Positional variations in germinal cell growth in pigment-chimeric eyes of Xenopus: posterior half of the developing eye studied in genetic chimerae and in computer simulations.

Growth of germinal cells at different angular positions within the posterior portion of the embryonic frog eye has been examined by orthotopically transplanting small groups of germinal cells from pigmented (stage 30-38) donor embryos into albino (stage 28-36) hosts and then serially photographing the polyclonal-cell progeny domain (typically a black sector) in the pigmented retinal epithelium of the living, growing eye. Far-ventral (6 o'clock) germinal cells formed a narrow sector along the ventral fissure, but ventral germinal cells at a position just posterior to the fissure (7 o'clock on a right eye) were seen to expand rapidly their angular territory on the germinal zone and formed huge sectors that widened toward the front of the older larval eye. Posterior (8, 9, and 10 o'clock) germinal cells were seen to shift their angular positions gradually toward dorsal and formed sectors that appeared to veer dorsalward nearing the front of the older eye. Dorsal (11 o'clock) germinal cells showed attenuative growth, forming sectors that narrowed approaching the front of the older eye. A simulation model of the growth dynamic was used to examine how expansive growth ventrally drives the positional variations in growth. When far-ventral germinal cells were programmed to retain the 6 o'clock position and ventral (7 o'clock) germinal cells were programmed to divide symmetrically at a high probability to produce two daughter germinal cells, not only were the observed ventral chimeric patterns simulated, but also simulated were the attenuative growth of dorsal transplants and the dorsal displacement and veering seen in the growth of posterior transplants.