Topology, kinetics and inheritance in clonal colonies of bone marrow stromal cells
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Bone marrow stromal cells (BMSCs), whose populations contain multipotent skeletal stem cells with relevant therapeutic applications, are known to produce very heterogeneous colonies upon in vitro culture, a trait that may severely hinder the clinical usefulness of BMSC-based therapies. Therefore, reaching a better insight on the nature of such heterogeneity, as well as on the factors determining it, is important. Here, by using time-lapse microscopy, we study the structure of N = 28 human BMSC colonies from six donors, each colony derived from a single cell, and trace their lineage trees up to the seventh generation. We confirm the presence of very significant inter-colony and intra-colony heterogeneities, both in the topology of the lineages and in the replicative kinetics of the colonies. We also find that topology and kinetics are strongly correlated, consistent with the existence of regulating factors linking the sub-population of inactive cells, which uniquely determine a lineage’s topology, and that of active cells, which are the sole responsible for the proliferation rate of the colony. Finally, we submit each colony to an entropy-based inheritance test, which measures the degree of non-random clustering of inactive cells within the same branches of the lineage, and find a clear signature of hereditary transmission of the probability of emergence of inactive cells in the largest majority of the experimental lineages.
Author summary
The topological structure of the lineage tree produced by the growth of a stem cell colony is uniquely determined by the number of its inactive cells and their position along the tree’s branches. Inactive cells do not divide (be it because they are temporarily quiescent, or because they are permanently senescent or differentiated), and therefore do not contribute to the statistics of the division times of the colony, while the active portion of the population proliferates with a kinetics that characterises the colony’s growth as a whole. Being these two phenotypes (active vs inactive) well-separated from each other, there are no a priori reasons to expect a specific connection between the structure of the inactive part of the population and the replicative speed of the active part. However, we do find strong evidence of such connection in BMSCs: colonies with many inactive cells are also characterized by a slow growth of their active cells population, and vice versa . Interestingly, our experimental results cannot be explained by random fluctuations in the lineage structure, as an entropy-based inheritance test indicates that the probability of emergence of inactive cells has a strong hereditary character.
