Limb connective tissue is organized in a continuum of promiscuous fibroblast identities during development

Although connective tissues play critical roles in development, our understanding of connective tissue fibroblast developmental programs lag behind that of other components of the musculoskeletal system mainly because fibroblasts are highly heterogeneous and poorly characterized. Combining single-cell RNA-sequencing-based strategies, including trajectory inference, and in situ hybridization analyses, we address the diversity of connective tissue fibroblasts and their developmental trajectories during chicken limb foetal development. We show that fibroblasts switch from a positional information to a lineage diversification program during their development. Muscle connective tissue and tendon contain several fibroblast populations that emerge asynchronously. In fine, these populations map to distinct layers, prefiguring the adult fibroblast layers. Populations that are close in transcriptional identity are found in neighboring layers. Altogether, we propose that the limb connective tissue is organised in a continuum of promiscuous fibroblasts identities, allowing for the robust and efficient connection of highly different tissues such as muscle, bone and skin. Single cell mRNA profiles of whole forelimbs in wild type chicken embryos at two developmental stages (E7 and E9)