Effect of wound healing and tissue transplantation on the navigation of axons in organ-cultured embryonic chick eyes

Calcium-dependent neuroepithelial contractions expel damaged cells from the developing brain

Both developing and adult organisms need efficient strategies for wound repair. In adult mammals, wounding triggers an inflammatory response that can exacerbate tissue injury and lead to scarring. In contrast, embryonic wounds heal quickly and with minimal inflammation, but how this is achieved remains incompletely understood. Using in vivo imaging in the developing brain of Xenopus laevis, we show that ATP release from damaged cells and subsequent activation of purinergic receptors induce long-range calcium waves in neural progenitor cells. Cytoskeletal reorganization and activation of the actomyosin contractile machinery in a Rho kinase-dependent manner then lead to rapid and pronounced apical-basal contractions of the neuroepithelium. These contractions drive the expulsion of damaged cells into the brain ventricle within seconds. Successful cell expulsion prevents the death of nearby cells and an exacerbation of the injury. Cell expulsion through neuroepithelial contraction represents a mechanism for rapid wound healing in the developing brain.

Electric axon guidance in embryonic retina: galvanotropism revisited

In addition to well-known mechanisms of chemical guidance, growing axons in the nervous system are directed by an extracellular electric field in a process known as galvanotropism. The galvanotropic behavior of nerve cells in vitro was first demonstrated as long ago as 1920. However, it remains unknown whether embryonic nerve tissues generate a similar electric field in order to guide growing axons. The present study reveals that an extracellular voltage gradient exists in the embryonic retina and that this gradient guides the axons of newborn retinal ganglion cells towards their targets. These findings indicate an important role for galvanotropism in the initial orientation of axons that extend over long distances, and provide insight into the mechanisms underlying the proper extension of developing axons in the embryonic brain.

Extraembryonic origin of circulating endothelial cells

Circulating endothelial cells (CEC) are contained in the bone marrow and peripheral blood of adult humans and participate to the revascularization of ischemic tissues. These cells represent attractive targets for cell or gene therapy aimed at improving ischemic revascularization or inhibition of tumor angiogenesis. The embryonic origin of CEC has not been addressed previously. Here we use quail-chick chimeras to study CEC origin and participation to the developing vasculature. CEC are traced with different markers, in particular the QH1 antibody recognizing only quail endothelial cells. Using yolk-sac chimeras, where quail embryos are grafted onto chick yolk sacs and vice-versa, we show that CEC are generated in the yolk sac. These cells are mobilized during wound healing, demonstrating their participation to angiogenic repair processes. Furthermore, we found that the allantois is also able to give rise to CEC in situ. In contrast to the yolk sac and allantois, the embryo proper does not produce CEC. Our results show that CEC exclusively originate from extra-embryonic territories made with splanchnopleural mesoderm and endoderm, while definitive hematopoietic stem cells and endothelial cells are of intra-embryonic origin.