Growth and development of the mouse retinal pigment epithelium: II. Cell patterning in experimental chimaeras and mosaics

The retinal pigment epithelium (PE), with pigmentation as a cell-autonomous marker, was analyzed in three types of mice: (1) congenic pigmented ↔ albino chimaeras, (2) X-inactivation mosaics (Cattanach's translocation), and (3) mosaics homozygous for the pink-eyed unstable mutation, which contain rare fully pigmented cells. In 10 chimaeric and 34 X-inactivation eyes, the proportionate mix in the right and left eyes of an individual animal was similar, the mix was approximately constant in all parts of a given eye, average patch size was larger toward the periphery of the PE, and peripheral patches tended to be elongated in the radial dimension. In all 44 whole mounts from pink-eyed unstable mutants, patches of 1–12 pigmented cells, each representing a single clone, were scattered throughout the PE; they tended to be larger with increasing distance from the optic nerve head. The collective data are consistent with significant cell mixing prior to specification of the two eye fields, during early organ-forming stages, and during later development of the PE. The tendency of peripheral patches to orient radially reflects the edge-biased pattern of cell proliferation in the PE. Cell mixing appears to be more prominent posteriorly in the PE sheet; growth proceeds anteriorly for more generations.