Figure 1. Structure of the skin and components of the HFSC niche.

Skin is composed of epidermis, dermis, and hypodermis layers. Below the hypodermis layer is the subcutaneous muscle layer. The hair follicle is a downward protrusion from the epidermis. HFSCs reside in the outer layer of the bulge region (dark green), and in telogen, they are quiescent. Melanocyte stem cells (black star) are also in the bulge. The inner bulge layer is a non‐HFSC population (light green). Above the bulge is the sebaceous gland (orange). The venous annulus is around the upper bulge (dark purple). Sensory nerves innervate the middle bulge (dark blue). Arrector pili muscle (dark brown) connects the bulge and the epidermis. The lymphatic capillaries (yellow line) are along the side of the hair follicle and connect to the lymphatic collecting (yellow line) vessels that are at the bottom of the hair follicle and parallel to the epidermis. Hair germ (gray) locates below the bulge, housing primed HFSCs. Below the hair germ is the dermal papilla (yellow). Around the dermal papilla is the dermal sheath (teal). The hair follicle is closely associated with blood vessels (red), and at this stage, blood vessels concentrate beneath the hair germ and form a horizontal plexus in telogen, with few dispersed blood vessels in the dermis. Dermal fibroblasts (beige) fill the empty space in the dermis. In the hypodermis are adipose tissue (mature and immature adipocytes, yellow and brown, respectively). Immune cells (light blue) also reside in the skin, with some being more mobile than others. (B) In early anagen, HFSCs are activated. The hair germ proliferates and differentiates to make progenitor or transit‐amplifying cells that enclose the DP and make the matrix. Melanocyte stem cells migrate downward from the bulge. The sebaceous gland is expanded. Adipocytes undergo hyperplasia and hypertrophy. The horizontal vasculature plexus in telogen (A) proliferates and becomes more vertical toward the epidermis in anagen (B). The lymphatic capillary increases its caliber and disassociates from the bulge at anagen. The dermal sheath also extends to enclose the bottom of the hair.