Labeling of developing vascular endothelium after injections of rhodamine-dextran into blastomeres of Xenopus laevis.

The goal of this work has been to label endothelial cells with fluorescent marker and to record their behavior during angiogenesis in vivo. Single blastomeres in 16-128-cell-stage embryos of pigment-deficient Xenopus laevis were injected intracellularly with 5% tetramethyl-rhodamine dextran. Subsequently, the embryos and tadpoles were examined with an epifluorescence microscope, a silicon-intensified target (SIT) camera, and video recordings. Clones that would include endothelium could be selected as early as stages 33-36 on the basis of heavy labeling in the ventral mesodermal core of the tail. Strands of fluorescent cells and early vessels appeared in the tail at stages 39-41. Subsequently, groups of endothelial cells were followed in case histories in the tail and in the aortic arches and gills of tadpoles. Two main results were that the patterns of fluorescent endothelial cells were stable in established arteries, veins, and capillaries for at least 2-12 days, and that labeled endothelial cells migrated distally in elongating sprouts. In addition, it was inferred that endothelium was derived from multiple blastomeres, probably in the ventral vegetal regions. Only small fractions of total endothelium were labeled from any single blastomere. None of the early blastomeres produced exclusive clones of vascular endothelium; other labeled cell types in various clones included muscle fibers, lymphatics, mesodermal stellate cells, blood cells, gut, proctodeum, and some epidermis, in addition to endothelial cells. The method of intracellular marking of blastomeres is recognized as a direct approach for charting lineage and fate tables in embryos of Xenopus and other species. The present study extends the period of observation in vivo for up to 2 weeks in the growing tadpole and focuses on endothelial cells during angiogenesis. Even though fluorescent dextran was apparently packaged in vesicles and metabolized, individual cells and small groups could be identified and followed with time. This method provides excellent opportunities for addressing problems in vascular development in the living animal.