Reelin-expressing neurons in the postnatal and adult human hippocampal formation. 
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.