Clonal architecture of the mouse hippocampus.

Experimental mouse chimeras have proven useful in analyzing the cell lineages of various tissues. Here we use experimental mouse chimeras to study cell lineage of the hippocampus. We examined clonal architecture and lineage relationships of the hippocampal pyramidal cells, dentate granule cells, and GABAergic interneurons. We quantitatively analyzed like-genotype cohorts of these neuronal populations in the hippocampus of the most highly skewed chimeras to provide estimates of the size of the progenitor pool that gives rise to these neuronal groups. We also compared the percentage chimerism across various brain structures to gain insights into the origins of the hippocampus relative to other neighboring regions of the brain. Our qualitative analyses demonstrate that like-genotype cohorts of pyramidal cells are aligned in radial arrays across the pyramidal cell layer, whereas like-genotype cohorts in the C-shaped dentate gyrus colonize either the outer shell or inner core of the granule cell layer in a symmetrical manner. Clonally related populations of GABAergic interneurons are dispersed throughout the hippocampus and originate from progenitors that are separate from either pyramidal or granule cells. Granule and pyramidal cells, however, are closely linked in their lineages. Our quantitative analyses yielded estimates of the size of the progenitor pools that establish the pyramidal, granule, and GABAergic interneuronal populations as consisting of 7000, 400, and 40 progenitors, respectively, for each side of the hippocampus. Last, we found that the hippocampal pyramidal and granule cells share a lineage with cortical and diencephalic cells, pointing toward a common lineage that crosses the di-telencephalic boundaries.