Cell Lineage Analysis in Xenopus Embryos

Cell lineage studies reveal what kinds of tissues descend from a single cell or specific region of an embryo. By defining precisely from which cells the various tissues and organs arise one can elucidate the mechanisms that control body organization, understand morphogenetic movements, and test the influence of exogenously applied gene products on these events. Because of easy accessibility, complete fate maps of the early cleavage stages of Xenopus have been published (1 –4 ). However, fate maps only describe the developmental path taken by a cell under normal, intact embryo conditions. Such studies cannot describe the full developmental potential of a cell or the times or mechanisms by which its fate is determined. The fate expressed by a cell, that is, the different tissue types that descend from it, is usually influenced by a number of factors, which may include maternal determinant molecules, cell-cell interactions, growth factor signals, and position within a morphogen gradient. Cell lineage tracing in Xenopus , therefore, is an essential technique to test the fate of a cell as it develops under novel experimental conditions (5 ). Lineage tracing also is an important tool for labeling host tissues for use in tissue recombinant experiments. The ability to recognize the origin of embryonic tissues was critical for interpreting the pioneering experiments of embryonic inductions (6 ,7 ). These early studies used pigmentation differences between donor and host species, but modern lineage labeling is more long lasting and reliable and allows tissues to be recombined within the same species.