Mesenchymal progenitor cells in red and yellow bone marrow.

Marrow cavities in all bones of newborn mammals contain haematopoietic tissue and stromal microenvironment that support haematopoiesis (haematopoietic microenvironment), known as red bone marrow (BM). From the early postnatal period onwards, the haematopoietic microenvironment, mainly in tubular bones of the extremities, is replaced by mesenchymal cells that accumulate lipid drops, known as yellow BM, whereas haematopoietic tissue gradually disappears. We analysed the ability of mesenchymal cell progenitors in red and yellow BM to produce bone and haematopoietic microenvironment in vivo after transplantation into normal or haematopoietically deficient (irradiated and old) recipients. We found that (1) normal substitution of red with yellow BM results from a gradual loss of mesenchymal stem cells (MSCs) capable of developing bone and haematopoietic microenvironment; (2) the mesenchymal cell population in tubular bones still containing active haematopoietic tissue gradually becomes depleted of MSCs, starting from a young age; (3) haematopoietic microenvironment is incapable of self-maintenance and its renewal depends on the presence of precursor cells; (4) the mesenchymal cell population remaining in areas with yellow BM contains cells able to develop functionally active haematopoietic microenvironment in conditions of haematopoietic insufficiency. Our data also indicate the possible existence of bi-potential stromal precursor cells producing either bone in normal, or bone together with active haematopoietic microenvironment in irradiated or old recipients. This study opens a spectrum of opportunities for the extension of haematopoietic territories by substituting the fat contents of BM cavities with haematopoietic tissue, thereby improving haematopoiesis compromised by cytotoxic treatments, irradiation, ageing, etc.