Interdigital tissue remodelling in the embryonic limb involves dynamic regulation of the miRNA profiles.

Next-generation sequencing in combination with quantitative polymerase chain reaction analysis revealed a dynamic miRNA signature in the interdigital mesoderm of the chick embryonic hinlimb in the course of interdigit remodelling. During this period, 612 previously known chicken miRNAs (gga-miRNAs) and 401 non-identified sequences were expressed in the interdigital mesoderm. Thirty-six microRNAs, represented by more than 750 reads per million, displayed differential expression between stages HH29 (6 id) and HH32 (7.5 id), which correspond to the onset and the peak of interdigital cell death. Twenty miRNAs were upregulated by at least 1.5-fold, and sixteen were downregulated by at least 0.5-fold. Upregulated miRNAs included miRNAs with recognized proapoptotic functions in other systems (miR-181 family, miR-451 and miR-148a), miRNAs associated with inflammation and cell senescence (miR-21 and miR-146) and miRNAs able to induce changes in the extracellular matrix (miR-30c). In contrast, miRNAs with known antiapoptotic effects in other systems, such as miR-222 and miR-205, became downregulated. In addition, miR-92, an important positive regulator of cell proliferation, was also downregulated. Together, these findings indicate a role for miRNAs in the control of tissue regression and cell death in a characteristic morphogenetic embryonic process based on massive apoptosis.

The spatiotemporal expression patterns of MSC-associated markers contribute to the identification of progenitor subpopulations in developing limbs.

During limb development, skeletal tissues differentiate from their progenitor cells in an orchestrated manner. Mesenchymal stromal cells (MSCs), which are considered to be adult undifferentiated/progenitor cells, have traditionally been identified by the expression of MSC-associated markers (MSC-am) and their differentiation capacities. However, although MSCs have been isolated from bone marrow and a variety of adult tissues, their developmental origin is poorly understood. Remarkably, adult MSCs share similar differentiation characteristics with limb progenitors. Here, we determined the expression patterns of common MSC-am throughout mouse hindlimb development. Our results demonstrate that MSC-am expression is not restricted to undifferentiated cells in vivo. Results from the analysis of MSC-am spatiotemporal expression in the embryonic hindlimb allowed us to propose five subpopulations which represent all limb tissues that potentially correspond to progenitor cells for each lineage. This work contributes to the understanding of MSC-am expression dynamics throughout development and underlines the importance of considering their expression patterns in future MSC studies of the limb.