The medullae of four normal rats were processed by a modified Bielschowsky silver impregnation technique for boutons. Some sections were examined by light microscopy, and others further processed for electron microscopy. The ultrastructure of boutons in the left hypoglossal nucleus was studied to find the sites of silver grain deposition in heavily and lightly impregnated tissue.
It was found that the highest silver grain frequency was over synaptic vesicles, mitochondria being far less frequently impregnated. The specificity of silver grains for synaptic vesicles was more marked in heavily impregnated tissue than in lightly impregnated tissue.
In heavily impregnated tissue all boutons, irrespective of type, were impregnated. Almost all boutons possessed some silver grains in lightly impregnated tissue, although the grain frequency was less than in heavily impregnated tissue.