Reelin‐expressing neurons in the postnatal and adult human hippocampal formation

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Abstract
Reelin plays a major role in the development of laminated brain structures. In the developing neocortex and hippocampus, Reelin is secreted by Cajal‐Retzius cells in the marginal zone. In the present report, we characterize Reelin‐immunoreactive neurons in the perinatal and adult human hippocampal formation. Two main populations of cells are described: Cajal‐Retzius cells and interneurons. Cajal‐Retzius cells are defined as neurons that coexpress Reelin and p73, a nuclear protein of the p53 family. Colocalization experiments of p73 with calcium‐binding proteins indicate that most Cajal‐Retzius cells express calretinin, but not calbindin. Cajal‐Retzius cell density decreases dramatically during the postnatal period, although a few Reelin/p73‐positive neurons are still found in the adult. At birth, Reelin‐positive, p73‐negative neurons are present in all layers of the hippocampal formation. Their morphology and localization indicate that they belong to a heterogeneous population of interneurons. They are numerous in the strata lacunosum‐moleculare and radiatum of CA1‐CA3, in the hilus, and in the molecular layer of the dentate gyrus, but less common in stratum oriens and alveus, and rare in the principal cell layers. Subpopulations of Reelin‐positive interneurons express calretinin or calbindin. The packing density of Reelin‐positive cells decreases postnatally, which may be related to the disappearance of Cajal‐Retzius cells and to the growth of the hippocampal formation. The presence of Reelin‐immunoreactive cells in the adult hippocampal formation indicates that Reelin is not restricted to development but that it may have additional functions in adult life.