Quantitative lineage analysis of the frog's nervous system. I. Lineages of Rohon-Beard neurons and primary motoneurons.

After injection of horseradish peroxidase into single blastomeres in Xenopus embryos at 2- to 512-cell stages, all of the descendants could be traced and counted at tailbud stages. Progenitors of Rohon-Beard neurons and of primary spinal motoneurons were identified, and all neurons of each type that originated from individual progenitors were counted. From these data were derived the geometric mean of the number (N) of Rohon-Beard neurons (or primary motoneurons) that descended from a single progenitor at each generation from the first to ninth, the mean number of progenitors of each generation, and the probability that, following mitosis, daughter cells continue in the Rohon-Beard (or primary motoneuron) lineages (the continuation probability). On a log-log plot a straight line fitted to the N values intercepts the abscissa at the 13th generation for Rohon-Beard progenitors and the 16th generation for primary motoneuron progenitors. This indicates the number of generations before the entire set of each type of neuron is finally produced. The Rohon-Beard neurons and primary motoneurons each originated from a separate group of progenitors at the 9th generation (512-cell stage), but those progenitors gave rise to mesodermal, endodermal, and ectodermal cells in addition to neurons. Nevertheless, the continuation probability significantly greater than 0.5 shows that there is a bias toward recruitment of progenitors into lineages leading to production of those two types of neurons.(ABSTRACT TRUNCATED AT 250 WORDS)