Establishment of polarity in the egg can be described as the earliest step in embryonic patterning. A single cell, the fertilized egg, contains the information that will ultimately specify the entire organism. As the egg divides during the early stages of embryonic development, different cells acquire different fates. One mechanism to account for this difference in developmental potential among the cleavage cells is the sequestering of molecules in the egg that could specify such potential. It is a long-standing proposal that localization of such determinants prior to fertilization can provide the basis for pattern formation in the early embryo. While there is evidence for the existence of localized cytoplasmic determinants in many systems, the identities of such determinants remain largely unknown in vertebrates, and the mechanisms for establishment of polarity in the oocyte are poorly understood.
In eggs of the frog, Xenopus laevis, cell polarity is readily apparent along the animal/vegetal (AV) axis, which represents an axis of developmental potential whereby specific cell fates are assigned with respect to the AV axis. What is not known are the identities of all the presumptive maternal determinants, and how they come to be properly localized in the oocyte. Such determinants can be localized as mRNA or protein; localized mRNAs are well documented in the Xenopus oocyte, and some are thought to play roles in axial patterning. While it is apparent that not all of the localized maternal components are RNAs, little progress has been made on identifying localized maternal protein determinants.
The goal of the proposed research is to identify and characterize localized maternal protein determinants in the Xenopus egg. The foundation for this work has been laid by Dr. Mowry's success in devising a subtractive immunization scheme to obtain monoclonal antibodies specific for proteins that are localized in the frog egg. Focusing their efforts on two of these vegetally localized proteins, three specific objectives are proposed: I. They will characterize the localized proteins at both the molecular and cytological levels. II. They will probe functions of the localized proteins in patterning and polarity. III. They will investigate the mechanism by which these proteins are restricted to the vegetal cortex.
These experiments are designed to investigate how developmental signals are spatially distributed to underlie patterning in the vertebrate embryo, and may also provide insight into how into how polarity is established in the oocyte.
