The hair can be divided into four parts: hair nipple, hair follicle, hair root, and hair shaft from bottom to top. The physiological characteristics and function of hair mainly depend on the hair papilla, hair follicle and sebaceous glands below the scalp epidermis.
The hair follicle is the part of the hair root in the dermis layer, which is composed of the inner hair root sheath, the outer hair root sheath, and the hair bulb. The inner hair root sheath is a hard, straight, thick-walled keratinized tube that determines the shape of the cross-section of hair growth. The lower part of the inner hair sheath is three layers: the HUXLEY sheath, the HENLE sheath and the inner hair root sheath surface layer. Before hair keratinization, the inner hair root sheath grows with the hair, and its source is the cells that propagate at the bottom of the hair follicle. Near the epidermis, the inner hair root sheath is detached from the epidermis and hair follicles.
The hair nipple is the lowermost part of the hair follicle, with capillaries and nerve endings. At the bottom of the hair follicle, epidermal cells divide and differentiate. These epidermal cells differentiate in different ways, forming different components of hair (such as cortex, epidermis, medulla, etc.), and the outermost cells form the inner hair root sheath. At this stage, the cells are soft and nonkeratinized.
The function of the sebaceous glands is to secrete sebum. The sebum is extruded through the sebum tube. When the hair passes through the sebum tube, the sebum extruded from the sebum tube is taken away. Sebum provides natural protection for hair, giving it shine and water resistance. The piloerection muscle is a small muscle organ connected to the epidermis. It depends on the external physiological environment. The piloerection muscle can stretch or contract. The temperature drop or the action of adrenal hormones can pull the hair follicles to a higher position and make the hair stand up.
Fur texture is the main component of hair, which is composed of elongated cells parallel to the long axis of the hair. In these cells there are about 10 mm of tension filaments and interfiber matrix, these components determine the main physiological traits of hair. The filaments are composed of fibrin, and 50% of the protein is in a helical structure, and the matrix is composed of non-spiral proteins rich in cysteine. These proteins are synthesized at the lower end of the hair follicle, and cysteine is converted into cystine at the final stage of synthesis.
The physical properties of hair are related to its chemical composition. Soaking the hair in water will soon swell. The weight after swelling is about 40% higher than the dry weight before soaking. This phenomenon of swelling with water shows that the hair is almost purely a protein component, and the lipid content is very small. The hair has a strong duality, because the arrangement of the filaments in the cell is parallel to the long axis of the hair.
