The Questions of Developmental Biology

In some instances, angiogenesis may be the major way of making blood vessels. In the
forelimb bud, for instance, the capillary network is probably derived by the sprouting of cells
from the aorta (Evans 1909; Feinberg 1991). Within this capillary network, a central artery
(which becomes the subclavian artery) forms as the major feeding vessel. Blood returns to the
body through marginal veins that form from the anterior and posterior capillaries (Figure 15.18).
The organ-forming regions of the body are thought to secrete angiogenesis factors that promote
sprouting by enabling the mitosis and migration of endothelial cells into those areas. VEGF
(mentioned earlier as a vasculogenesis factor) also promotes the migration of endothelial cells
into the organs from preexisting blood vessels on the organs' surfaces.
Several organs make their own angiogenesis
factors. The placenta is one organ whose function depends
on redirecting existing blood vessels into it. When the
placenta is first being formed, it induces angiogenesis by
secreting proliferin (PLF), a factor that resembles growth
hormone. When the placental blood vessels have become
established (after day 12 in the mouse), the placenta
secretes proliferin-related protein (PRP), a peptide that
acts as an inhibitor of angiogenesis (Jackson et al. 1994).
Ovarian follicle cells and placental cells also secrete leptin,
a hormone that is involved in appetite suppression in the
adult. However, it can also act locally to induce
angiogenesis and cause endothelial cells to organize into
tubes (Figure 15.19; Antczak et al. 1997; Sierra-Honigmann
et al. 1998).