Anatomy, Histology and Growth Cycle of the Hair Follicle

HEALTHY HAIR TO HAIR DISEASES PROTECTION DIAGNOSIS AND TREATMENT GUIDES, Selda Pelin Kartal, Editör, Turkiye Klinikleri, Ankara, ss.3-14, 2026

ABSTRACT

The hair follicle is a highly specialized cutaneous mini-organ distinguished by its complex structural

organization, unique regenerative capacity, and lifelong cyclic activity. Hair follicle morphogenesis

begins during embryonic development through reciprocal epithelial–mesenchymal interactions that

establish follicular architecture and determine the definitive number and distribution of follicles in

human skin. After birth, hair follicles do not form de novo but instead undergo continuous cyclic

remodeling through the anagen, catagen, and telogen phases. This chapter provides an integrated overview

of the anatomy, histology, and growth cycle of the human hair follicle. The organization of the

infundibulum, isthmus, and lower follicle is described, together with the histological features of the

outer and inner root sheaths, hair shaft, dermal papilla, dermal sheath, sebaceous gland, and arrector

pili muscle. Particular emphasis is placed on the bulge region as a key epithelial stem cell niche and on

the dermal papilla as the principal mesenchymal regulator of hair cycling. In addition, the molecular

and cellular mechanisms governing hair follicle cycling are summarized, highlighting the roles of

Wnt/β-catenin signaling, growth factors, hormonal regulation, oxidative stress, and immune–mesenchymal

interactions in coordinating anagen initiation, catagen regression, and telogen maintenance.

Keywords: Hair follicle; Anatomy; Histology; Growth