Early cell fate decisions and cell positioning in the mouse embryo. (360G-Wellcome-064421_Z_01_A)

I propose three complementary approaches to study how early events can help to specify polarity of the mouse embryo. I first propose to study developmental cues in the egg whose animal-vegetal polarity dictates one axis of the future blastocyst, and the position of sperm entry determines the second axis. The sperm entry position sets the first cleavage plane and conveys a division advantage upon the cell inheriting it. I will investigate the cytoplasmic events underlying these two consequences of sperm entry and test their relative importance in establishing the embryonic abembryonic polarity of the blastocyst. I also propose to study why, although the animal pole of the egg can be removed without affecting development, its duplication is inhibitory. Secondly I will combine lineage tracing and transplantation studies to ask how polarity of the blastocyst, set up by the above processes, is transformed to give organised signalling centres in the postimplantation embryo. My focus will be to discover the origins of visceral endoderm with potential to signal to the epiblast of the egg cylinder. To determine when such signalling centres become active, I will first concentrate on transplantation experiments that test the ability of anterior visceral endoderm precursors to repress posterior gene activity. The extent of such experiments could be broadened by better knowledge of the patterns of gene expression from the blastocyst onwards. The third part of my proposal aims to identify genes that are expressed asymmetrically along axes of the blastocyst and/or at the earliest times within postimplantation signalling centres or their progenitors. My attention will centre upon finding genes that are differentially expressed in the animal and vegetal halves of the blastocyst or become uniquely expressed in progenitors of anterior visceral endoderm. These will be identified through the construction of subtractive cDNA libraries and by screening microarrays. I will select genes with expression patterns likely to be meaningful in the development of signalling centres and examine the consequences of both their ectopic expression and loss of expression using dsRNAi. In the longer term I propose to analyse how the patterns of expression of these or other genes are rebuilt following perturbation of development. Thus not only do I hope my work wil contribute to a molecular understanding of how asymmetries are established and transmitted to later stages of the embryo in normal development, but also will provide insight into the remarkable regulative properties of the mammalian embryo.