Imaging hematopoietic stem cells in the marrow of long bones in vivo.

Hematopoietic stem and progenitor cells (HSPCs) are located in the bone marrow in zones of residence specialized in supporting them which are referred to as niches. It is in such a specialized niche that normal HSPCs are maintained to perform their self-renewal and differentiation duties in a highly controlled manner. One challenge in dissecting the functional significance of the complex cellular and molecular interactions in the niche is to link the types and qualities of cell-cell contacts to the intracellular signaling components involved in cell regulation. Attempts to study the interactions of HSPC with their niche eventually have to be performed in their natural location in vivo, as isolation of the cells from bone -marrow will disrupt the HSPC-niche interactions and thus not reveal functionally critical cell-cell -contacts. Intravital imaging of individual cells in the bone marrow has just recently been introduced, almost exclusively focusing on imaging inside the marrow of the calvaria. However, calvarial marrow is functionally distinct from marrow of long bones, the major source of HSPC for both physiology and study. To overcome these limitations, we developed a novel method for multiphoton intravital imaging of HSPC in the marrow of long bones.