Embryology, Histology, and Physiology of the Hair Follicle

The document from 2017 provides an in-depth analysis of the hair follicle's development, structure, and growth cycle. It explains that hair follicles, sebaceous glands, and apocrine glands originate from the same embryonic cells and highlights the importance of this for understanding related neoplasms. The hair follicle is divided into the infundibulum, isthmus, and lower segment, with the lower segment undergoing changes during the hair growth cycle, which includes anagen, catagen, and telogen phases. The document details the histological features of these segments, the intense proliferative activity of the follicle, and the role of various cells and structures, such as the follicular papilla and matrix cells, in hair growth. It also discusses racial differences in hair follicles, the keratinization process, and the composition of the hair shaft. Additionally, the document advocates for the use of horizontal sections in the histopathologic examination of scalp biopsies for alopecia, as it allows for a more comprehensive analysis of hair follicles. It also describes the development and life cycle of sebaceous and apocrine glands, as well as the innervation and anatomy of the arrector pili muscle.