Evidence for physiological growth of hippocampal mossy fiber collaterals in the guinea pig during puberty and adulthood.

By means of Timm's procedure and computer-assisted morphometry, the left and right hippocampi of 69 hybrid guinea pigs from nine age levels (P5, P10, P20, P40, P80, P160, P320, and P610, and P1100) were analyzed for postnatal growth of recurrent hippocampal mossy fiber collaterals (RMFC) terminating below, within, and above the dentate granule cell layer. Postnatal growth of RMFCs showed, in both sexes, a first peak at P40, with stainable mossy fiber boutons covering the cell bodies of large neurones, some of which were reminiscent of basket cells. No significant changes of the density of mossy fiber collaterals were noticed from P40 to P160. At P320 a remarkable expansion of RMFCs was noted in a few animals, and by P610 all animals showed highly proliferated RMFCs which densely covered cell bodies and dendrites of target cells. The oldest group (P1100) showed an equal or slightly lowered density of RMFCs. We conclude that the growth of recurrent mossy fiber collaterals occurs in two spurts. The first completes just before sexual maturity. The second spurt occurs in the mid-life period, between P160 and P610.