Abstract
We have quantified hippocampal spine structure at the light and ultrastructural levels in cell cultures approximately 1- 3 weeks old and in the brains of rodents 5 and 21 d old. The number of spines bearing synapses increases with age in cultures and in brain, but the structures are similar in both. In culture, about half of the synapses are formed on spines and the remainder are formed on dendritic shafts. In the 5-d-old brain, about half of the synapses occur on dendritic shafts, by 3 weeks of age only approximately 20% of synapses are found on dendritic shafts, and in the adult shaft synapses are very rare.