Effect of methotrexate on neuroepithelium in the rat fetal brain

Pregnant rats were treated with 30 mg/kg of methotrexate (MTX) on gestation day 13, and fetal brains were examined histopathologically from 6 to 48 hr after the treatment. In the telencephalon of the control group, there were few pyknotic neuroepithelial cells throughout the experimental period. Six hr after MTX treatment, several pyknotic neuroepithelial cells scattered throughout the telencephalic wall. At 12-36 hr, pyknotic neuroepithelial cells increased significantly and were diffusely distributed throughout the telencephalic wall. Neuroepithelial cells were eliminated and showed sparse cell density at 36 hr in the telencephalon. Almost all fetuses died at 48 hr. Most of the pyknotic neuroepithelial cells were positively stained by the TUNEL method and positive for cleaved caspase-3. While mitotic and phospho-histone H3-positive neuroepithelial cells were located along the ventricular layer of telencephalon in the control group, they were rarely observed in the same region at 6-36 hr in the MTX-treated group. MTX induced few pyknotic changes to neuroepithelial cells in the metencephalon, compared to other parts of brain. The distribution of apoptotic neuroepithelial cells and the time-course changes of the indices of apoptotic and mitotic neuroepithelial cells were different from those of other DNA-damaging chemicals reported previously. The difference may reflect the disparity in mechanisms of apoptosis and the inhibition of cell proliferation in neuroepithelial cells induced by MTX. To our knowledge, this is the first report demonstrating histopathological findings of fetal brain damage induced by MTX.