Postnatal Neurogenesis in the Developing Hippocampus
Each of the specimens pictured in the grid pattern below were injected with ³H-thymidine on the days indicated. All specimens were killed on postnatal day (P) 60 with an overdose of pentobarbitol and subsequent perfusion with 10% neutral formalin. The brains were blocked in the sagittal plane, were placed in Bouin’s fixative for 24 hours, then transferred back to formalin until embedding in paraffin. Serial 6 µm sections were saved from medial to lateral. Here we show sections of the dorsal hippocampus lateral to the midline, but still overlying a mid-lateral part of the thalamus (not shown in the pictures). The labelled cells are easily identified by the small black dots overlying the nucleus. The precursors of these cells absorbed the injected label, incorporated it into their DNA, and have retained that label throughout their entire lives. We analyzed the proportion of labeled cells that ocurred after the onset of each injection series. Because of the multiple injections used, our assumption was that all labeled cells were formed AFTER injections began, unlabelled cells were formed BEFORE injections began. There is a consistent labelling pattern in the dentate granular layer: labeled cells are always placed DEEP to unlabelled cells. This non-random distribution of cells according to age is called a NEUROGENETIC GRADIENT. By clicking on the grid of thumbnail sections below, a new window will open and you can view the section at high magnification with high resolution cellular detail. The labeled photo to the right shows the different parts of the hippocampus and some surrounding structures. These structures are nearly the same in all of the photos, so only one section is labeled as an example. To download a pdf of the labeled picture, click on the link below.