Inherited prenatal hydrocephalus in the H–Tx rat: a morphological study

The H–Tx rat has inherited hydrocephalus which is present at birth. In order to investigate the onset and early stages of hydrocephalus, the heads of fetuses from 16 to 21 days gestation and at 1 day after birth, were serially sectioned using conventional wax histology. Lateral and third ventricle volumes were measured with a graphics tablet and microcomputer. Hydrocephalus was first detected at 18–20 days gestation by enlarged lateral ventricles and it was sometimes accompanied by a large third ventricle. Most hydrocephalics had a non–patent cerebral aqueduct between the third ventricle and the posterior collicular recess and the remainder (about 25%) had an aqueduct which was patent but with a smaller lumen than in non–hydrocephalic littermates. Some fetuses prior to 18 days gestation with normal lateral ventricles also had nonpatent aqueducts. Abnormal aqueducts were lined by ependymal cells which were ventrally displaced by thickening of the overlying midbrain; also the subcommissural organ was foreshortened. Infusion with fluorescent markers confirmed that the flow pathway through the aqueduct was obstructed in many hydrocephalic rats. It is concluded that the hydrocephalus may be the result of abnormal brain development in the midline region of the dorsal mesencephalon, leading to aqueduct closure.