3 TYPES OF SEROTONIN-CONTAINING AMACRINE CELLS IN TADPOLE RETINA HAVEDISTINCT CLONAL ORIGINS

In the Xenopus tadpole there are three different serotonin-containing amacrine cells: large, brightly fluorescent (LB), and small, dimly flu orescent (SD) cells in the inner nuclear layer and displaced (DIS) cel ls in the ganglion cell layer. To reveal the potential roles of region al cues and lineage factors in their determination, quantitative maps were made of the spatial distribution and blastomere origin of each ce ll type. LB and SD cells were evenly distributed across the four retin al quadrants, arguing against a hypothesis that these cells are induce d differentially by quadrant-specific cues. Blastomere progenitors of the 32-cell embryo are biased to produce only subsets of serotonin ama crine cells: 1) all nine progenitors of one retina produced some SD ce lls, but only eight produced LB, and only five produced DIS cells; and 2) there are overlapping but distinct subsets of blastomere progenito rs for each serotonin subtype. This bias is not simply a reflection of the size of a clone in the retina; significant quantitative differenc es were observed between the proportion of serotonin progeny and the p roportion of the entire retina produced by six of the nine retinal pro genitors. This bias also is not simply a reflection of the spatial dis tribution of a blastomere clone in the retina; the number of LB descen dants in each retinal quadrant was statistically different from its pr ogenitor's total contribution to the quadrant. These results indicate that the development of the three different serotonin-containing amacr ine cells in the retina is biased by membership in specific blastomere clones. (C) 1997 Wiley-Liss, Inc.