Fig. 7. A model of BMP requirements of embryonic and adult blood compartments. (A) The three blood compartments are specified at different BMP levels. The anterior VBI (aVBI) precursors start out at zero or very low BMP, being derived from C1 and D1 blastomeres in the DMZ, which are fated to become the Spemann organiser, a region high in BMP antagonists such as noggin, chordin and follistatin. The DLP compartment, which is derived from the VLMZ, is specified at intermediate levels of BMP, while the posterior VBI (pVBI) is specified at high BMP levels, being derived from the VMZ on the opposite side of the embryo from the organiser. (B) Even the Spemann organiser-derived aVBI population, however, require BMP at some point in their development. Our model suggests that the aVBI precursors (blue cells, stage 10) that represent leading edge dorsal mesoderm rapidly escape the organiser region and migrate towards the animal pole where fate mapping (Bauer et al., 1994; Vodicka and Gerhart, 1995) shows that, from approx. stage 11 onwards, they come into contact with the ectoderm of the animal cap, which is rich in BMP (dots, stage 10 and 12). By stage 14, blood precursors in the dorsal leading edge mesoderm have migrated all the way to a ventral position and have become specified as blood and endothelial progenitors.