Single cell analysis of commitment in early embryogenesis.

Fate maps of amphibian embryos tell us the destination of certain areas at later stages of development. After studying Vogt's fate maps, Spemann wrote (in 1938) that "the question which at once calls for an answer is whether this pattern of presumptive primordia in the beginning gastrula is the expression of a real difference of these parts, whether they are already more or less predestined or 'determined' for their ultimate fate, or whether they are still indifferent and will not receive their determination until a later time." Until recently answers to this question have relied upon explant experiments, which indicate that by the late blastula stage ectoderm, mesoderm and endoderm regions are distinctly determined. By using a method involving single cell labelling and transplantation, it is now possible to pinpoint more accurately the time during early embryogenesis at which individual blastomeres become committed. In the vegetal pole, determination towards endoderm is a gradual process beginning during the middle blastula stage (stage 8) and completed by the beginning of gastulation (stage 10). This method offers the possibilities of comparing the committed and the uncommitted state and studying at a molecular level the mechanisms of cell determination.