The effects of 13-cis-retinoic acid, which is clinically very effective in the treatment of severe acne, on sebaceous glands in humans was studied by means of histology, planimetry, and autoradiography. Histologically and planimetrically, one finds a marked decrease of size of sebaceous glands of up to 90% of the pretreatment values after 12 weeks of treatment. Additionally, the ratio of the so-called "differentiating cell pool (DCP)" vs. the so-called "undifferentiating cell pool (UCP)" changed from 2:1 to 1:7, probably indicating a disturbance of differentiation (lipid-production) of sebocytes. The labeling index of sebocytes also regressed significantly under therapy with 13-cis-retinoic acid.

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The proliferation response and changes in cellularity of mouse tongue epithelium were studied after single doses of X-rays and during 3 weeks of daily irradiation. A single dose of 13 Gy resulted in minimum cellularity (70% of control values) on days 3-5 and complete restoration on day 7. Mitotic activity ceased for 1 day followed by normal-to-supranormal values until day 15. A wave of abnormal mitoses was observed with a peak at days 4-7. Daily irradiation with 3 or 4 Gy induced neither major structural nor visible cellular damage. Cellularity decreased to approximately 60% during week 1 and subsequently remained at 60-70%. The proliferation activity was reduced to approximately 8% by day 2. Mitotic activity during weeks 2 and 3 was subnormal-to-normal, with a dose-dependent increase to normal counts during the first weekend and a distinct overshoot over the second weekend respectively. A proliferation model is presented to explain the present findings and previous functional measurements of changes in tissue tolerance. Its major features are accelerated symmetrical stem cell divisions and abortive divisions of sterilized cells.

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