The goal of this study was to determine whether dendritic regression occurs in granule neurons of the rat dentate gyrus during late postnatal development. In vitro hippocampal slices were prepared from rats between the ages of 14 and 60 days, and granule neurons in one portion of the suprapyramidal blade were labeled by intracellular injection of horseradish peroxidase. The dendrites of filled neurons were analyzed in both two and three dimensions directly from 400 microns thick whole-mounts. Results showed that the molecular layer expanded by approximately 50% between days 14 and 60. At every age examined, granule cell dendrites reached the top of the molecular layer, suggesting that dendrites continued to grow during this time period. In contrast, the number of dendritic segments per neuron decreased from an average of 36 to 28. Three-dimensional measurements showed that total dendritic length and surface area per granule cell did not change, suggesting that the overall dendritic tree size of granule neurons may be regulated during late postnatal development in the rodent.

Figure 1: Micrographs of HRP-filled granule neurons in whole-mounts of hippocampal slices from a 14-day-old rat (A) and a 53-day-old animal (B). Both cells were located in the suprapyramidal blade. Note that some dendrites are not in focus (solid arrows). Axons can be seen exiting the somata (open arrows). Numerous red blood cells that stain during the reaction with diainobenzidine are present (*). Final magnifications are identical for both micrographs. Bar = 100 mm

Figure 2: Camera lucida drawings of granule cells from a 14-day-old rat (A) and a 51-day-old animal (B). Note that all dendrites, except one branch (*) in the younger animal, extent to the top of the molecular layer (dashed upper line). Final magnifications are identical for both drawings, ML, molecular layer, granule cell layer. Bar = 50 mm.

Figure 3: Micrographs of HRP-filled granule cell dendrites from a 14-day-old rat (A and C) and a 53-day-old animal (B and D). Dendrites shown are in (A) and (B) are from the middle portion of the layer. Asterisks indicate stained red blood cells. Final magnifications are identical for all micrographs. Bar = 20 mm

Figure 4: Compute-generated plots of granule neurons from a 14-day-old (A), a 40-day-old (B), and a 53-day-old rat (C). These plots were generated from three-dimensional data. Bar = 50 mm.

Figure 5: A comparison of total dendritic length and the number of dendritic segments per cell with increasing age of the rat. An asterisk (*) indicates a significant difference from the youngest age group. There were no significant differences in the number of segments among the 4 oldest age groups. Note that total length did no change significantly during the entire period. See Table 1 for number of cells analyzed. Values are means +_ S.E.M.

Figure 6: Percentage of granule cell branch points in thirds of the molecular layer vs age. An asterisk (*) indicates a significant difference from the youngest age group. There were no significant differences among the 4 oldest age groups. Values are means +_ S.E.M.

Figure 7: Number of higher order segments (fifth through eighth) per granule neuron vs age. There was no significant differences among any of the age groups. Values are means +_ S.E.M.

Figure 8: Number of branches per neuron terminating within the molecular layer. An asterisk (*) indicates a significant difference from the youngest age group. There were no other significant differences among any of the age groups. Values are means +_ S.E.M.

TABLE 1: Granule cell parameters and molecular layer width versus age

Values given are means ± S.E.M.

* Significantly different from youngest age group (P < 0.05).