Biology Reference
In-Depth Information
FIGURE 7.2
Formation of the grex of D. discoideum by chemotaxis-driven aggregation of myxamoebal cells.
embryos of the frog Xenopus laevis, precursor cells of the vascular system arise in the lateral
mesoderm, 200
m
ref4
from where they will eventually be needed (
Figure 7.3
). They
migrate from these sites towards the axis of the embryo, specifically towards the hypochord
area, which lies just ventral to the notochord. Once they have converged on the midline, the
cells differentiate into endothelium and adhere to each other to form the dorsal aorta, the
vessel that carries high-pressure blood from the heart to the trunk of the body. The hypo-
chord expresses a diffusible growth factor (VEGF) and the precursor cells express a receptor
for it (flk-1), suggesting that the cells navigate up the concentration gradient of VEGF. This
hypothesis is strengthened by the observations that if an ectopic source of VEGF is applied
to an X. laevis embryo, the vessel precursor cells migrate towards this as well,
4
and that dorsal
aorta development fails in vegf
/
mice.
5
The same general pool of vascular precursors in
300
e
m
FIGURE 7.3
Morphogenesis of the dorsal aorta of Xenopus laevis by coalescence of migrating angioblasts, the
precursors of blood vessels, which arise in the lateral mesoderm.
