Snyaptic and dendritic development and mental defect.

The hypothesis that specific defects in synaptic and dendritic development of cerebral cortex may form the anatomical basis in some cases of mental defect has been examined by electron microscopy and by use of the Golgi-Cos method. Two types of abnormality have been identified to date. One is a specific lesion of presynaptic terminals, first reported by Gonatas and Goldensohn (14) in a child with mental retardation and myoclonic seizures. This lesion, consisting of massive proliferation of membranous structures in terminal axons, appears to be rare and may be the anatomical substrate of one or more genetically determined dementing illnesses in infancy. More commonly, cerebral cortex from the severely retarded shows defects in number, length, and spatial arrangement of dendrites and synapses, best demonstrated by the Golgi method. Such abnormalities have been found in six out of eleven brains from severely retarded individuals examined by us. The etiology of the retardation was unknown in the majority; two had other recognizable developmental malformations of brain. It is suggested that a number of different etiological factors, if active during the period of rapid synaptic and dendritic growth in cerebral cortex (i. e., from the last trimester of pregnancy to the end of the first postnatal year) may result in stunted development of these structures.