The unbearable lightness of bone marrow homeostasis.

The anatomical and functional dimensions of bone marrow topography have been at the forefront of modern bone and immunological research for many years and remain a source of complexity and perplexity due to the multitude of microhabitats within this microenvironment. In fact, research has uncovered fascinating functional aspects of bone marrow residents, and the bone marrow niche has been identified as the foremost reservoir of a variety of cells including hematopoietic, skeletal and endothelial stem/progenitor cells. The physical interactions of the marrow residents, combined with the release of cytokines and growth factors, organize well-defined operative compartments, which preserve bone and immune cell homeostasis. In a simplistic view, both the hematopoietic and bone marrow stromal (mesenchymal) stem/progenitor cell populations dwell at the interface between the endosteum and the bone marrow area (endosteal niche) and in the perivascular space (vascular niche). Indeed, the tantalizing hypothesis of bone marrow regulatory dependency on these niches is supported by current research insofar as the increase in the number of osteoblasts results in a concomitant increase in the hematopoietic population, indicating that the osteoblasts and the endosteal niche are key components of HSC maintenance. On the other hand, impaired function of the vascular niche compromises the endosteal niche's ability to support hematopoiesis. These fascinating discoveries indicate that there are strong ties between bone marrow inhabitants within the confines of the bone marrow itself. When these ties fail, niche-niche communication suffers and results in reduced bone formation, enfeebled hematopoiesis and unrestrained HSC migration through blood circulation. This study focused on the extraordinary homeostatic equilibrium and function of both bone and immune cells within the spatially defined microenvironment of bone marrow. But how important is the anatomically outlined scenery in which the bone marrow entity supports and hosts the hematopoietic elements?