Danevic, Christina Shirin; (1994) Cell lineage of pyramidal and nonpyramidal neurons in the rat cerebral cortex. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

We have utilized the techniques of retrovirally-mediated gene transfer, immunocytochemistry, and electron microscopy to study the generation of the two classes of cells in the rat cerebral cortex, the pyramidal and nonpyramidal neurons. These two neuronal types have different functions in the brain, pyramidal neurons are excitatory (utilizing glutamate) and project outside of the cortex, while nonpyramidal neurons are inhibitory (utilizing GABA) and terminate locally. An important question is whether these two neuronal categories arise from the same ventricular zone progenitor or whether, by the time of corticogenesis, the two are generated by different progenitors. The replication-defective retroviral vector used contained an E-coli [beta]-galactosidase gene insert which could be detected histochemically. The viral constructs were injected in low titers into the telencephalic ventricles of fetuses during the period of corticogenesis (E14-E21) where they infected ventricular cells. The beta-galactosidase gene was used as a heritable marker since it is passed on to the progeny of these cells. The fetuses were then allowed to continue their development until either two weeks postnatally or until adulthood (1-3 months). Animals were then perfused, the brains serially sectioned, then stained histochemically [beta]-galactosidase utilizing X-gal as the enzymatic substrate. The sections were then processed for electron microscopy and flat embedded in Araldite. Sections containing stained cells were serially drawn using camera lucida and clusters of clonally-related cells were defined. Cells in each cluster were then identified using postembedding immunocytochemistry to localize GABA or glutamate in semithin sections. In some cases the findings were confirmed by light and/or electron microscopy. Our results showed two different outcomes for the two age groups studied. In the adult brains, the vast majority of neurons in a group were composed of only one cell type, either all pyramidal or all nonpyramidal neurons. The two week old animals, on the other hand, showed very different results. Many of the clones contained both pyramidal and nonpyramidal neurons. The possible explanations for this discrepancy between young and adult animals such as cell death and cell transformation are discussed.

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