Early neurogenesis in Xenopus: the spatio-temporal pattern of proliferation and cell lineages in the embryonic spinal cord.

Lineage tracing techniques and pulse labeling experiments were used to reveal the lineages and the pattern of proliferation of neural precursors in the Xenopus neural plate. After gastrulation there is a wave of mitosis; most cells of the neural plate undergo a single division during this wave. After this first division, many cells leave the cell cycle and differentiate as primary neurons. In the stage 35/36 hatching larvae, clones of primary neurons usually contain only two cells. The remainder, most of which arise from the superficial layer, are predominantly the precursors of secondary neurons. They are mitotically quiescent until stage 20, and then undergo another one to two rounds of division during embryonic life. Secondary precursors and primary neurons are never part of the same clone, although, in individual clones, primary neurons are frequently of different types. By the neural plate stage, separate precursors seem to exist for primary and secondary neurons, but the precursors of primary neurons themselves are pluripotent.