On the occurrence of neuronal sprouting in the frontal cortex of a patient with Down's syndrome.

We report on the autopsy of a 52-year-old Down's syndrome patient with a 9-year history of progressive dementia. Histologically, there was a marked neuronal loss in the cerebral cortices with numerous neurofibrillary tangles and senile plaques and congophilic angiopathy. The second cortical layer was spongy in appearance and the frontal and temporal white matter showed marked myelin pallor. Immunohistochemistry for MAP2 of the frontal and temporal cortices revealed great reduction in the number of large pyramidal neurons. On the other hand, a much stronger MAP2 immunoreactivity than that seen in the controls was observed in the second layer, which consisted of many small pyramidal neurons with features of aberrant sprouting. They were recognized in the frontal cortices but not in the temporal cortices, and were not immunolabeled with phosphorylated-tau nor MAP5. On double immunolabeling with MAP2 and beta-amyloid antibodies, these neurons were found to be not directly associated with the formation of senile plaques. Ultrastructurally, MAP2 was immunolocalized in an amorphous granular form in the neuronal somata and processes, and paired helical filaments were not immunolabeled. The observation may indicate the occurrence of a remarkable plastic response of small pyramidal cortical neurons in this patient with Down's syndrome.