Distribution of Corticogeniculo‐Neurons and Projection of Geniculocortical Fibers in Methylazoxymethanol‐lnduced Microcephalic Rats

Abstract

Abstract In the rat with methylazoxymethanol (MAM)‐induced microcephaly, the distribution of corticogeniculate neurons and the projection of genicurocorti‐cal fibers were examined using the horseradish peroxidase (HRP) tracing technique. In agreement with the previous studies, HRP‐labeled corticogeniculate neurons in the MAM‐rats were irregularly distributed and reduced in number in the occipital cortex and neurons in the LGNd also showed developmental deficits such as reduced cell number and abnormal distribution. It was first demonstrated that in the MAM‐rats, the density of HRP‐labeled geniculocortical fibers observed in the corpus calosum was remarkably low than the control. Moreover, HRP‐labeled fiber plexus in the MAM‐rats revealed a layer‐like appearance similar to the control. However, its major location did not coincide with layer IV because of the obscure lamination pattern in the superficial and intermediate layers. Besides, in the MAM‐rats, the density of the HRP‐labeled geniculocortical afferents in the plexus was abnormally high and these afferent terminals showed uneven distribution compared with more uniform distribution in the control. From these observations, it is suggested that there is a possibility of abnormal neuronal contacts between the geniculocortical afferent and the target cortical neurons in the visual cortex.