Cells and Structures Involved in Hair Follicle Regeneration: An Introduction

Abstract

Introduction The hair follicle spans across the skin epidermis, dermis, and hypodermis and undergoes repeated cycles of growth (anagen), regression (catagen), quiescence (telogen), and shedding (exogen). Research over the past decades have increasingly shown that hair regeneration not only depends on hair follicle stem cells (HFSCs), but also involves coordinated interactions with surrounding cells and structures. Methods PubMed, Web of Science, and Google Scholar were used to find peer-reviewed articles examining HFSCs, fibroblasts, adipocytes, immune cells, lymphatics, blood vessels, nerves, and exosomes in the context of hair regeneration. Results Distinct populations of HFSCs exist in the bulge and secondary germ of the hair follicle. During hair cycling, HFSCs interact with specialized dermal fibroblasts called dermal papilla cells and dermal sheath cells, which have hair-inducing properties. Dermal adipocyte differentiation and remodeling, including fluctuations in dermal adipose thickness, are associated with changes in HFSC activity. Perifollicular macrophages, mast cells, cytotoxic T cells, and regulatory T cells modulate hair growth and have been implicated in various hair disorders. The organization of lymphatic and blood vessels relative to the hair follicle may be correlated with phases of the hair cycle. Substance P and cutaneous norepinephrine signaling may also regulate hair growth. Intercellular communication in the hair follicle microenvironment may be mediated by extracellular vesicles called exosomes, which deliver a variety of cargo in a cell-specific manner. Conclusions Understanding the interdependence of cells and structures involved in the hair growth cycle, as well as mediators of intercellular signaling, are essential for the study of hair regeneration.KeywordsHair follicle stem cellBulgeSecondary germFibroblastDermal papillaDermal sheathPreadipocyteAdipocyteImmune cellMacrophageMast cellLymphocyteT cellLymphatic vesselBlood vesselNerveExtracellular vesicleExosome