Fate-mapping experiments in the mouse have revealed that the primitive streak can be divided into three functional regions: the proximal region gives rise to germ cells and the extra-embryonic mesoderm of the yolk sac; the distal region generates cardiac mesoderm and node-derived axial mesendoderm; and the middle streak region produces the paraxial, intermediate and lateral plate mesoderm of the trunk. To gain insight into the mechanisms that mediate the assembly of the primitive streak into these functional regions, we have cloned and functionally identified the gene disrupted in the amnionless (amn) mouse, which has a recessive, embryonic lethal mutation that interferes specifically with the formation and/or specification of the middle primitive streak region during gastrulation. Here we report that the gene Amn encodes a novel type I transmembrane protein that is expressed exclusively in the extra-embryonic visceral endoderm layer during gastrulation. The extracellular region of the Amn protein contains a cysteine-rich domain with similarity to bone morphogenetic protein (BMP)-binding cysteine-rich domains in chordin, its Drosophila melanogaster homolog (Short gastrulation) and procollagen IIA (ref. 3). Our findings indicate that Amn may direct the production of trunk mesoderm derived from the middle streak by acting in the underlying visceral endoderm to modulate a BMP signaling pathway.

The primitive streak is the defining feature of the gastrulating mouse embryo. Currently, little is known in the mouse about the mechanisms that mediate the assembly of the primitive streak or about the signaling pathways that specify the different types of mesoderm and endoderm generated from the streak. To gain insight into primitive streak assembly and function, we have conducted a detailed phenotypic characterization of amnionless, a transgene-induced insertional mouse mutation that arrests embryonic development during 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. Significantly, 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 left and right arrow amnionless-/- blastocyst chimeras indicate that this proposed separate pathway for middle streak formation is dependent on amnionless gene functions in the visceral endoderm.

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