A Picture is Worth a Thousand Words

That is a truism if there ever was one. The field of stem cells, not just hematopoietic stem cells, is defined by the actual pluripotent or multipotent cells and the necessary niches in which they reside. Elegant experiments with Drosophila melanogaster, Caenorhabditis elegans, zebrafish, and other organisms clearly demonstrated the importance of a single nurse cell (a specialized cell in Drosophila that contributes to the formation of oocytes and acts as the niche) in determining self-renewal and differentiation of the stem cells. Signals imparted from these cells directly contribute to the fate of the stem cells, whether by direct signaling or gradients of secreted factors and extracellular matrix. The importance of niches for hematopoiesis has been established through alterations of osteoblasts or vascular beds (with limited knowledge of localization) and through the use of immunohistochemistry (without knowledge of function).   

Before describing the findings, it is useful to describe the available technology. Two-photon microscopy is a relatively novel approach that allows intravital microscopy. Living tissues can be visualized to a depth of approximately 1 mm. The concept is based on the idea that two photons of low energy can excite a fluorophore in a quantum event resulting in the emission of a fluorescent photon that is of higher energy than either of the two photons. These low-energy photons do not cause damage outside the focal area. Because two photons are needed to excite the fluorophore, the fluorescence is much greater when the beam is tightly focused and, therefore, there is less background noise. The fluorescence is collected by a photomultiplier tube and recorded as a pixel. Scanning through the desired region forms the image. Using infrared light with long wavelengths to generate the photons results in less scatter and high-resolution images. This technology allows movement away from static images to actual movies.  

The paper by Lo Celso, et al., from David Scadden’s laboratory, now brings into clearer focus the relationships between the stem cells and the microenvironment. Using both confocal and two-photon video microscopes, these investigators were able to image the homing of individual hematopoietic cells in the calvarium of an irradiated mouse and its interaction with the blood vessels, osteoblasts, and endosteal surfaces. They observed that the interactions between the stem/progenitor cells and the osteoblasts in microvessels were dynamic and non-random. Different hematopoietic cell subsets localized to distinct locations, depending on the stage of differentiation and guided by autonomous and non-autonomous factors. When expansion or engraftment was studied, the hematopoietic stem cells localized to areas in close proximity to the osteoblasts.