Cell lineage analysis reveals multipotent precursors in the ciliary margin of the frog retina.

The eyes of lower vertebrates grow throughout life by the proliferation of cells in the ciliary margin. To determine the range of cell types that descend from single cells of the ciliary margin in Xenopus laevis embryos, a cell lineage study was performed. Precursor cells in the periphery of the eyes were labeled by intracellular injection of rhodamine dextran, and the proliferation of these cells resulted in clones of labeled descendants. The number of cells per clone was quite variable (range, 1 to 104 cells). This broad range of sizes (compared to clones derived from optic vesicle cells) and their distribution suggest that there may be two types of ciliary margin cells with different proliferative fates: self-renewing stem cells and cells that undergo a limited number of divisions. Labeled descendants in the neural retina differentiated into all of the major cell types, including glia. One-quarter of the clones had labeled descendants in both the neural retina and the pigmented epithelium. These observations suggest that ciliary margin cells are multipotent for all neural, glial, and pigmented epithelial cell types of the eye. Thus, the molecular events that regulate the commitment of specific cell types must occur late in the cell lineages of the ciliary margin.