Cellular proliferation in the exencephalic brain of the mouse embryo.

Autoradiographic studies were conducted on the midbrain and cerebral hemispheres of abnormal (Lp/Lp) exencephalic embryos and normal littermates at 12--18 days of gestation. At 12 days of gestation the percentage of cells labeled (labeling index, LI) in the abnormal brains was similar to that in the normals; however, by 14 and 16 days, the LI had decreased in the normal brains but remained the same in the abnormals. Although an accurate labeling index could not be determined in the 18-day abnormals because of the necrosis and severe histological distortion, incorporation of tritiated thymidine continued to occur, but at an apparently reduced level from earlier stages. On the basis of differences in the LI at 14--16 days of gestation, it appears that cells in the abnormal brains retain their ability to proliferate for longer periods of development than is the case in normal brains.