The stages of embryonic hair morphogenesis and the postnatal hair cycle are presented.
Top, Embryonic morphogenesis.
The placode, germ and peg stages represent initiation of hair follicle formation, and result in subsequent formation of the hair placode, germ and peg. The bulbous peg represents a stage of cytodifferentiaiton, when the matrix cells (MC) complete differentiation into hair shaft (HS), inner root sheath (IRS) and companion (Cp) layers and when the outer root sheath (ORS), including the sebaceous gland (SG) and bulge, is formed. The mature hair follicle consists of an upper permanent part, that includes the bulge and the SG and is supported by ancestor pili muscle (APM), and the lower cycling part, which lies below the bulge area.
Middle and bottom, Hair cycle.
The mature hair follicle enters the catagen (destructive) phase, where the lower cycling part of hair follicle undergoes apoptosis and is replaced by a regressing epithelial column. The hair follicle moves upward towards the skin surface, forming a structure known as club hair, which will eventually be shed. During the subsequent telogen (resting) phase, the dermal papilla (DP) detaches from the epidermal part of the hair follicle and rests under the club hair. During the anagen (growth) phase, the bulge stem cells repopulate the cycling compartment of the hair follicle and create the new hair germ.
Center, Hair follicle cell layers.
The IRS consists of medulla (M), cortex (Co) and cuticle (Cu) layers.
The HS consists of the hair shaft cuticle (Ch), the Henle's layer (He) and the Huxley's layer (Hu).
The companion layer (Cp) separates the IRS from the ORS.