The inner root sheath consists of four layers: the cuticle, Huxley’s layer, Henle’s layer, and the companion layer. The cells of the inner root sheath cuticle partially overlap with the cuticle cells of the hair shaft, anchoring the hair shaft tightly to the follicle. Inner root
sheath cells produce keratins 1/10 and trichohyalin that serve as an intracellular “cement” giving strength to the inner root sheath to support and mould the growing hair shaft, as well as guide its upward movement. The transcription factor GATA-3 is critical for inner root sheath differentiation and lineage. Mice lacking this gene fail to form an inner root sheath [25]. The inner root sheath separates the hair shaft from the outer root sheath, which forms the external concentric layer of epithelial cells in the hair follicle.