A role for the extraembryonic yolk syncytial layer in patterning the zebrafish embryo suggested by properties of the hex gene

Recent studies in mouse suggest that the extraembryonic endoderm has an imp ortant role in early embryonic patterning [1]. To analyze whether similar m echanisms operate in other vertebrates, we cloned the zebrafish homologue o f Hex, a homeobox gene that is expressed asymmetrically in the mouse viscer al endoderm [2]. Early expression of zebrafish hex is restricted to the dor sal portion of the yolk syncytial layer (YSL), an extraembryonic tissue. By the onset of gastrulation, hex is expressed in the entire dorsal half of t he YSL, which directly underlies the cells fated to form the neural plate. We show that hex expression is initially regulated by the maternal Wnt path way and later by a Bmp mediated pathway. Overexpression experiments of wild -type and chimeric Hex constructs indicate that Hex functions as a transcri ptional repressor and its overexpression led to the downregulation of bmp2b and wnt8 expression and the expansion of chordin expression. These finding s provide further evidence that the zebrafish YSL is the functional equival ent of the mouse visceral endoderm and that extraembryonic structures may r egulate early embryonic patterning in many vertebrates.