Histologic Changes in the Skin of the Rhino Mouse (hrrhhrrh) Induced by Retinoids

The effects of four retinoids, all-trans-retinoic acid (RA), 13-cis-retinoic acid (13-cis-RA), aromatic retinoid, and arotinoid ethyl ester (arotinoid) were examined on the skin of the rhino mouse. Rhino mouse skin is characterized by the presence of keratin-containing utricles attached to the epidermis, and subcutaneous cysts, both of which are derived from hair follicles. The utricles were examined in horizontal sheets of epidermis, and in vertical histologic sections. After applications of 0.1 ml of 0.1% RA, 0.1% 13-cis-RA, 0.1% aromatic retinoid, or 0.001% arotinoid in acetone to the dorsal skin for 5 days a week for 2 weeks, there was a reduction of utricle diameter to 45%, 52%, 30%, and 31% of acetone-treated controls, respectively. Histologic examination of the epidermis revealed that a dose-dependent hyperplasia and thickening of the epidermis and the stratum granulosum was induced by the retinoids when given at subtoxic doses, being most marked after arotinoid or RA. The thickness of the walls of the utricles also increased with increasing dose of retinoid, paralleling the changes in the epidermis. After doses of 0.1% RA and 0.001% arotinoid, the utricles resembled the hair follicles of hairless mice. Hyperplasia of the epidermis appears to be a primary effect of retinoids or rhino mouse skin. Hyperproliferation of the utricle wall is accompanied by a reduction in the size of the utricle lumen, preceding eventual differentiation of the utricles to normal-looking pilar units.