Hair Development, Growth, and Loss 40 effective approaches for the prevention and treatment of a variety of hair growth disorders and hair follicle regeneration. Functioning epidermis and hair follicles require maintenance of tight yet flexible contact between epithelial cells, melanocytes, dermal fibroblasts and other cell populations inside the skin, which unfortunately cannot yet be feasibly achieved using an in vitro system. The mouse model is the most straightforward and best- studied system for the investigation of the molecular mechanisms that cause the abnormalities in hair structure, growth, and/ or pigmentation, thus the mice have been extensively studied as an ideal model to identify the genetic controls of growth and regeneration of skin and hair follicles.4 This chapter will review basic knowledge of the development, growth, and disorders of human and murine hair follicles, and summarize recent advances in the field. (Please note that several terms are defined in the section titled Concluding Notes and Term Definitions at this chapter’s close.) Introduction: In adult humans, there are two major hair types: heavily pigmented terminal hairs on the scalp, eyebrow, eyelash, beard, axillary and pubic area, and vellus hairs that are most seen in facial and trunk skin.5 Mice have more hair types, including pelage overcoat and undercoat hairs, vibrissae, eyelashes, and hairs around the feet, tail and genital areas. Identifying various hair types is very important when analyzing the hair phenotype of different mouse models, since distinct molecular mechanisms are involved in the formation of different types of hair follicles.6 Hair is composed of the hair follicle and the hair shaft (HS). The HS is generated by the hair follicle, which is a self-renewing mini-organ that consists of epidermal and dermal compartments (Figure 1). The epidermal compartment can be divided into a permanent upper portion that does not cycle visibly, and a non- permanent lower portion, which is continuously regenerated during successive hair cycles. The permanent upper portion can be divided into two regions: the upper segment and the middle segment. The upper segment extends from the entrance of the sebaceous gland duct to the opening of the hair canal and is called