Ectopic neurones in the hippocampus of the postnatal rat exposed to methylazoxymethanol during foetal development.

Ectopic neurons have been detected in the hippocampus of postnatal hooded rats aged 5-24 days. These rats were exposed to methylazoxymethanol acetate (MAMac) during foetal development by injecting the mother rats with this neurotoxin. At birth, the hippocampus of rats exposed to MAMac showed a normal cytoarchitecture; ectopic neurones became prevalent from 5 days onwards. These ectopic neurones were restricted to subfields CA 1-2 of Ammon's horn, and evidence is presented which suggests that these ectopic sites are formed by neuronal emigration from stratum pyramidale. Ectopic neurones have been shown to occur in genetic abnormalities of man, and in the mutant mouse 'reeler'. MAMac is a powerful methylating agent especially for guanine which is present in DNA and RNA. It is postulated that viable cells with an altered DNA-RNA state may in some way be predisposed to the formation of ectopic cell clusters after a latent period. The movement of neurones from stratum pyramidale provides a convenient animal model for investigating the mechanisms by which ectopic neuronal sites are formed by abnormal migratory patterns.