Neuronal proliferation in the 9-month-old rodent-radioautographic study of granule cells in the hippocampus.

Nine-month-old rats were injected with 5 microCi 3H-thymidine (3H-Tdr) and allowed to survive for 20 days. In light-microscopic radioautographs, labeled cells were found in the granule cell layer of the hippocampus. Analysis of electron micrographs of the labeled cells, taken from re-embedded 1.5 micron radioautographic sections, clearly demonstrated their neuronal nature with synapses along their cell bodies and dendrites. Our results indicate that 0.025% of the granule neurons are heavily labeled in the dorsal hippocampus. Electron microscopy of re-embedded light-microscopic radioautographic sections confirms that granule neurons in the rodent are newly formed up until 9 months after birth.