Prefrontal neocortical disturbances in mental retardation

Morphometrical analysis of the frontal lobe neocortex of seven selected cases of mental retardation of unclassified aetiology and pathology, showing mild dysplastic changes of the neocortex on routine workup, were compared with nine normal controls and seven cases with Down's syndrome. In comparison with the normal controls, the group with 'unclassified' mental retardation showed an increased percentage of disoriented pyramidal neurons in layers III and IV-V (8-17%, P < 0.01), an abnormal distribution of small pyramidal cells with a shift from superficial to deeper layers of the prefrontal cortex (P < 0.01), and an increased cortical thickness (+38%, P < 0.05) of Brodmann area 10, as well as a tendency to a decreased gyration of the frontal lobe sulci (-14%, P = 0.18). However, no statistically significant macroscopical differences either in frontal lobe gyration or in the size of the cerebrum and its frontal lobes were found between these two groups. On the other hand, the Down's syndrome group had a significantly decreased gyration (-36%, P < 0.01). These findings may indicate an inhibited and disordered migration of putative small pyramidal neurons in cases of 'unclassified' mental retardation.