ASYMMETRICAL BLASTOMERE ORIGIN AND SPATIAL DOMAINS OF DOPAMINE AND NEUROPEPTIDE-Y AMACRINE SUBTYPES IN XENOPUS TADPOLE RETINA

Amacrine cells are located almost exclusively in the inner nuclear lay er (INL) of the retina, but they express a variety of neurotransmitter s. To begin to elucidate the relative roles of the local environment a nd cell lineage in determining the different neurotransmitter subtypes of amacrine cells, we combined lineage tracing and immunocytochemical techniques to map the spatial distribution and clonal origin of dopam ine (DA) and neuropeptide Y (NPY) amacrine cells in Xenopus tadpole re tina. At the earliest period of neurotransmitter expression, both DA a nd NPY amacrine cells were distributed preferentially in center and in termediate annular regions, and in anterior and dorsal quadrants. Most of the DA and NPY cells first emerged as scattered cells and later as clusters (of 2 or more cells) that increased in number and size up to premetamorphic stages. These results suggest that DA and NPY amacrine subtypes may be influenced by environmental cues localized to specifi c regions of the retina. Lineage analysis showed that the percentage o f DA or NPY amacrine cells produced by most blastomere progenitors is significantly different from that predicted by the number of cells in the retina produced by those blastomeres. Only two blastomeres produce d over 90% of the DA amacrine cells and only four produced 97% of the NPY amacrine cells. Some retinal progenitors did not contribute at all to these two amacrine subtypes. There also is a marked asymmetry in t he blastomere origin of DA and NPY amacrine cells. Two retinal progeni tors produced significant numbers of NPY but very few DA amacrine cell s. This analysis provides evidence that blastomere origin restricts th e developmental choices of retinal progenitors. (C) 1995 Wiley-Liss, I nc