Meninges are a neural stem cell niche during development and in adulthood

Leptomeninges, which include arachnoid and pia mater, cover the entire CNS and are filled with cerebrospinal fluid produced by choroid plexi. In a recent study we have shown that leptomeninges covering the parietal cortex contain nestin+ cells that can be isolated, expanded in vitro and induced to differentiate in neuronal and glial lineages. We named Leptomeningeal Stem/progenitor Cell (LeSC) these cells with neural differentiation potential. With this work we intend to characterize in more details this cell population and its niche during development. Confocal immunomicroscopy on rat brain tissue slices was used to describe changes in distribution and proliferation rate of nestin+ cells and in the expression profiles of cellular and extracellular matrix antigens during development (E20, P0, P8, P15) and adulthood. The nestin+ cell layer is identified as early as E20 and is located outside the basal lamina (stained by laminin) in the leptomeninges. The number and density of these cells decreased with the animal age. Their ultrastructural properties were analysed by standard and immune-electron microscopy. Leptomeningeal nestin+ cells are distinct from NG2+ pericytes, GFAP+ astrocytes and CD31+ endotelial cells. Stem cell niche is a specific site in adult tissues where stem cells reside, self-renew and produce progeny capable of differentiation and functional specialization. We analysed the expression pattern of stem cell markers Sox2, Pax 6 and Oct4. Among leptomeningeal nestin+ cells, we identify a subpopulation co-expressing Oct4 . Furthermore, cluster of proliferating (Ki67)/ nestin+ cells were found. Moreover, we observed in the context of leptomeingeal tissue a significant fraction of differentiated neural progenitor cells. To further characterize leptomeninges as neural stem cell niche, we assessed the in vitro stem cell properties. As expected from the ex-vivo observation, the number of colony forming unit and cell growth rate, decreased with rat age. It is to note that LeSC can be expanded very efficiently even from adult animals. Under differentiating conditions, nestin+ cells expanded from all rat ages underwent neural differentiation. Interestingly, the in vitro-generated neurons displayed typical neuronal morphology and phenotype as well as functional properties as shown by synapse formation and responses to depolarizing agents. In addition to neurons, we could observe GFAP+ astrocytes and O4+ oligodendrocytes. In conclusion we show that the leptomeninges are a putative new neural stem cell niche capable of housing and maintaining up to adulthood a population of stem/progenitor cells (LeSCs) expressing stemness related genes.