Cell patterning in pigment-chimeric eyes of Xenopus: local cues control the decision to become germinal cells

Between 2.5 and 4 days of development, cell proliferation in the Xenopus eye becomes confined to a narrow ring of germinal cells at the front rim of the eye cup. Continued growth of the eye (which lasts until well beyond metamorphosis) is by the continued proliferation of cells in this germinal zone. To determine what factor(s) promotes cell division in this region of the eye long after it ceases at the back of the eye (near the optic nerve), we have transplanted small groups of eye cells from pigmented donor embryos into the eyes of albino hosts, transposing cells from the mitotically quiescent back of the eye to the germinal zone and vice versa. Regardless of their position of origin in the donor eye, only implants into the host germinal zone behaved like germinal cells--as assayed in the living growing eye by the addition of black tissue to the pigment retinal epithelium. Conversely when donor germinal cells were implanted into the back of the host eye, they ceased dividing once they became integrated into the eye and remained as a tiny black spot on the back of the host eye. This suggests that local environmental cues, rather than intrinsic cellular determinants, specify the fates of eye cells ensuring that cells on the eye rim will continue to function as germinal cells while others will withdraw from the cell cycle.