A question of fate

clusters of coloured balls are three-dimensional models of the developing embryo of the nematode worm, Caenorhabditis elegans. The embryos develop according to a standard pattern, the cell lineage of which was originally worked out by directly observing cells in thousands of embryos under the microscope. But with the advent of 'four-dimensional microscopy', the entire development of one embryo can now be recorded in three-dimensional time-lapse video, down to the single-cell level. This information has been used to create a database of cell positions, cell cleavages and final cell fates, and to generate three-dimensional models like the ones above, showing where any particular cell came from and what it will become. The smaller images above are models of (anticlockwise from the top left) a wild-type four-cell-stage embryo, a 12-cell-stage, a 51-cell-stage, a 185-cell-stage and a premorphogenetic stage embryo with 385 cells. The nuclei are colour-coded according to their lineage, making it clear how cells of a particular lineage spread out to form defined clumps at the premorphogenetic stage. In the large picture, the progress of a particular lineage in both time and space — from the blastomere called ABp at the four-cell stage (orange ball near the centre) via the precursors to the final hypodermal cell, TL, (orange ball on the right) — is projected onto the premorphogenetic stage. Using this technique it has been shown that normal development of the C. elegans embryo is much more variable than originally anticipated. The technique has also been used to compare the development of normal and mutant embryos. For more information, see Schnabel R. A question of fate