Development of afferent fiber lamination in the infrapyramidal blade of the rat dentate gyrus.

In the rat dentate gyrus, the lateral perforant path, the medial perforant path, and the major part of the hilar projection to the molecular layer share the lamination domain, mainly in the outer one-third of the molecular layer, the middle one-third, and the inner one-third, respectively. To reveal the order of the afferent fiber lamination and to have an indication of how the synaptic sites on dendrites are determined, we investigated the ontogeny of afferent fiber lamination in the dorsal hippocampus by injecting 1, 1'-dioctodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) into the entorhinal cortex and hippocampus in vivo. Fibers from the contralateral hilar region were found under the pia mater of the infrapyramidal blade at postnatal day 3 (P3), whereas the entorhinal afferent fibers were absent in the infrapyramidal blade. Then the medial and the lateral perforant path appeared under the pia mater in the infrapyramidal blade as riding on top of the preexisting laminae by P7 and by P11, respectively. Based on the established knowledge that most entorhinal layer II neurons simultaneously innervate both the suprapyramidal blade and infrapyramidal blade by branching, it is assumed that the medial and lateral perforant path in the suprapyramidal blade await an appropriate timing for sprouting of interstitial branches into the infrapyramidal blade. The granule cells in the infrapyramidal blade had dendritic growth cones by P11. Calretinin immunohistochemistry revealed Cajal-Retzius cells in the infrapyramidal blade even at P14. Under the pia mater, axon growth cones of ingrowing afferent fibers may interact with the dendritic growth cones or the Cajal-Retzius cells, and determines the synaptic sites on the granule cell dendrites.