ORGANIZATION OF THE EMBRYONIC AND EARLY POSTNATAL MURINE HIPPOCAMPUS .I. IMMUNOCYTOCHEMICAL CHARACTERIZATION OF NEURONAL POPULATIONS IN THESUBPLATE AND MARGINAL ZONE

Immunocytochemical techniques were used to characterize the neuronal p opulations in the hippocampal subplate and marginal zone from embryoni c day 13 (E13) to postnatal day 5 (P5). Sections were processed for th e visualization of microtubule-associated protein 2 (MAP2) and other a ntigens such as neurotransmitters, neuropeptides, calcium-binding prot eins and a synaptic antigen (Mab SMI81). At E13-E14, only the ventricu lar zone and the primitive plexiform layer wererecognized. Some cells in the later stratum displayed MAP2-, gamma-aminobutyric acid (GABA)- and calretinin immunoreactivities. From E15 onwards, the hippocampal a nd dentate plates became visible. Neurons in the plexiform layers were immunoreactive at E15-E16, whereas the hippocampal and dentate plates showed immunostaining two or three days later. Between E15 and E19 the following populations were distinguished in the plexiform layers: the subventricular zone displayed small neurons that reacted with MAP2 an d GABA antibodies; the subplate(prospective stratum oriens) was poorly populated by MAP2- and GABA-positive cells; the inner marginal zone ( future stratum radiatum) was heavily populatedby multipolar GABAergic cells; the outer marginal zone (stratum lacunosum-moleculare) displaye d horizontal neurons that showed glutamate- and calretinin immunoreact ivities, their morphology being reminiscent of neocortical Cajal-Retzi us cells. Thus, each plexiform layer was populated by a characteristic neuronal population whose distribution did not overlap. Similar segre gated neuronal populations were also found in the developing dentate g yrus. At perinatal stages, small numbers of neurons in the plexiform l ayers began to express calbindin D-28K and neuropeptides. During early postnatal stages, neurons in the subplate and inner marginal zones we re transformed into resident cells of the stratum oriens and radiatum, respectively. In contrast, calretinin-positive neuronsin the stratum lacunosum-moleculare disappeared at postnatal stages. At E15-E19, SMI8 1-immunoreactive fibers were observed in the developing white matter, subplate and outer marginal zone, which suggests that these layers are sites of early synaptogenesis. At P0-P5, SMI81 immunoreactivity becam e homogeneously distributed within the hippocampal layers. The present results show that neurons in the hippocampal subplate and marginal zo nes have a more precocious morphological and neurochemical differentia tionthan the neurons residing in the principal cell layers. It is sugg ested that these early maturing neurons may have a role in the targeti ng of hippocampal afferents, as subplate cells do in the developing ne ocortex. (C) 1994 Wiley-Liss, Inc.