Lymphangioblasts in the avian wing bud.

The development of the lymphatics has not yet been studied experimentally. Descriptive studies could not answer the question whether the lymphatics are exclusively derived by sprouts of the early embryonic lymph sacs, or whether lymphangioblasts in the mesenchyme contribute to the lymphatic system. We have studied the development of the lymphatics in quail-chick chimeras. In 6.5-day-old quail embryos, the endothelium of the jugulo-axillary lymph sac can be demonstrated with the QH1 antibody. In contrast to the jugular vein and the aorta, the lymph sac is irregularly shaped and does not possess a media of smooth muscle cells, and, the lymph sac endothelium starts to express the vascular endothelial growth factor receptor-3 (VEGFR-3). Cells of the quail paraxial mesoderm grafted into chick embryos integrate into the endothelium of the jugular lymph sac, strongly indicating the existence of lymphangioblasts. In the wing of 10-day-old quail embryos, VEGFR-3-positive lymphatics are accompanying all major blood vascular routes. On day 3.5 of development, that is about one day before the first occurrence of the jugulo-axillary lymph sac, we grafted distal wing buds of chick embryos homotopically into quail embryos. The chimeric wings were analyzed on day 10. The VEGFR-3 and QH1 double staining revealed that the lymphatics were formed by both chick and quail endothelial cells. This result shows that the lymphatics of the wing do not exclusively develop from sprouts of the lymph sacs, but also by recruitment of local lymphangioblasts.