Origine e sviluppo degli oligodendrociti nel cervelletto

Glial cells constitute a large fraction of the mammalian brain. In particular, oligodendrocytes are responsible for the myelination process, a mechanism that enables fast, saltatory impulse propagation. The current knowledge about the developmental origin, the clonal relationship and the spatial distribution of cerebellar oligodendrocytes is still fragmentary. The general tenet is that the majority of oligodendrocytes has an extracerebellar origin in the ventricular region of the metencephalic ventral rhombomere 1 (r1), while only 6% derive from the cerebellar ventricular zone. Oligodendrocyte precursors enter the cerebellum through the velum medullare and disperse throughout the parenchyma; they first occupy the deep white matter (DWM), surrounding the cerebellar nuclei, and progressively invade the nascent cortical lobuli, where they settle both in the axial WM and in the cortical layers. This study aims at investigating the ontogenesis of the oligodendrocyte lineages in the cerebellum, in terms of spatial segregation, dispersion within the cerebellar layers and cellular composition along the distinct maturation stages. In order to achieve this outcome, we applied an innovative clonal analysis system called Star Track, based on the random, stable and inheritable expression of 6 fluorescent proteins. A mixture of plasmids coding for the fluorophores was injected in the IV ventricle of E12 and E14 embryos and electroporated in utero in order to mark single progenitors lining the ventricle. Animals were then sacrificed at P30 and a software recently developed in collaboration with the Polytechnic of Turin allowed to identify each clone. Here, I will show the analysis of the 79 clones found in two electroporated cerebella. These clones, very heterogeneous in size, were found along the whole medio-lateral (M-L) axis of the cerebellum, from the vermis to one of the hemispheres. Nevertheless, while in the first animal a higher percentage of cells was located in the hemisphere, in the second one we found the majority of cells in the vermis. This opposite behaviour results in slightly different features of clones in the two animals: although clones in both cerebella were mainly gathered in the deepest part of the cerebellum, where the DWM and the deep nuclei are located, clones in the second cerebellum were distributed also in the cerebellar median lobuli compared to the others. In line with this result, more than a half of the analysed oligodendrocytes were located in the WM, while the contribution of other layers was less significan. Finally, immunofluorescence and morphological analysis were performed to evaluate the clones composition: most of them were heterogeneous, containing oligodendrocyte progenitor cells (OPCs), premyelinating and myelinating cells. However, the relative abundance of the cells at each stage was different between the two animals, with a prevalence of premyelinating in the first one and of OPCs in the second one. This may be due to the fact that oligodendrocytes are more prone to differentiate where they have more axons to myelinate, such as in the cerebellar hemispheres where the WM tracts are overall bigger compared to those in the vermis. Here we provide the first description of the developmental, differentiation and spatial distribution patterns for oligodendrocyte lineages in the cerebellum, highlighting that distinct rates of proliferation/maturation may exist between cells located in distinct cerebellar regions