Comparative studies on effects of all-trans-retinoic acid and all-trans-retinoyl-beta-D-glucuronide on the development of foetal mouse thymus in an organ culture system

The thymus is a target organ of retinoid teratogens. Retinoids with a generally reduced teratogenic potency should therefore also exert reduced adverse effects on thymus development. The effects of all-irans-retinoic acid (a-tRA) and all-trans-retinoyl-beta-glucuronide (a-tRAG) on the in vitro development of thymic lobes of 15-day-old mouse foetuses were compared in an organ culture system. Both compounds were added to the medium at concentrations ranging from 10(-7) to 10(-5) M. The culture period was 6 days. The investigations showed a concentration-dependent effect of both substances on the proliferation of the lymphatic cells. At 10(-5) M the number of thymocytes was significantly reduced to values of about 70% of the controls by either of the retinoids (P less than or equal to 0.05). Results of flow cytometry showed significant differences concerning the differentiation markers CD4 and CD8 after the culture period. The presence of 10(-6) M a-tRA induced a significant increase in the percentages of CD4(+)CD8(-) cells and a significant decrease of CD4(+)CD8(+) cells. At 10(-5) M a-tRA an additional significant increase in the percentages of CD4(-)CD8(-) cells was found. In contrast, after treatment with a-tRAG, percentages of these populations were in the same range as the controls. Light and electron microscopic investigations revealed a depletion of lymphatic cells and an increase of intracytoplasmic vacuoles in the thymic epithelial cells at 10(-6) and 10(-1) M of either retinoid. HPLC analyses revealed a remarkable degree of retinoid isomerization and (in the case of a-tRAG) of hydrolysis. Compared with the culture medium, retinoids were accumulated in the thymic lobes. Possibly a-tRAG acts by way of limited hydrolysis to retinoic acid.