, Volume 155, Issue 1, pp 27-45

Fine structure of the photoreceptor of the ascidian tadpole during development

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The eye of the tadpole larva of Amaroucium constellatum consists of 15–20 photoreceptor cells, a single pigment cup cell, and three lens cells. Each photoreceptor cell during its development gives rise apically to a neck process and basally to an axon, which is thought to extend to the cerebral ganglion. At the tip of the neck a sensory cilium differentiates to produce the photoreceptor membranes. This process begins as a balloon-shaped evagination of the cell membrane above the ciliary basal body. This evagination infolds to form the presumptive photosensitive lamellae which increase in number, length, and regularity as the eye matures. Microvilli also arise from the tip of this cell and intermingle with the ciliary membranes. The neck process itself elongates, narrows, and becomes filled with microtubules during development. The pigment cell, initially long, flat, and electron lucent, develops into a cupshape. Its cytoplasm becomes packed with pigment granules, forming an effective light shield. The development of the lens cells involves formation of the lens vesicles and elongation of the cells into the lumen of the pigment cup. Differentiation of ascidian photoreceptor cells is compared with that of the vertebrate rod and cone.

This investigation was supported in part by NIH Grants number GM01981, RR06084, and EY00443.1 wish to acknowledge Dr. A. L. Bell for his assistance during this investigation, Drs. S. W. Smith, T. H. Goldsmith, and R. Wehner for critical reading of the manuscript, and Jeri Cohen for technical assistance.