The Outer and Inner Root Sheaths of the Hair Follicle

The ORS and IRS of the Hair Follicle

The hair follicle is surrounded by two main structures below the surface of the skin - the inner root sheath and the outer root sheath. The inner root sheath is quite thin and the outer root sheath is quite thick. These two sheaths wrap around the hair follicle like blankets. In the above photo of scalp hematoxylin and eosin stained histology, the IRS and ORS are shown. The hair shaft is often not seen on these standard slides as it gets washed away during processing in the lab. But right in the middle (of the hole in the middle) is normally where the hair shaft is found.

The inner root sheath (IRS) is actually not one structure but rather has several layers - a Henle layer, Huxley layer and cuticle layer. The IRS has several important roles including helping shape and mould the newly developing hair shaft and also helping with proper keratinization. The inner root sheath helps keep the hair fiber firmly glued into the scalp.

The outer root sheath (ORS) is continuous with the skin epidermal layer. The epidermal layer essentially dips down to join up with the ORS in the region where hair follicles come out of the skin. The ORS is thickest about 1/3 of the way to the bottom. This region of the ORS contains a further thickened region know as the “bulge.” The bulge region of the ORS contains the hair follicle “stem cells.” Stem cells are cells which have the potential to become other types of cells including cells of the IRS, ORS and matrix. In essence these stems of the bulge have the potential to forms new hairs.