Developmental changes in the topographical distribution of cells contributing to the lateral olfactory tract.

Developmental changes in the locations of cells in the olfactory bulb which contribute axons to the lateral olfactory tract (LOT) were investigated using retrograde transport of horseradish peroxidase (HRP). HRP was placed in the olfactory tubercle, LOT, and/or piriform cortex of golden hamsters aged 3,7,8 and 20 days. The numbers of labeled mitral and tufted cells were counted on equidistant sections through the olfactory bulb. In the younger animals (days 3 and 7), there was a distinct localization of labeled cells in the medial quadrant of the bulb. In the 3-day-old group 49% of all labeled cells were located in the medial quadrant, but by day 8 the per cent of cells in all 4 quadrants was equivalent. Between days 3 and 20, the average perimeter of the mitral cell layer doubled. There was a similar increase in the average relative total number of labeled cells per bulb. When pairs of brains from the different age groups with HRP placements of similar size and location were compared, the older brain always had more labeled cells. There was no relation of the number or distribution of labeled cells to the location of the injection. The number of labeled cells correlated positively with the size of the injection within each age group. We believe this is the first report of a developmental topographical gradient of the cells of origin of the LOT. This localization may be related to early topographical differences in functional activity which reflect a limited exposure to odors in the first week of life.