Hair follicles are the skin organs used to grow hair, and they are also the only organs in mammals that cycle through life and are distributed on the entire body surface skin except palms, soles and lips.
Hair follicles can help the epidermis to maintain the body from damage from the external environment, and play an important role in physiological tissue renewal and trauma repair .
At present, studies have found that the hair follicles with a larger contact area with the pili muscle last longer, while the hair follicles with a smaller contact area with the pili muscle are more likely to be stimulated by androgen and turn into vellus hairs .
The basic structure of mammalian hair follicles is all the same. Hair follicles are composed of connective tissue sheath (CTS), inner root sheath (IRS), outer root sheath (ORS), hair bulb (Hair bulb) and hair shaft (Hair shaft), etc. .
The hair bulb is surrounded by some dense dermal fibroblasts, and the invaginated part is called the dermal papilla (Dermal papilla, DP), which is an inducible structure for sending and receiving signals . It is composed of Dermal papilla cells (DPC) in the upper part of the dermis, which can induce hair follicle regeneration . The inner root sheath is a rigid structure that surrounds the hair shaft, determines the shape of the hair shaft, and consists of the sheath, Huxley’s layer, and Henle’s layer. Outside the inner root sheath, the outer root sheath is composed of thick cells that surround the entire inner root sheath and hair shaft. The outermost part of the hair follicle is the connective tissue sheath, which consists of several layers of fibroblasts that separate the follicle from the dermis .
Hair follicles are actually composed of two major parts, the epithelium (hair matrix and outer root sheath) and the dermis (hair papilla and connective tissue sheath), which contain more than 20 types of cells, including keratinocytes, melanocytes, Langerhans cells (Langerhans cell), Merkel cell (Merkel cell), dermal sheath cells and dermal papilla cells .
Most mammalian hair follicles are divided into two categories, namely primary hair follicles (PF) and secondary hair follicles (SF). The primary hair follicle occurs earlier, the hair bulb is large, the hair follicle is longer and thicker, it has two developed sebaceous glands (Sebaceous gland, SG), and the hair shaft generally has medulla. Secondary hair follicles occur later, with small hair bulbs, short and thin hair follicles, no or only one underdeveloped sebaceous gland, and no medulla in the hair shaft. Most adult mammalian hair follicles are distributed in groups, and each hair follicle group basically consists of one to five primary hair follicles and several secondary hair follicles surrounding the primary hair follicles.