1.1.4.1　Hair and hair follicles

The hair is made up of keratinocytes arranged concentrically and keratinized.Palms，soles，flexion of fingers（or toes）and their nodal extensions，lips，nipples，glans，medial foreskin，labia minora，medial labia majora，clitoris are hairless，called hairless skin；other parts with hairs of different lengths，diameters and colors are called hairy skin.Generally，hair can be divided into two types：bristles and vellus hair.The bristles are hard，medulla，and dark in color.There are two types of bristles：①long hair，such as hair，beard，armpit hair，and pubic hair；②short hair，such as eyebrows，eyelashes，nose hair and ear hair，is usually shorter compared with long hair.The vellus hair is soft，without medulla，and lighter in color，mainly seen on the face，limbs，and trunk.The part of the hair that is visible on the body surface is the hair shaft，and the part that is located inside the skin is the hair root.The ends of the hair roots swell in the shape of onions and are called hair bulbs.From the longitudinal section，the hair shaft can be divided into medulla，cortex，and hair scalp from inside to outside.The medulla is the central part of the hair，which is composed of 2-3 layers of cubic cells，and the cytoplasm is lightly stained.At the end of the hair，there is usually no medulla；the cortex is the main part of the hair，which is composed of several layers of fusiform epithelial cells；the hair coat is a thin and transparent layer of keratinocytes，overlapping each other like a roof tile（Figure 1-6）.

Hair follicles are located in the dermis and subcutaneous tissues.They are necessary structures for hair growth and are composed of epithelial cells and connective tissue.The sebaceous gland ducts usually open up into the upper part of a hair follicle，called the infundibulum.The part below the opening of the sebaceous gland ducts to the place where the hairy muscles attach is called the hair follicle isthmus；the end of the hair follicle is mostly spherical，called the hair bulb.The hair follicles are composed of inner and outer hair root sheaths and connective tissue sheaths.The cells of the first two layers of hair root sheaths all originate from the epidermis，while the connective tissue sheaths originate from the dermis.(https://www.daowen.com)

The hair growth cycle can be divided into anagen，catagen，and telogen，corresponding to about 3 years，3 weeks，and 3 months，respectively（Figure 1-7）.Hair in all parts does not grow and fall at the same time.About 80％of all hairs are in the anagen.Normal people can lose 70-100 hairs per day，and there is also an equal amount of hair regeneration.Hair traits are related to a variety of factors including genetics，health，hormone levels（e.g.，sex hormones，thyroid hormones，glucocorticoids），drugs，and climate.