Topological defects govern mesenchymal condensations, offering a morphology-based tool to predict cartilage differentiation

A critical initial stage of skeletal morphogenesis involves formation of highly compact aggregates of mesenchymal cells, known as mesenchymal condensations, appearing as regularly-spaced pattern of spots. Conventional computational models to understand their patterning have been based on chemotaxis, haptotaxis, and reaction-diffusion equations. In this work, we investigate the mesenchymal condensations from a different perspective, namely topological defects within liquid crystal-like pattern. Using bone marrow-derived mesenchymal stromal cells (bm-MSCs), we observed emergence of cellular swirls in confluent in-vitro cultures, followed by appearance of mesenchymal condensations at the centers of the selfassembled swirls. Specifically, the condensations appeared at the comet-like (+1/2) and spiral-shaped (+1) topological defect sites within the swirl pattern. Next, with the rationale that cellular swirls precede skeletal morphogenesis, and supported with the qualitative observation that swirl pattern-features are donor-specific, we probed the correlation between swirl pattern and the chondrogenic differentiation outcome of bm-MSCs. Towards this, we first generated and imaged cellular swirls systematically across 5 donors by controlling seeding density, culture vessel geometry, and culture duration. We observed that the swirl pattern features quantified as variance of coherency correlated strongly with the cartilage matrix proteins, sulfated glycosaminoglycan and collagen-II, quantified from the standard in-vitro chondrogenic differentiation assay. Our work shows that swirl-pattern quantification provides a novel and powerful tool to predict efficacy of bm-MSCs for in-vitro cartilage regeneration. Significance StatementMesenchymal condensation is a critical stage in the formation of bone and cartilage, where the mesenchymal cells form high density cell clusters that are regularly spaced. In this work, we inspect the patterning of these condensations in-vitro from a novel perspective. We first show that at high density, bone-marrow-derived mesenchymal stromal cells (bm-MSCs) self-assemble to form cellular swirls resembling the vortices in a turbulent flow. This is followed by cell aggregations at the centers of the vortices, which show correspondence to mesenchymal condensations. Interestingly, we observed that the swirl pattern made by bm-MSCs isolated from human donors, varies from individual to individual and correlates with their propensity to differentiate into cartilage. This suggests that swirl pattern quantification via image analysis can be used to predict differentiation outcome, in context of regenerative cell therapy.