Cellular interactions and pattern formation in the visual system of the branchiopod crustacean, Daphnia magna. III. The relationship between cell birthdates and cell fates in the optic lamina.

The birthdates of nerve cells in the optic ganglion of the water flea Daphnia magna were determined using [3H]thymidine autoradiography. The analysis was aided by computer-assisted three-dimensional reconstruction and correlated with serial electron microscopy of the developing visual system. Previous work has shown that the projection from eye to optic ganglion is retinotopic at the level of single cells ( Macagno , E.R., V. LoPresti , and C. Levinthal (1973) Proc. Natl. Acad. Sci. U.S.A. 70: 433-437). Studies of both normal and perturbed development ( LoPresti , V.,E.R. Macagno , and C. Levinthal (1973) Proc. Natl. Acad. Sci. U.S.A. 70: 56-61; Macagno , E.R. (1978) Nature 275: 318-320) have suggested that retinotopy is a consequence of an invariant pattern of sequential growth and cellular interactions. We report here that the laminar cells, the first-order target cells of the visual projection, become postmitotic in an orderly fashion that correlates with their position and connectivity in the adult array. Furthermore, presumptive laminar cells complete their final division just prior to or during initial contact with the growing processes of the visual fibers with which they will form their mature retinotopic contacts. The correlation between times of final division of target cells and the arrival of visual fibers with which they interact provides further indirect evidence that sequential growth and cellular interactions are key factors in establishing normal connectivity in the visual system of arthropods.