The amn gene product is required in extraembryonic tissues for the generation of middle primitive streak derivatives

The primitive streak is the defining feature of the gastrulating mouse embr yo. Currently, little is known in the mouse about the mechanisms that media te the assembly of the primitive streak or about the signaling pathways tha t specify the different types of mesoderm and endoderm generated from the s treak. To gain insight into primitive streak assembly and function, we have conducted a detailed phenotypic characterization of amnionless, a transgen e-induced insertional mouse mutation that arrests embryonic development dur ing gastrulation. Our histological and molecular analyses, when examined in the context of the mouse gastrula fate map, lead to the model that middle streak formation is specifically impaired in the amnionless mutant. Signifi cantly, these observations argue that the formation of the middle streak is mediated by a pathway that is genetically separable from those that direct the specification of the distal and proximal streak regions. Intriguingly, our findings from wt ES cell <-> amnionless(-/-) blastocyst chimeras indic ate that this proposed separate pathway for middle streak formation is depe ndent on amnionless gene functions in the visceral endoderm. (C) 1998 Acade mic Press.