Genetic control of extraembryonic cell lineages studied with tetraploid<-->diploid chimeric concepti

The first differentiation event during mammalian embryogenesis is the commitment of blastomeres to the trophectoderm cell lineage. Much remains to be learned about the genetic control of this first cell lineage commitment and the subsequent events underlying the differentiation of all extraembryonic cell lineages. Because of the unique features of intrauterine embryonic development, the study of embryogenesis in lower organisms has shed little light on mammalian extraembryonic lineage differentiation. Rather, two major methods in developmental genetics have contributed to our understanding of genetic control of extraembryonic cell lineages. First, abnormalities in extraembryonic tissues have been described in many genetically engineered mutant mouse lines. However, the histological description of these abnormalities does not demonstrate whether the observed defect is the primary cause of embryonic lethality. Second, tetraploid<-->diploid aggregation experiments have been used to generate chimeric concepti with distinct genotypes in the extraembryonic tissues and the embryo proper. This experimental approach has provided the definitive demonstration of the crucial role of several transcription factors, growth factors and cytoskeleton proteins in extraembryonic tissue formation. The present review summarizes the origin of tetraploid<-->diploid aggregation experiments and it usefulness for the study the genetic control of extraembryonic cell lineages.