P76. Imaging hematopoietic regeneration in real-time

Although we have learned a great deal about the phenotype and function of hematopoietic stem and progenitor cells, we have remained largely in the dark about the dynamic behavior of these cells in context of their native microenvironment. Here we describe a strategy that combines the use of transgenic mice with live microscopy to allow in vivo imaging of hematopoietic cell behavior in real time. The high spatial and temporal resolution of the system has allowed us to visualize the living hematopoietic tissue with exceptional clarity and track the architecture and dynamics of cell division, cell migration and cell death. Using this approach we have compared the kinetics of regeneration following two distinct forms of injury: chemotherapy and radiation. While chemotherapy led to a clear loss of hematopoietic cells, much of the microenvironment remained intact allowing for rapid recovery; in contrast, radiation exposure resulted in extensive destruction of both hematopoietic cells and the microenvironment, and was associated with delayed recovery. Imaging of the early events that occurred in each type of injury revealed an unexpected activation of myeloid cells in the bone marrow following chemotherapy, but not radiation. Importantly, ectopic delivery of these cells following radiation markedly enhanced both short-term hematopoietic recovery as well as long-term survival of mice receiving a limiting transplant dose, identifying myeloid cells as an important component of injury repair and regeneration. Thus, the development of this high-resolution in vivo imaging approach provides a unique view within the living organism and can be a powerful tool for gaining new insight into the regulation of fundamental biological processes such as homeostasis and regeneration.