Development of the extraembryonic lineages

The first cell types that form in the mammalian embryo, the trophectoderm and the primitive endoderm, give rise to extraembryonic lineages critical for survival of the embryo in the uterine environment. Recent studies have begun to identify transcription factors and cell signalling pathways important to the establishment and maintenance of these lineages. In some instances, the factors involved are specific to extraembryonic cell types, but in other cases, the same molecules are used elsewhere in the development of the embryo itself. Delineation of embryonic versus extraembryonic function can be achieved by careful use of targeted mutagenesis and chimeric analysis.