Anatomy and Biology of Hair at Different Ages

Abstract

Often considered as a mini-organ, human hair displays complex functions. Adult hair is divided into two parts: the hair shaft, composed of dead, fully keratinized epithelial cells visible on the surface of the scalp, and the root, which includes the hair follicle and its appendages, the sweat and sebaceous glands as well as the arrector muscle, to form the pilosebaceous unit. The follicle presents a continuous cycle of growth and regression, controlled by an environment requiring surrounding niches for hair follicle stem cells and various signaling pathways. To achieve such a complex organization between hair follicles and the surrounding environment, sophisticated morphogenesis is required during embryonic development. Indeed, hair development begins around the eighth week of fetal development and consists of three phases, induction, organogenesis, and cytodifferentiation. This process requires close interaction between the ectoderm and the mesoderm via growth factors, cytokines, neuropeptides, neurotransmitters, and hormones. The first hair emerges in successive waves and presents different morphological and growth characteristics from the terminal hair, which appears between 12 and 18 months. Comprehension of these phenomena is essential to understand the mechanisms of drug incorporation into hair, as well as the difficulties of interpretation of the concentrations, particularly in early childhood.