Hair Development, Growth, and Loss 42 derived from progeny of the stem cells after their exit and downward movement from the bulge, and contiguous with the basal layer of the epidermis. The IRS is composed of three layers: Henley’s layer, Huxley’s layer, and the cuticle. The innermost structure is the central HS, which is compacted into a fiber-like structure and can also be divided into three layers— the cuticle, the cortex, and the medulla. There is a companion layer that is tightly bound to the IRS, thus allowing the maximum flexibility in movement between ORS and the upwards-moving IRS. The cuticle layer of IRS interlocks with the hair shaft cuticle, anchoring the shaft in the follicle and enabling both layers to move during hair follicle growth.5,9 The dermal portion of the hair follicle consists of the dermal papilla (DP) and dermal sheath (DS), which are specialized fibroblasts of mesenchymal origin. However, expression of neuronal markers by DP cells and their transcription profiles suggest a neural crest origin. By tracing the progeny of neural crest stem cells, investigators confirmed that a portion of the DP cells is derived from the neural crest.10 DP and DS are separated from the epithelial portion of the hair follicle by a basement membrane. The DP, which consists of a small cluster of packed fibroblasts at the base of each hair follicle, is well recognized as the commanding center to drive the hair cycle. The DP is also believed to influence the size of hair bulb size, the diameter, and length of HS. The DS, or connective tissue sheath, lines the epithelium of the hair follicle from the bulge Figure 2 Cross-section of the mature human hair follicle CO = Cortex CU = Cuticle CI = cuticle inner root sheath IRS = Inner Root Sheath hu = Huxley’s Layer he = Henley’s Layer ORS = Outer Root Sheath D = Dermis, F = fibroblast image courtesy of Karen Lammers
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