Bone Marrow-derived Cells Contribute To Multilineage Reconstitution And Blastema Stage-specific Upregulation Of A Transient Scaffold In Regenerating Mouse Digit Tips
In 2005, 1.6 million Americans lived with a debilitating amputation and this figure is predicted to double by 2050. But the ability of a mammal to recapitulate a complex limb structure is not impossible. Evidence of children and mice re-growing digit tips following amputation midway through the terminal phalanx (P3) exists. The hallmark of this phenomenon is development of a blastema housing undifferentiated cells capable of being re-programmed to replicate the missing part. Our central goal is to understand specific components of this process for application into pro-scarring injuries. The mouse digit anatomy is prominently outlined by microfilaments containing ER-TR7, and antigen derived from fibroblast reticular cells (FRCs) of the thymus shown to facilitate intercellular communication to promote lymphoid organogenesis. A unique blastema characteristic is the upregulation of an ER-TR7+ scaffold stemming from half of the blastema population which reverts to its pre-existing pattern after regenerate differentiation concludes. We measured a correlation between ER-TR7 and type III collagen (COL3) at the transcriptional and protein levels both in vitro during induction of ER-TR7 in primary P3 cells and throughout digit regeneration. Co-expression with COL3 sheds light on ER-TR7 identity and allows testing various approaches to manipulation of the scaffold through the better understood mechanism of COL3 regulation. Furthermore, we aimed at determining the origin of ER-TR7+ blastema FRCs. Using bone marrow (BM) transplantation, we generated eGFP+ BM chimeras to study the fate of BM-derived cells (BMDCs) after amputation based on the hypothesis that in the regenerate, multipotent BMDCs contribute to various cellular phenotypes including FRCs. So we tested co-immunolocalization of eGFP with antigens particular to fibroblastic, hematopoietic, endothelial, osteoblastic, and mural cells. Many BMDCs homed to the injury throughout regeneration. But hematopoietic BMDCs were limited to inflammation whereas mesenchymal BMDCs expanded and were primed as ER-TR7+ FRCs in the P3 BM niche prior to homing to the blastema site, where they amounted to nearly 50% of cells. Moreover, BMDCs differentiated into endothelial, osteoblastic, and smooth muscle subpopulations and although diluted by pre-existing progenitors by the endpoint of regeneration, BMDCs persisted as part of various structures thus contributing to long-term function.