Comparison of electron microscopy and silver staining for the detection of the first entorhinal synapses to develop in the dentate gyrus.

The development of the projection from the entorhinal cortex to the dentate gyrus (perforant path) has been studied with the electron microscope. The projection was lesioned in baby rats 5--13 days old and the dentate gyrus examined after 6--72 hr. Degenerating synapses first appeared in small numbers in the dentate neuropile at 7 days and in greater numbers in progressively older animals. There was a sixteen-fold increase in the number of synapses undergoing degeneration between 7 and 13 days. This investigation provides a calibration for the reduced silver method which has been used to trace developing axons (Singh, 1977). By this method the first signs of Wallerian degeneration, after cutting axons in the perforant path, were seen in the dentate neuropile at 9 days.