Electron microscopic study on cell differentiation in anagen hair follicles in mice

The dorsal and sinus hair follicles from 7-day-old mice of several strains were ultrastructurally examined to study their cell differentiation. The findings were compared with those in human anagen hair follicles. The keratinized cells in the Henle's and Huxley's layers gradually lost their cell volume and finally became thin and lamellar in mice. The outer root sheath (ORS) was composed of two cell layers in the mouse hair bulb. The cells in the outer layer gradually increased in number upwards; the completed outer layer consisted of 2-3, and over 10 cell layers in the mouse dorsal and sinus hair follicles, respectively. These cells produced many membrane-limited, highly electron-dense granules or cementsomes, and finally underwent a keratinization toward the hair canal. The innermost cell (IMC) layer of the ORS formed a unique single-cell layer in mice as well as in humans, although the mouse IMC were much thinner and contained fewer tonofilaments. Accumulation of tonofilaments in the inner side of the cytoplasm and keratohyalin production, but no cementsome production occurred in the mouse IMC. The mouse IMC started to keratinize after the degeneration of the keratinized inner root sheath (IRS) cells. These findings suggest that the lower hair follicle is composed of the IRS layers, the IMC layer, and the outer layer of the ORS in either mice or humans, although there are some morphological differences in cell differentiation in each layer between the two species.