Light and electron microscope studies of cell sorting in combinations of chick embryo neural retina and retinal pigment epithelium.

A recent hypothesis, proposed by A. S. G. Curtis, asserts that the formation of homogeneous tissues within heterotypic aggregates occurs during the initial phases of cellular aggregation rather than by a process of cell sorting in mixed aggregates. The present studies of the early stages of aggregation of mixed suspensions of dissociated chick embryo pigmented retinal epithelial and neural retinal cells do not support Curtis' hypothesis. The early aggregates formed from these mixed cell suspensions are disordered mixtures of the two cell types. Establishment of homogeneous neural retinal and pigmented retinal epithelial tissues occurs much later in the aggregates. The hypothesis of Moscona that the process of formation of type-specific tissues in mixed aggregates requires a specificity of cell adhesion was examined by attempting to demonstrate cell contact specificity at the electron microscope level in heterotypic aggregates undergoing cell sorting. Specificity of cell contact was not observed. Instead, fine structural studies of cell contact interactions demonstrate that pigmented retinal epithelial cells have broad areas of cell contact and specialized contact junctions with neural retinal cells within the aggregates.