Essential knowledge points for laser hair removal: the anatomy and physiological basis of hair
Hair consists of three parts: the hair shaft exposed on the epidermis, the hair roots below the epidermis, and the hair follicles. The hair shaft is composed of three layers: hair medulla, hair cortex and hair cuticle from the inside to the outside. The hair root is wrapped in the hair follicle, and the end is swollen into a ball, called a hair bulb. The center of the hair bulb is concave and connected to the dermal papilla of the hair follicle, called the dermal papilla. It contains rich capillaries and nerve tissue, which can nourish the hair bulb and has sensory functions. The upper part of the hair follicle is the infundibulum and isthmus, and the lower part is the bulb and stem. The hair follicle is divided into three layers from the inside to the outside: the inner and outer hair root sheaths and the connective tissue sheath. Hair is generally divided into vellus hair, terminal hair and intermediate hair. The vellus hair is short, thin and soft, usually without medullation and melanin; the terminal hair is long, thick and hard, with marrow and melanin; the shape of the middle hair is between the vellus hair and the terminal hair.
The terminal hair bulb is located between the dermis and subcutaneous tissue. During the growth period, it can reach as deep as the subcutaneous tissue. It is mainly composed of immature hair matrix cells and contains a small amount of melanocytes. The hair matrix cells are arranged regularly and in a palisade-like pattern just above the hair papilla. They constantly produce new cells and gradually move upward. Some of them differentiate into the hair shaft, and the other part differentiate into the inner hair root sheath. Hair matrix cells have a strong metabolism, are not affected by day and night, and are rapidly renewed. Hair growth is completed through continuous cell division. It is generally believed that the size of the dermal papilla and hair bulb is directly related to the thickness of the hair they produce. The hair papilla may regulate and control the rhythm of the hair follicle growth cycle, playing a very important role in the growth and stability of hair follicles. If the dermal papilla shrinks or is damaged, hair will stop growing and gradually fall out.
The hair growth cycle is generally divided into growth phase, regression phase and resting phase. Each hair follicle undergoes cyclic changes independently, and even adjacent hair follicles are not in the same growth cycle. Hair growth and shedding are determined by the cyclic changes of hair follicles from the growth phase to the resting phase. In addition, nerves and various hormones (androgens, growth hormone, thyroxine, adrenosebum hormone, etc.) are also involved in the regulation of hair follicles and hair growth.
In the next issue we will explain to you: Classification of hirsutism
