An ectopic stromal implant model for hematopoietic reconstitution and in vivo evaluation of bone marrow niches.

In adults, hematopoiesis takes places in the bone marrow, where specialized niches containing mesenchymal nonhematopoietic cells (stroma) harbor the hematopoietic stem cell (HSC). These niches are responsible and essential for the maintenance of HSCs. Attempts to expand HSCs fail to keep the general properties of stem cells, which depend on several niche components difficult to reproduce in in vitro culture systems. Here, we describe a methodology for in vivo study of hematopoietic stroma. We use stroma-loaded macroporous microcarriers implanted in the subcutaneous tissue of experimental animals and show that the ectopic stroma implant (ESI) is able to support hematopoiesis. Moreover, lethally irradiated mice can be rescued by ESI preloaded with HSCs, showing that they function as an ectopic bone marrow. ESI is also shown as a good system to study the role of different niche components. As an example, we used stromas lacking connexin 43 (Cx43) and confirm the importance of this molecule in the maintenance of the HSC niche in vivo. We believe ESI can work as an ectopic bone marrow allowing in vivo testing of different niches components and opening new avenues for the treatment of a variety of hematologic conditions particularly when stromal cell defects are the main cause of disease.