39 Chapter 2 Hair Development, Growth, and Loss Yuhang Zhang, PhD College of Pharmacy and Department of Dermatology, University of Cincinnati Chao-Chun Yang, MD Department of Dermatology, College of Medicine, National Cheng Kung University, Taiwan Biology of the Hair Follicle Hair plays a significant role in our lives. Over the centuries and across different cultures, decoration and styling of scalp hair have been important to the appearance of men and women. Beginning to develop in the first trimester, the total number of scalp and body hairs can reach 5 million at birth. The number of scalp hairs is around 90,000–130,000 and varies between ethnic groups. No new hair follicles are formed after birth, although the size, color and cycling pattern of the hair may change throughout one’s life.1 As the size of the body increases, however, the density of the hair follicles on the skin surface decreases. The mature hair follicle not only has its role as an important symbol of social interaction, it also serves a wide range of functions including thermoregulation, physical protection and sensory activity.2 Hair follicles are reservoirs of epidermal keratinocytes and melanocytes and contribute to the process of wound healing or repigmentation of the epidermis. Hair follicles are also the sites of origin of many neoplasias, including basal cell carcinoma and pilomatricoma.3 Full characterizations of the molecular pathways and involved molecules that control hair follicle formation and cycling are critical to the development of