Digits in a dish: An in vitro system to assess the molecular genetics of hand/foot development at single-cell resolution.

models allow for the study of developmental processes outside of the embryo. To gain access to the cells mediating digit and joint development, we identified a unique property of undifferentiated mesenchyme isolated from the distal early autopod to autonomously re-assemble forming multiple autopod structures including: digits, interdigital tissues, joints, muscles and tendons. Single-cell transcriptomic analysis of these developing structures revealed distinct cell clusters that express canonical markers of distal limb development including: , , and (phalanx formation), and (perichondrium), , , and (joint interzone), and (interdigital tissues), (muscle progenitors), (articular perichondrium/articular cartilage), and and (tenocytes/tendons). Analysis of the gene expression patterns for these signature genes indicates that developmental timing and tissue-specific localization were also recapitulated in a manner similar to the initiation and maturation of the developing murine autopod. Finally, the digit system also recapitulates congenital malformations associated with genetic mutations as cultures of mutant mesenchyme produced defects present in mutant autopods including digit fusions, reduced phalangeal segment numbers, and poor mesenchymal condensation. These findings demonstrate the robustness of the digit system to recapitulate digit and joint development. As an model of murine digit and joint development, this innovative system will provide access to the developing limb tissues facilitating studies to discern how digit and articular joint formation is initiated and how undifferentiated mesenchyme is patterned to establish individual digit morphologies. The digit system also provides a platform to rapidly evaluate treatments aimed at stimulating the repair or regeneration of mammalian digits impacted by congenital malformation, injury, or disease.