Down's syndrome: occurrence of ALZ-50 reactive neurons and the formation of senile plaques.

We have histopathologically investigated the hippocampal formation in 4 individuals with Down's syndrome (DS), 7 control individuals, and 3 individuals dying after being in coma 3-7 days. Adjacent sections of brain were stained by the Bielschowsky method and by ALZ-50 immunocytochemical methods. ALZ-50 immunoreactive neurons were found in each individual with DS and only in the control infants. Neither ALZ-50-immunoreactive features nor abnormal silver-positive features stained by the Bielschowsky method were found in the adolescent or young adult controls or coma patients. Diffuse form senile plaques (SP) were found only in the oldest DS individual. The data suggest that ALZ-50 reactive neurons persist during the life of an individual with DS and may precede the formation of SP.