Cellular interactions and pattern formation in the development of the visual system of Daphnia magna (crustacea, branchiopoda) : I. Interactions between embryonic retinular fibers and laminar neurons

Abstract

Retinular cells in the Daphnia compound eye were deleted at specified developmental stages and in adults to test the hypothesis that growing retinular fibers trigger the differentiation of laminar neurons, thereby recruiting them as synaptic targets and organizing them into a particular structural pattern. Deletion of retinular neurons was accomplished by producing small lesions with either ultraviolet or laser microbeams. The effects of the lesions on the eye and optic ganglion were assayed quantitatively using computerized techniques for the three-dimensional reconstruction of biological structures from serial light or electron micrographs. The following results were obtained: (1) In animals with embryonic retinular lesions, the number of laminar neurons found in adults was reduced and was roughly proportional to the number of surviving retinular cells which sent fibers to the lamina. When retinular lesions were made in adults, however, no evidence of laminar cell loss was found. (2) A reduced number of higher-order neurons (medullary neurons) was found only in animals with very large embryonic retinular lesions. (3) When retinular cells were deleted before their fibers could grow into the lamina and contact immature target cells, the laminar cells deprived of these contacts failed to differentiate morphologically before degenerating at late embryonic stages. (4) In a number of cases, optic cartridges were formed with abnormal numbers of retinular fibers and laminar neurons. These observations support our hypothesis and increase our understanding of the role of cellular interactions in the development of the arthropod visual system.