No abstract

Dopamine (DA) is an important neuromodulator in the visual system. The release of DA in the retina largely depends on environmental lighting conditions. Most previous studies have assessed the effect of illumination on retinal DA or its metabolites using homogenates or in vitro preparations. This study was designed to investigate the effect of transitions between lighting conditions--from dark to steady or flickering light and vice versa--on retinal DA release in zebrafish using in vivo microdialysis. The transition from dark to flickering light increased DA release, whereas the transition from flickering light to dark decreased it. This latter effect depended on time of day within the light period, e.g., it was strongest in the late afternoon. When using steady light, none of these effects were seen. Our study also demonstrates that in vivo microdialysis can successfully be applied to the investigation of retinal DA release in zebrafish.

We have examined the process by which small groups of pigmented germinal cells transplanted orthotopically from stage 30-38 donor embryos into stage 28-38 albino hosts contribute new postmitotic cells to the pigmented retinal epithelium of the growing larval eye in Xenopus. In the great majority of chimeric eyes, the transplant healed to occupy a small arc-territory at the intended dorsal or anterior position on the host germinal zone. Over the course of subsequent weeks, the transplanted germinal cells added new mitotically quiescent cells to the distal rim of the pigmented retinal epithelium and so gave rise to an elongating black sector on the growing larval eye. Cellular details at the boundaries of the graft-derived sector were stable over time; the accumulation of such landmarks provided a summary record-in the proximodistal axis of the older eye-of the growth history of the transplant. Case-to-case variation among both groups of transplants suggested a measure of indeterminancy in the details of germinal cell growth.With a few notable exceptions (1)(2)(3)(4), the germinal cells of developing vertebrate organs remain a fascinating but largely unstudied cellular specialization. The developing eye in the clawed frog Xenopus has a discrete germinal zone (GZ) with certain unique advantages for transplantation studies using heritable cell markers-a strategy of proven value in a wide range of developing systems (5-8). A thin ring of pigmented and underlying neuroepithelial germinal cells on the front rim of the optic cup perpetuates itself while adding annuli of new postmitotic cells to the pigmented retinal epithelium (PRE) and neural retina (9)(10)(11)(12)(13)(14). This annular growth pattern, as revealed by 3H-labeled thymidine autoradiography in the neural retina (10, 11) and by preliminary observations on the growth of pigment-chimeric eyes (13,15,16), yields an orderly spatiotemporal pattern of cell birthdates along the proximodistal axis of the older eye ( Fig. 1 a and b).Nevertheless, in the frog eye (as in most vertebrate solid organs), very little is known about the origin and detailed fates of germinal cells, including (i) the mechanics of cell division by which they contribute new postmitotic cells to the PRE, (it) the extent to which their mitotic patterns and near-neighbor relationships may change as development proceeds, and (iii) the reproducibility oftheir growth patterns from embryo to embryo. We address these issues in the present study, in which we have followed the growth of small groups of pigmented germinal cells, transplanted orthotopically from pigmented stage 30-38 donor eyes into the GZ of stage 28-38 albino host eyes. (XBL) of these same XB female breeders crossed with albino XL males used in the host spawnings (22). All phenomena described in this paper (including Fig. 6) were observed in transplants of all four genetic backgrounds; both in or from embryos (hosts or donors) reared from egg-laying only at room temperature (22 ± 2°C) and in embryos prechilled at 12-13°C for...

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.