p73 and Reelin in Cajal-Retzius cells of the developing human hippocampal formation.

In the fetal human hippocampus, Cajal-Retzius (CR) cells coexpress p73, a p53-family member involved in cell survival and apoptosis, and the glycoprotein reelin, crucial for radial migration. We distinguish two populations of putative CR cells. (1). p73/reelin expressing cells appear around 10 gestational weeks (GW) at the cortico-choroid border in the temporal horn of the lateral ventricle (the ventral cortical hem) and occupy the marginal zone (MZ) overlying the ammonic and dentate primordia. (2). Additional p73-positive cells appear from 14 GW onward in the neuroepithelium near the dentate-fimbrial boundary and spread toward the pial surface, flanking the migrating secondary dentate matrix. From 13 to 17 GW, large parts of the dentate gyrus are almost devoid of CR cells. p73/Reelin-positive CR cells appear in the MZ of the suprapyramidal blade at 16 GW and around 21 GW in the infrapyramidal blade. The p73-positive cells of the dentate-fimbrial boundary express reelin when they are close to the pial surface, suggesting that they differentiate into CR cells of the infrapyramidal blade. Reelin-positive, p73-negative interneurons are prominent in the prospective strata lacunosum-moleculare and radiatum of cornu ammonis as early as 14 GW; in the dentate molecular layer and hilus they appear around midgestation. We propose that CR cells of the human hippocampal formation belong to two distinct cell populations: an early one derived from the ventral cortical hem and mainly related to migration of the ammonic and dentate plates and a later appearing one derived from the dentate-fimbrial neuroepithelium, which may be related to the protracted neurogenesis and migration of dentate granule cells, particularly of the infrapyramidal blade.