Lineage Tracing and Fate Mapping in Xenopus Embryos.

Fate mapping approaches reveal what types of cells, tissues, and organs are derived from specific embryonic cells. Classical fate maps were made by microscopic techniques using embryos comprising small numbers of transparent cells. More complex and opaque embryos require use of a vital or lipophilic dye that labels small groups of cells. Intracellular injection of a lineage tracer that labels the injected cell and all of its descendants can be used to mark a single cell in Xenopus embryos, whose large cells are easy to microinject and usually cleave in regular patterns. Intracellular lineage tracers must be neutral compounds that do not interact with cellular processes that might change the developmental fate of the injected cell, be small enough to diffuse quickly throughout the cytoplasm before the cell divides so that all descendants are labeled, and be large enough to not diffuse to adjacent cells via gap junctions. They should not be diluted by cell division or intracellular degradation, and should be easily detected by histochemical reactions (enzymes) or direct imaging (fluorescent compounds). Several types of lineage tracers have been used, including small, fluorescently tagged dextrans and mRNAs encoding enzymes or fluorescent proteins, described here. Many lineage tracers can be combined with cell type-specific mRNA and protein expression assays, making lineage tracing a powerful tool for testing the function of genes and cell fate commitment.