Artificial humanized bone microenvironment acts as a natural hematopoietic niche. (A) To generate a humanized bone environment, RAG2^−/−γ_c^−/− mice were implanted with human MSC-loaded BCP scaffolds (s). Eight weeks after implantation, this led to the formation of human bone (b) in the vascularized (v) open spaces that was capable of supporting mouse hematopoiesis. Representative H&E-stained slides are shown. (B) CD34⁺ UCB cells engraft and differentiate in humanized ossicles. RAG2^−/−γ_c^−/− mice implanted with MSC-loaded BCP scaffolds and injected with CD34⁺ cells at week 8 and analyzed at week 16 revealed ossicles with human CD45⁺ cells (CD45). These included B cells (CD20), T cells (CD3), and myeloid cells (CD15). Undifferentiated CD34⁺ cells (CD34) were detected adjacent to both bone (b) and vessels (v). Shown are representative images of immunohistochemical stainings for human CD markers.