Neurogenesis of glutamic acid decarboxylase immunoreactive cells in the hippocampus of the mouse. I: Regio superior and regio inferior.

The neurogenetic gradients of neurons showing glutamic acid decarboxylase (GAD) immunoreactivity were determined in the regio superior and in the regio inferior of the mouse hippocampus. Pregnant C57Bl mice received pulse injections of (3H)thymidine from E11 through E17 (E0 being the day of mating). Distributions of (3H)thymidine-labeled, GAD-positive neurons in the different strata of the hippocampus proper were recorded in adult animals. GAD-positive neurons in this region are generated prenatally. Radial gradients of neurogenesis of GAD-positive cells are characterized by two main features: 1) with the exception of the stratum lacunosum-moleculare and its interface with the stratum radiatum, GAD-positive neurons of the plexiform strata are generated before those destined for the pyramidal layer; 2) within the pyramidal layer, GAD-positive cells are positioned according to an inside-out sequence. In the transverse axis, neurogenesis of GAD-positive cells follows a regio inferior to regio superior gradient. This gradient is due to prolonged neurogenesis of GAD-positive cells for the pyramidal layer in the regio superior. Given the selective laminar disposition of the GABAergic interneurons in the hippocampus, the present authors explored whether or not the diverse types of these interneurons could have specific birth dates and concluded that no relationship exists between birth dates and adult phenotypes of GAD-immunoreactive cells in the mouse hippocampus proper.