Clonal Analysis Of Neocortical Gliogenesis

Abstract

Proper neural function relies on the regulation of the diversity and the proportion of neuronal and non-neuronal cell types. Glia, non-neuronal neural cells, have emerged as key players in the development, function, and maintenance of the central nervous system (CNS). Glia constitute as much as 50% of the cellular population in the CNS. As our understanding of the array of glial functions increases, little is understood about the physiological basis and developmental origin of glial abundance and diversity. ` The mammalian neocortex is a highly organized structure that plays a crucial role in higher-order brain functions such as cognition and integrating sensory input. Radial glial progenitors (RGPs) that line the cerebral ventricles and are positive for the transcription factor Emx1 produce neocortical excitatory neurons and glia in a sequential manner. Previous lineage tracing has revealed that at the conclusion of neurogenesis 16% of RGPs proceed to gliogenesis. During development, these RGPs generate discrete columnar structures that contain neurons, astrocytes, and oligodendrocytes. To understand how the diversification of glia is mediated by individual progenitors, we performed clonal analysis of neocortical RGPs at their transition from neurogenesis to gliogenesis using Mosaic Analysis with Double Markers (MADM). In terms of lineage specification, our analysis uncovered three distinct types of gliogenic RGPs based on their potential to generate neural cell types. The proportion RGP subtypes is independent of time of exit from neurogenesis and location along the anterior-posterior axis. Quantitatively, MADM-based clonal analysis of neocortical gliogenesis revealed that the number of glia produced by an individual RGP is independent from the time exit from neurogenesis occurs. Our analysis shows that the overall ratio of Emx1-derived glia to neurons is 1:5. Our analysis suggests that although the majority of neocortical astrocytes are Emx1-RGP derived, a significant proportion of oligodendroglia are derived from other origins. We also found that different programs regulate the proliferation of astrocyte-restricted and oligodendrocyte-restricted glial precursors, with oligodendrocyte precursors having a larger proliferative capacity compared to astrocyte precursors. In terms of distribution and structure, analysis of the laminar distribution of astrocyte clones suggests separate lineages for Layer-1 and white matter (WM) astrocytes and that astrogliogenesis, similar to neurogenesis, proceeds in an inside-out fashion. Most oligodendrocyte clones are clustered into a single layer and the majority of RGP-derived oligodendrocytes are destined for deep cortical layers, whereas RGP-derived astrocytes are more evenly spread through the cortical width. Our results reveal how the production of neuronal and non-neuronal cell types are developmentally regulated in individual progenitors. In particular, shedding insight on the regulation of lineage specification of glial types, establishment of neuron:glia ratio, and the distribution and structure of RGP-derived glia in the neocortex.