Hematopoietic assays developed by Lopez et al in the axolotl.¹  (A) CFU assays indicate that CFU-E and CFU-GEMM colonies are generated from adult axolotl liver (L; green) and spleen (S; orange), but not thymus (T; dark blue), bone marrow (BM; purple), or kidney (K; light blue), when stimulated with pokeweed mitogen-stimulated axolotl spleen cell-conditioned media (PWM-SCM) and human EPO. (B) After the sites of hematopoietic progenitors in adult axolotls were identified, GFP⁺ hematopoietic stem and progenitor cells were isolated from the liver (L; green) and spleen (S; orange), and engrafted in white adult axolotls. (Left) However, rates of graft-versus-host disease (GVHD) were high. (Right) Harvesting GFP⁺ hematopoietic stem and progenitor cells from 3- to 7-month-old axolotls reduced the incidence of GVHD, leading to long-term engraftment of progenitors. (C) Because irradiation can impair regeneration in axolotls, the authors developed an intracardiac injection assay. GFP⁺ hematopoietic stem and progenitor cells were isolated from adult axolotl and injected into larvae before 3 months of age. This allowed long-term reconstitution of recipients (up to 2 years later) that could be measured by antibody staining, morphological examination of hematopoietic cells, and functional assays. Importantly, the incidence of GVHD using this assay was reduced to 0%.