Determinants of cell fate during zebrafish embryogenesis

The mechanisms underlying how the cells within a vertebrate embryo acquire specific fates are not well understood. From previous studies on zebrafish, two models emerged that describe the cell fate potentials of early cells (blastomeres). In one model, the tissues that will be populated by the descendants of any blastomere cannot be predicted. In the second model, tissue fates are strictly and reproducibly segregated into specific blastomeres by the initial cleavages. The work presented here supports some aspects of each of the above models. I show that in zebrafish, blastomeres do not have predictable prospective tissue fates, and there is no correlation between the position of a blastomere and the specific set of tissues derived from it. However, the position of a blastomere influences two aspects of the cell fates adopted by its descendants. Specifically, the position of a blastomere affects which germ layer the majority of the descendants populate, and affects the number of different tissues the descendants tend to populate. The inability to predict the specific tissues derived from any blastomere is consistent with some aspects of the first model, whereas the tendency for some blastomeres to populate different subsets of tissues is consistent with predictions from the second model. Based on these results, I suggest that if maternally supplied determinants are used to initiate pattern formation in the embryo, there are specific constraints as to where the determinants might be localized. If maternal determinants establish embryonic patterning they must be either localized within the cells that give rise to the marginal region of the gastrula, or sequestered within the yolk cell. In order to determine when and where zebrafish express a putative localized cell fate determinant, I examined the expression of the zebrafish homolog of Xenopus Vg1. Although Vg1 RNA is restricted to vegetal blastomeres in Xenopus, I found that Vg1 RNA is present throughout all blastomeres in zebrafish. This finding is consistent with the idea that although zebrafish blastomeres show tendencies to populate specific subsets of tissues, all blastomeres have equivalent cell fate potentials