Hair transplantation is a relatively safe and mature surgery, its complication rate is low, mainly seen in the following: 1, bleeding: mainly because the scalp blood supply is rich and caused by improper operation, surgery time is too long. Therefore, it is necessary to routinely check the patient’s blood clotting before surgery and be familiar with the anatomical level of the head to ensure gentle surgical operation and improve the efficiency of implantation. 2. Infection: As long as the aseptic operation specifications are strictly followed and antibiotics are applied prophylactically, the occurrence can generally be avoided. 3. Swelling: The swelling of the operated area is the main concern 1-2 days after surgery, and develops to the forehead and periocular area in 3-5 days. As long as the amount of anesthetic solution for swelling is paid attention to during the operation, it can generally subside in 1 week. 4. scar growth: mainly seen in the donor area, so the amount of skin taken from the occipital area should be controlled during the operation, and patients are advised to avoid activities that increase the tension of the occipital scalp such as bending the neck and waist after the operation. 5. epidermal-like cysts: mostly caused by the adhesion of the epithelium of the transplanted hair graft implanted under the skin of the recipient area, which can generally be cured by incision and drainage under local anesthesia, and 6, endogenous hair and foreign body reaction: implantation of hair embryo containing only part of the hair follicle, hair growth will lose the original epithelial orifice, resulting in the forced growth of hair in the subcutaneous tissue, stimulating the tissue to produce a foreign body reaction. The local area may show inflammatory reactions and even form abscesses repeatedly. Therefore, intraoperative screening of hair embryos should be done, and postoperative anti-inflammatory and pus-removal treatment can be given for the above conditions. V. Research progress The medical field has made great progress in the research of hair transplantation. Due to the high demand for hair follicles in the surgery of many patients with hair loss, the surgery time is long, which is bound to increase the pain of patients. Some foreign doctors recommend multiple surgical procedures, but multiple occipital hair harvesting also causes pain and affects the healing of the occipital wound. In 1990, Philpott was the first to successfully culture free human hair follicles. It was found that the hair follicles were suspended in Williams’ medium and epidermal growth factor (EGF) promoted the formation of “club hair”, which could facilitate the transition from the anagen phase to the degenerative phase. In 1996, Jinjin Wu et al. at the Southwest Hospital of the Third Military Medical University reported the successful in vitro culture of hair follicles from cadaveric scalp preserved in liquid nitrogen. The study showed that hair follicles were cultured in 10% neonatal bovine serum DMEM medium spiked with insulin and hydrocortisone, and insulin and hydrocortisone had a role in maintaining the growth pattern of hair follicles, and DNA synthesis was mainly the result of hair matricellular division and proliferation. In 2003, Lindenbaum et al. reported that the addition of insulin, thyroxine and growth hormone to MCDB153 medium to make CCM gel can be used topically to treat androgenic alopecia, which accelerates hair growth by stimulating epithelial regeneration, increases hair growth rate and reduces hair loss. ”Clonal therapy” is an autologous cell expansion technique, which mainly involves the culture and expansion of isolated hair follicle stem cells. With the rapid development of tissue engineering and cloning technology, the screening and cloning of hair stem cells has become possible and has been successfully tested on animals.Cotsareli et al. found that hair follicle stem cells were present in the augmentation site and they also found that by cutting the hair follicle horizontally, both parts of the isolated follicle could regenerate new hair follicles, which led to the expansion of hair follicle volume.Jahoda [14] et al. studied that human hair In 2001, Swinehart reported the successful results of hairline implantation using the flat-fractioned follicle cloning method. 2009, a scholar at Shantou University School of Medicine proposed the Efficient Follicular Regeneration Unit (EFRU) and the Minimum Follicular Regeneration Unit (SFRU), which are expected to regenerate more donor hair follicles for hair loss treatment. In the clinical application of hair transplantation surgery, the main hair transplantation devices are: Choi hair transplants, Boudjema hair transplants, Carousel hair transplants, P-FUE perforators and Calvitron hair transplantation machines. Among them, Carousel hair transplantation machine is a one-step process that perforates the recipient area and implants the hair, with a speed of up to 40 grafts per minute, which is 6 times faster than manual operation. The principle of the Calvitron is to improve the efficiency of the implantation. In terms of donor preparation, the French doctor developed a hair germ cutting machine to improve the preparation efficiency, but the disadvantage is that some of the hair follicles are damaged and the donor is wasted to some extent. In addition, the clinical application of laser devices such as ultra-pulsed CO2 for perforating the implant area has the advantages of high speed, less damage and less bleeding. In the classical surgical method, the multi-blade knife has been widely used to perforate the jewel knife or diamond knife, which not only improves the surgical efficiency, but also greatly reduces the damage to the operating area and achieves better clinical results. VI. Outlook The clinical application of hair transplantation has become more and more widespread, from the traditional unit follicle transplantation for hair loss to single follicle microscopic implantation for eyebrow, eyelash, and beard loss, as well as suture needle implantation method for eyelash reconstruction [20] and other techniques to promote the continuous development and progress of hair transplantation. It can be expected that in the near future, in terms of basic research: breakthroughs will be made in the regulation and treatment of baldness genes, and the technology of hair follicle stem cell cloning will be gradually promoted to the clinic after perfecting human experimental research. In terms of clinical practice: new hair germ preparers and hair implanters will be improved and perfected, follicular complex injectable transplantation methods may be further validated and applied, surgery and medicinal solution with treatment to form a systematic program, and allogeneic hair follicle transplantation technology will bring hope to some patients with special hair loss.