Haematopoietic stem cell (HSC) niches, although proposed decades ago, have only recently been identified as separate osteoblastic and vascular microenvironments. Their interrelationships and interactions with HSCs in vivo remain largely unknown. Here we report the use of a newly developed ex vivo real-time imaging technology and immunoassaying to trace the homing of purified green-fluorescent-protein-expressing (GFP(+)) HSCs. We found that transplanted HSCs tended to home to the endosteum (an inner bone surface) in irradiated mice, but were randomly distributed and unstable in non-irradiated mice. Moreover, GFP(+) HSCs were more frequently detected in the trabecular bone area compared with compact bone area, and this was validated by live imaging bioluminescence driven by the stem-cell-leukaemia (Scl) promoter-enhancer. HSCs home to bone marrow through the vascular system. We found that the endosteum is well vascularized and that vasculature is frequently localized near N-cadherin(+) pre-osteoblastic cells, a known niche component. By monitoring individual HSC behaviour using real-time imaging, we found that a portion of the homed HSCs underwent active division in the irradiated mice, coinciding with their expansion as measured by flow assay. Thus, in contrast to central marrow, the endosteum formed a special zone, which normally maintains HSCs but promotes their expansion in response to bone marrow damage.
In addition to stimulating skeletal muscle growth and repair, Wnt7a/Fzd7 signaling increases the polarity and directional migration of myogenic progenitors and improves the efficacy of muscle stem cell therapy.
Neural crest cell (NCC) invasion is a complex sculpting of individual cells into organized migratory streams that lead to organ development along the vertebrate axis. Key to our understanding of how molecular mechanisms modulate the NCC migratory pattern is information about cell behaviors, yet it has been challenging to selectively mark and analyze migratory NCCs in a living embryo. Here, we apply an innovative in vivo strategy to investigate chick NCC behaviors within the rhombomere 4 (r4) migratory stream by combining photoactivation of KikGR and confocal time-lapse analysis of H2B-mRFP1 transfected NCCs. We find that the spatial order of r4 NCC emergence translates into a distal-to-proximal invasion of the 2nd branchial arch. Lead and trailing NCCs display similar average cell speeds and directionalities. Surprisingly, we find that lead NCCs proliferate along the migratory route and grow to outnumber trailing NCCs by nearly 3 to 1. A simple, cell-based computational model reproduces the r4 NCC migratory pattern and predicts the invasion order can be disrupted by slower, less directional lead cells or by environmental noise. Our results suggest a model in which NCC behaviors maintain a spatially-ordered invasion of the branchial arches with differences in cell proliferation between the migratory front and trailing NCCs.
SUMMARYDuring development and regeneration, directed migration of cells, including neural crest cells, endothelial cells, axonal growth cones and many types of adult stem cells, to specific areas distant from their origin is necessary for their function. We have recently shown that adult skeletal muscle stem cells (satellite cells), once activated by isolation or injury, are a highly motile population with the potential to respond to multiple guidance cues, based on their expression of classical guidance receptors. We show here that, in vivo, differentiated and regenerating myofibers dynamically express a subset of ephrin guidance ligands, as well as Eph receptors. This expression has previously only been examined in the context of muscle-nerve interactions; however, we propose that it might also play a role in satellite cell-mediated muscle repair. Therefore, we investigated whether Eph-ephrin signaling would produce changes in satellite cell directional motility. Using a classical ephrin 'stripe' assay, we found that satellite cells respond to a subset of ephrins with repulsive behavior in vitro; patterning of differentiating myotubes is also parallel to ephrin stripes. This behavior can be replicated in a heterologous in vivo system, the hindbrain of the developing quail, in which neural crest cells are directed in streams to the branchial arches and to the forelimb of the developing quail, where presumptive limb myoblasts emigrate from the somite. We hypothesize that guidance signaling might impact multiple steps in muscle regeneration, including escape from the niche, directed migration to sites of injury, cell-cell interactions among satellite cell progeny, and differentiation and patterning of regenerated muscle.
Neural crest (NC) cells invade the vertebrate embryo in ordered migratory streams, yet it is unclear whether cells communicate to maintain spacing and direction. Here, we examined NC cell communication in detail, using optical highlighting and photobleaching to monitor cell contact dynamics. We observed cytoplasmic transfer between NC cell neighbors through thin cellular bridges. The transfer of molecules between NC cells was bi-directional, not at equal rates, and independent of bridge dynamics. The cytoplasmic transfer was prevalent in recently divided NC cells. Molecular simulations, based on Brownian motion and measured cell volumes, predicted that simple diffusion could not account for observed cytoplasmic transfer rates. Cell tracking revealed that exchange of cytoplasmic material preceded the re-orientation of cells to the direction of migration. Our data suggest a mechanism by which NC cells communicate position information through the formation of cellular bridges that allow exchange of cytoplasmic material through active transport.
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