Hematopoietic stem cells are responsible for the continued production of blood and immune cells throughout life. These cells are maintained and regulated by cells within their immediate environment, or niche. Our lab has identified Leptin receptor-expressing perivascular stromal cells as important components of the hematopoietic stem cell niche. In addition to the perivascular niche cells, the peripheral nervous system also regulates hematopoietic stem cell function. Our preliminary data suggest reciprocal communication between peripheral nerves and a subset of perivascular niche cells that is required for the maintenance of bone marrow innervation. We will use quantitative three-dimensional imaging techniques we have developed to understand the physical interactions of cells and nerve fibers within the hematopoietic stem cell niche. We will also use genetic manipulation to determine the functional requirement of signals from the perivascular niche cells for innervation of the bone marrow and regulation of stem cell function. To test whether signals from peripheral nerves directly or indirectly regulate the hematopoietic stem cell niche we will use optogenetics to activate the nerves and assess the effect on stem cell maintenance and localization. These studies will provide insights into the inter-cellular regulation of hematopoiesis. This will improve our understanding of how interactions between perivascular niche cells and peripheral nerves regulate normal hematopoiesis, which might offer new therapeutic strategies for hematopoietic disorders ranging from anemia to cancers of the blood-forming system.