DISTRIBUTION OF NEURONAL HETEROTOPIAE FOLLOWING PRENATAL EXPOSURE TO METHYLAZOXYMETHANOL

The three-dimensional distribution of neuronal heterotopiae induced in rat brains by prenatal exposure to the cytotoxic drug, methylazoxymet hanol acetate, has been examined by computer reconstruction techniques . Three types of heterotopiae may be identified in mature mt brains ex posed between E11 and E16: Layer I heterotopiae, periventricular heter otopiae, and hippocampal heterotopiae. The distributions of Layer I he terotopiae and periventricular heterotopiae show clear temporospatial gradients; such that with subsequent age of exposure, Layer I heteroto piae are situated progressively more medially, dorsally, and rostrally , and periventricular heteotopiae are situated progressively more rost rally. Periventricular heterotopiae are most extensive following expos ure to the agent on E14. For both of these heterotopiae there is a cha racteristic pattern of distribution for each gestational age of exposu re to the agent. By contrast, hippocampal heterotopiae, consisting of misplaced pyramidal neurons in subfields CA1 and CA2 of Ammon's horn, did not show significant changes in distribution with different ages o f exposure to the drug. The significance of these temporospatial gradi ents for mechanisms underlying the production of the heterotopiae is d iscussed.