Asparaginase allergies: it's all in the genes.

embryonic liver, followed by the embryonic spleen. These studies open the door for further in-depth analyses of the axolotl hematopoietic system. By using GFP transgenic donors, hematopoietic cells can be easily visualized in adults, which is advantageous for future studies characterizing the role of specific cell types in wound repair and regeneration, as well as visualizing homing and engraftment. Additionally, the authors show that HSPC populations are roughly 1000-fold enriched in the lymphoblastic population of the spleen, positioning this fraction for further prospective isolation approaches. In addition, their finding that HSPCs reside in the peripheral layer of the liver indicates that the axolotl liver and spleen likely provide different hematopoietic niches for different functions. Comparison of the signaling processes occurring between these 2 sites, as well as among other niches in different vertebrate animals, may provide insight into the conserved core network of HSC support molecules. Finally, the axolotl is likely an excellent system to understand the role of hematopoietic cells not only in the regeneration of limbs, spinal cord, and other organs, but also in the regeneration of the hematopoietic system itself. For example, as mammals age, they have less ability to robustly generate lymphoid cells, with HSCs becoming skewed toward myeloid outputs. It would be interesting to investigate if this trend exists in the neotenic axolotl, which in the adult form retains juvenile traits. Overall, this pioneering work in the axolotl now provides another excellent model system in which to compare and contrast the evolution of vertebrate hematopoiesis. Conflict-of-interest disclosure: The authors declare no competing financial interests. n