Secretion of inhibin beta A by endoderm cultured from early embryonic chicken.

Although several reports have indicated a role for endoderm in the regulation of heart development, the mechanism remains unknown. To begin characterization of endoderm-secreted proteins, explants from postgastrulation (Hamburger-Hamilton stage 5/6-8) chicken embryos were cultured in defined medium. Fluorography of SDS-PAGE gels revealed a pattern of synthesized, secreted proteins that was independent of time in culture or embryonic stage when explants were removed. Approximately 10 labeled bands were detected, the most prominent of which migrated at 17, 25, and 200 kDa. ELISA analysis revealed that while acidic and basic fibroblast growth factor-like antigens were barely detectable, fibronectin and inhibin beta A were very reactive. Western blot analysis verified the presence of fibronectin and, most remarkably, inhibin beta A, activin dimers of which have recently been implicated in Xenopus mesoderm induction (Smith, Price, Van Nimmen, and Huylebrock (1990). Nature 345, 729.)