Hypoblastic tissue and fibroblast growth factor induce blood tissue (haemoglobin) in the early chick embryo

ABSTRACT We have investigated the temporal and the causal basis of blood tissue specification in the chick embryo. Earlier workers have shown that the prospective blood-forming area is specified in a horseshoe-shaped area at the posterior side of the embryo. We found that cultured explants from the posterior marginal zone at stages XI to XIII (consisting of the posterior marginal zone and part of Koller’s sickle) have a high propensity to form haemoglobin (Hb), which could be inhibited at stage XI by adding antibody against basic fibroblast growth factor (bFGF) to the neutral culture medium; this treatment had no effect from stage XII onwards. The same result was found when whole embryos were cultured with an antiserum raised against bFGF, or with heparin. In another series of experiments, we found that cultured pieces from the inner-core of stage XIII epiblasts (with or without hypoblast tissue) were able to form Hb, whereas inner-core pieces from the pre-hypoblast stages, namely stages X and XI, did not form Hb. The capacity to form Hb, however, could be conferred upon the inner-core pieces from stage X epiblasts if bFGF at a concentration of 75-150 ng/ml was added to the culture medium. Furthermore, and most pertinently, the capacity to form Hb could be conferred on stage X inner-core pieces when they were co-cultured with hypoblast from a stage XIII embryo in a sandwich explant. Thus the inductive role of the hypoblast appears to be mediated via bFGF. This conclusion was confirmed by our finding that the antibody to bFGF inhibited the formation of Hb in these hypoblast sandwich explants. We propose that the erythropoietic cell lineage, i.e. blood tissue, is determined in the chick embryo before stage XII by a bFGF-type of inductive signal from the emerging hypoblast adjacent to Koller’s sickle, in the area encompassing the posterior marginal zone. The morphogenetic changes that take place in the epiblast and hypoblast between stages X and XIII are considered in the discussion, especially with respect to how the posterior horseshoe of blood-forming tissue arises in response to the inductive signal.