Development of the hippocampal region in the rat. II. Morphogenesis during embryonic and early postnatal life.

Morphogenesis of the entire hippocampal region was examined in normal rats from embryonic (E) day EIO to E22 and on postnatal (P) days PI, P7, and P21, and was correlated with autoradiographic datings (Bayer, '79c). This region is postulated to form from two connected primordia in the telencephalon, easily recognizable on E16. One lies in the dorsomedial wall and generates a portion of the subiculum, Ammon's horn and the dentate gyrus. The other lies in the dorsoposterior wall and generates the entorhinal cortex and part of the parasubiculum and presubiculum. A cortical plate appears in the presumptive entorhinal region on E17; a cell-sparse fibrous zone appears in the middle of the cortical plate on E19. Lamination in the entorhinal cortex proceeds from lateral to medial and from deep to superficial, with the thickening oflayer III being the last to appear on E22. Lamination in the subiculum starts on E18, but the distinction between superficial laminae in the paraand presubiculum cannot be made until E22. The stratum pyramidale is small on E20 in presumptive Ammon's horn, then greatly lengthens between E21-PI. In early dentate gyrus development, cells migrate from the neuroepithelium near the outgrowth of the fimbria and adjacent to the choroid plexus toward a subpial location. The ectal limb ofthe granular layer appears at the two extremes of the dentate gyrus on E20; theendallimb appears perinatally and develops rapidly to become morphologically similar to the ectal