Relationships between neuronal birthdates and cytoarchitecture in the rat inferior olivary complex

The correlation between birthdates of neurons and their ultimate location within the inferior olivary nucleus (ION) was investigated in the rat by the 5-bromodeoxyuridine (BrdU) method. We performed injections every 4 hours throughout the ION generation period, and were thus able to demonstrate that 1) neurons are distributed in the adult ION following characteristic gradients that define subdivisions identical to those established by hodological studies; and 2) ION neurons born at the same time tend to be arrayed in small clusters in the adult structure. Implications of these findings for the mechanisms of olivary neuron migration, selective aggregation, and elaboration of projectional topography are discussed. This study provides direct evidence that one of the factors governing the elaboration of the cytoarchitecture of a neuronal nucleus is the temporal sequence of generation of its neurons.