Developmental neuron death in the rat superior cervical sympathetic ganglion: cell counts and ultrastructure

Counts of neurons of the rat superior cervical ganglion (SCG) were made at two days before birth and at several postnatal ages. There is a significant decline in the number of apparently normal neurons over the first postnatal week, with the number falling from 39 500 at 3 days to 26 500 at 7 days. Cell numbers then remained constant up to day 60 when the number of neurons was 27 500. The incidence of degenerating neurons, identified by light and electron microscopy, was correlated temporally with the loss of normal neurons. The early manifestations of the neuron degeneration were chromatin clumping and the presence of free monoribosomes. Later stages were characterized by increased chromatin clumping, dense aggregations of monoribosomes, numerous intracytoplasmic vacuoles, and only short segments of rough endoplasmic reticulum. The ultrastructure of the majority of these dying neurons is similar to the 'nuclear' types of degeneration described by Pilar & Landmesser (1976) and Chu-Wang & Oppenheim (1978). Based on the presence of degenerating neurons coincident with the reduction in neuron numbers, we conclude that neuron death is an important aspect of early postnatal development in the rat SCG.