Alternative Sources Of Angioblasts
In addition to the lateral plate mesoderm, angioblasts may originate from other tissue. In the chick, somitic mesoderm may play a role in producing angioblasts, and in the mouse, the allantois has the potency to contribute angioblasts.
In the chick, there are two sources of angioblasts which seed the various organs (Figure 1; Pardanaud et al., 1996). The first source is paraxial mesoderm. The cephalic paraxial mesoderm provides angioblasts for the blood vessels of the head (Couly et al., 1995), while the somites of the trunk contains angioblasts that migrate to form the vessels of the body wall, limb, kidney, and dorsal portions of the aorta. The second source of angioblasts is the splanchnopleural mesoderm. These angioblasts colonize the visceral organs, gut, and the floor of the aorta. These latter angioblasts are actually hemangioblasts, because they not only generate endothelial lining, but also provide the blood cell precursors (Pardanaud et al., 1996).
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| Figure 1 Two sources of angioblasts in the chick embryo form the endothelia of separate regions. The angioblasts from the somites migrate through the intermediate mesoderm (kidney), somatopleure, and the roof and lateral regions of the aorta. The angioblasts from the splanchnopleure form the vessels of the gut and visceral organs as well as the floor of the aorta. The angioblasts at the floor of the aorta also produce blood cells. (After Pardanaud et al., 1996.) |
A third source of hemangioblasts may be the allantois. This organ has not usually been considered as a source of blood or endothelial cells, but new evidence suggests that it plays a role in supplying hemangioblasts to the embryo. Caprioli and colleagues (1998) have shown the emergence of both hematopoietic and endothelial precursors in the avian allantoic bud. They removed allantoic bud primordia from quail embryos before the allantois had been vascularized. These rudiments were grafted into the coelom of a chicken host. Quail hematopoietic and endothelial cells later were found in the bone marrow of the host. Because the graft was located at a distance from the limb bud, these cells could reach the bone marrow only by the circulatory pathway. The most likely explanation is that the allantois was providing hemangioblasts to the host embryo.
The allantois of mice may also contain hemangioblasts. Downs and Harmann (1977) marked the allantoises of 8 day (headfold stage) mouse embryos with a lacZ (β-galactosidase) transgene and transplanted the lacZ-expressing headfold-stage allantoises into different sites in 8-day non-transgenic host embryos. After 23 hours in culture, operated embryos were examined histologically for contribution of donor allantoic cells to the embryo. None of the allantoic regions contributed to paraxial mesoderm when placed into the fetus, but the allantoic cells colonized the endothelium and adjacent mesenchyme of the dorsal aorta.
Literature Cited
Caprioli, A., Jaffredo, T., Gautier, R., Dubourg, C., and Dieterlen-Lievre, F. 1998. Blood-borne seeding by hematopoietic and endothelial precursors from the allantois. Proc Natl Acad Sci USA 95: 1641-1646.
Couly, G., Coltey, P., Eichmann, A. and Le Douarin, N. M. 1995. The angiogenic potentials of the cephalic mesoderm and the origin of brain and head blood vessels. Mech. Dev. 53: 97-112.
Downs, K. M. and Harmann, C. 1997. Developmental potency of the murine allantois. Development 124: 2769- 2780.
Pardanaud, L., Luon, D., Prigent, M., Bourcheix, L.-M., Catala, M., and Dieterlen-Lievre, F. 1996. Two distinct endothelial lineages in ontogeny, one of them related to hemopoiesis. Development 122: 1363-1371.