An electron microscopical study of neuronal cell clustering in postnatal mouse striatum, with special emphasis on neuronal cell death.

In this study, electron microscopy was used to study cell clustering in the postnatal mouse striatum. From the date of birth (PO) through postnatal day 7 (P7), groupings of eight to ten striatal neurons were delimited easily in low magnification electron micrographs. Often, within individual groupings, adjacent neurons were separated only by a thin, 10 nm gap, and formed cell pairs or cell triads. Coincident with marked expansion of the striatal neuropil in the second postnatal week, striatal neurons formed more dispersed cell clusters consisting only occasionally of cell pairs or triads. Single, pyknotic neuronal nuclei were seen in clusters of normal neurons exhibiting different stages of maturation but were absent from clusters consisting only of well-differentiated neurons. The neuropil surrounding cell clusters with pyknotic neurons or that adjacent to neighboring cell clusters often contained degenerating dendrites and axon terminals. Whereas this naturally occurring neuronal cell death was present in the tissue throughout the first postnatal week, only degenerating dendritic and axonal profiles were seen in the P15 striatum. This latter fact suggests that the occurrence of pyknotic neuronal somata does not account entirely for the more localized degeneration of other neuronal profiles and raises the possibility that other degenerative processes may be occurring simultaneously in the tissue.