Previous studies have shown that anterior lateral plate endoderm from stage 6 chicken embryos is necessary and sufficient to enable precardiac mesoderm to complete its cardiogenic program in vitro, culminating in a rhythmically contractile multicellular vesicle (Sugi and Lough [1994] Dev. Dyn. 200:155-162). To identify cardiogenic factors, we have begun to characterize proteins that are secreted by endoderm cell explants. Fluorography of proteins from endoderm-conditioned medium revealed 1-2 dozen bands, the most prominent of which migrated at approximately 17 and 25 kD. The bulk of the 17-kD band, which migrates near FGFs and subunits of the transforming growth factor-β€ family, was identified by N-terminal sequencing as transthyretin (TTR). A component of the 25-kD band was identified by Western blotting as retinol binding protein (RBP). RT/PCR analysis revealed that mRNAs for both proteins are in the embryo as early as stage 3. In situ hybridization localized these mRNAs to the extraembryonic endoderm at stage 6, after which they were detected in endoderm overlying the embryo proper, including the developing heart. Later, RBP and TTR mRNA and protein were detected in cells associated with the developing heart. Western blotting of whole embryo proteins revealed the presence of RBP by stage 7, followed by sequential increases to stage 25; by contrast, content of RBP in isolated hearts peaked at stage 14, then declined. Immunohistochemistry revealed the presence of RBP protein in the extracellular matrix subjacent to lateral plate endoderm beginning at stage 8; upon formation of the definitive heart, intense staining was observed in the cardiac ''jelly.'' By contrast TTR was intracellular, first detected as subtle deposits in stage 6 embryonic endoderm, which by stage 8 were prominent in the dorsally invaginated endoderm subjacent to the precardiac splanchnic mesoderm. At stages 11-14, TTR was detected only in myocardial cells. Such localization of RBP and TTR may indicate a role in the transport and distribution of retinol and thyroid hormone, respectively, from yolk to embryo prior to establishment of the circulatory system, and is suggestive of a subsequent role in heart development.