Effects of prenatal gamma irradiation on the development of the corpus callosum of Swiss mice.

The temporal sequence of events related to the effects of prenatal gamma irradiation on the development of the corpus callosum and cerebral cortex was studied in Swiss mice. Pregnant females on gestational day 16 were exposed to a 60Co source receiving total doses of 2 or 3 Gy. The offspring were analyzed at both prenatal and postnatal days. One day after irradiation, a great number of pyknotic figures was seen along the whole extension of the cerebral wall, especially in the proliferative zones. At perinatal ages, the thickness of the proliferative zones was reduced and the glial sling was never identified. From 5 days after birth onwards, we observed a severe shrinkage of layers II + III and IV. The majority of the irradiated mice were totally acallosal (particularly when the 3 Gy dose was used), but some animals presented callosal remnants. These remnants were identified above the ventral hippocampal commissure, except for two animals in which a larger callosal remnant extended from the columns of the fornix to the dorsal hippocampal commissure. The presence of callosal remnants in animals irradiated with 3 Gy was dependent on the age at which the animals were analyzed since remnants were observed in some animals analyzed at perinatal ages, but never in older animals. Callosal defects can be explained at least by three factors: (1) Death of a great part of callosal neurons located at layer III. (2) Postnatal axonal elimination. (3) Absence of the glial sling. The callosal agenesis in the absence of the glial sling indicates that this structure may play a crucial role in guiding callosal axons. However, the presence of callosal remnants indicates that surviving callosal axons can use structures other than the sling to cross the midplane. Our data indicate that axons of the middle portion of the callosum can cross the midplane using the ventral hippocampal commissure as a guide. Additionally, the dorsal hippocampal commissure may play a role in directing axons of the posterior part of the corpus callosum.