Patterns of cell proliferation in the retina of the clawed frog during development.

Quantitative assays of the spatial pattern of cell production in the developing retina of Xenopus have been made using 3H-thymidine labelling and colcemid blockade of mitosis. Reconstructions were made from serial sections showing the position of every mitotic figure in the retina. After stage 54 the number of mitotic figures decreases at the dorsal margin of the retina and increases at the ventral margin. The ventral:dorsal ratio of mitoses reaches 10:1 by metamorphosis. Density of mitotic figures is maximum at the point of entry of the ophthalmic vessels at the ventral margin. In spite of asymmetrical production of retinal cells the cell density remains constant throughout the retina, probably as a result of displacement of retinal cells dorsally to compensate for the relatively greater proliferation ventrally. It is also proposed that the asymmetrical retinal growth serves to maintain the relationship between each point in visual space and corresponding points in the two retinae as the eyes are displaced dorsally on the head during metamorphosis.