Mutations in human SOX9 are associated with campomelic dysplasia (CD), characterised by skeletal malformation and XY sex reversal. During chondrogenesis in the mouse, Sox9 is co-expressed with Col2a1, the gene encoding type-II collagen, the major cartilage matrix protein. Col2a1 is therefore a candidate regulatory target of SOX9. Regulatory sequences required for chondrocyte-specific expression of the type-II collagen gene have been localized to conserved sequences in the first intron in rats, mice and humans. We show here that SOX9 protein binds specifically to sequences in the first intron of human COL2A1. Mutation of these sequences abolishes SOX9 binding and chondrocyte-specific expression of a COL2A1-driven reporter gene (COL2A1-lacZ) in transgenic mice. Furthermore, ectopic expression of Sox9 trans-activates both a COL2A1-driven reporter gene and the endogenous Col2a1 gene in transgenic mice. These results demonstrate that COL2A1 expression is directly regulated by SOX9 protein in vivo and implicate abnormal regulation of COL2A1 during, chondrogenesis as a cause of the skeletal abnormalities associated with campomelic dysplasia.

During early mouse embryogenesis, temporal and spatial regulation of gene expression and cell signalling influences lineage specification, embryonic polarity, the patterning of tissue progenitors and the morphogenetic movement of cells and tissues. Uniquely in mammals, the extraembryonic tissues are the source of signals for lineage specification and tissue patterning. Here we discuss recent discoveries about the lead up to gastrulation, including early manifestations of asymmetry, coordination of cell and tissue movements and the interactions of transcription factors and signalling activity for lineage allocation and germ-layer specification.

The investigation into lineage allocation and early asymmetries in the pre-and peri-implantation mouse embryo is gaining momentum. As we review here, new insights have been gained into the cellular and molecular events that lead to the establishment of the three lineages of the blastocyst, to the determination of the origin and the fates of the visceral endoderm in the peri-implantation mouse embryo, and to the generation of cellular and molecular activities that accompany the emergence of asymmetries in the pre-gastrulation embryo. We also discuss the continuing debate that surrounds the relative impacts of early lineage bias versus the stochastic allocation of cells with respect to the events that pattern the blastocyst and initiate its later asymmetries. IntroductionThe progression of the mammalian embryo from fertilization to gastrulation involves an ordered series of lineage specifications and axial asymmetries (Fig. 1) that result, first, in the development of the blastocyst (see Glossary, Box 1), with its embryonic-abembryonic axis ( Fig. 1; Box 2), and, later, in the formation of the embryo itself, with its anterior-posterior (AP), dorsal-ventral (DV) and left-right (LR) axes. In many invertebrates and vertebrates, asymmetries that are established in the egg correlate with the segregation of determinants that influence later lineage formation and axis development. However, in the mouse egg, the morphological asymmetries that exist, such as the position of the second polar body (see Glossary, Box 1) and the sperm entry point, do not clearly demarcate an asymmetric domain of, for example, signaling activity or of cell fate determinants in the fertilized mouse egg. Whether there is any instructive relationship between the asymmetry of the egg and the later asymmetry of the blastocyst and lineage allocation remains a controversial issue. It is well known that the pre-implantation mammalian embryo is highly regulative and resistant to the loss or addition of cells brought about by experimental manipulations. However, this does not preclude the existence of an, as yet, uncharacterized property that could bias developmental outcomes in the intact embryo.Whether any early asymmetries in the mouse egg and/or blastocyst relate to the orientation of the definitive body axes is even less certain. It is now clear that the AP patterning of the gastrulating embryo is initiated prior to gastrulation by spatially localized signals that emanate from regionally patterned extra-embryonic tissues. Some of these asymmetries may be set up as early as the blastocyst stage, linking pre-implantation patterning to post-implantation morphogenesis.Here, we review recent experiments that define the molecular components of lineage specification in the mouse blastocyst. We also review the ongoing uncertainty and debate that surrounds the relative importance of early cleavage patterns at the two-to four-cell stage and of symmetric versus asymmetric divisions at the eight-to 16-and 16-to 32-cell stage, and the importance of final cell po...