Cellular interactions and pattern formation in the development of the visual system of Daphnia magna (Crustacea Branchiopoda). II. Induced retardation of optic axon ingrowth results in a delay in laminar neuron differentiation

摘要

Groups of embryonic photoreceptors in one side of the compound eye of Daphnia were irradiated with an ultraviolet microbeam at a stage when the cells were postmitotic but had yet to elaborate axons. Immediately after irradiation, the embryos were placed under fluorescent illumination. On the average, 16 of the irradiated photoreceptors were killed by the exposure. Previous observations suggest that an approximately equal number were rescued by the post-irradiation fluorescent illumination. The schedule of differentiation of the rescued photoreceptors was affected such that their axons arrived at the target region in the optic lamina from 2 to 10 hr after they would have normally. Serial section electron microscopic analysis showed that differentiation of laminar neurons contacted by the delayed axons also was delayed by a length of time corresponding to the delay in axon arrival. These and previous observations indicate that the differentiation of laminar neurons is triggered by contact with optic axons and can be initiated over a period of several hours after these cells become postmitotic.