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. 2021 Feb 4;9:636737. doi: 10.3389/fcell.2021.636737

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Copyright © 2021 Barnes, Malhotra and Calvert.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Evolution of ciliary photoreceptors. (A) Schematic of an ascidian tadpole cerebral ganglion photoreceptor. Reproduced from Eakin and Kuda (1971). (B) Transmission electron micrographs of cerebral ganglion photoreceptors of Amaroucium constellatum larva. In the upper panel the section is perpendicular to the membrane lamellae. The lower panel is an image from a section parallel to the lamellar membranes. Reproduced from Barnes (1971). (C) Transmission electron micrograph of rod photoreceptors from mouse retina showing the CC, apical membrane region of the IS and nascent and enclosed discs of the OS. Note dense packing of nascent discs (arrowheads) and close juxtaposition of neighboring rods. Scale bar 1 μm. Reproduced from Ding et al. (2015). (D) Schematic of an amphibian rod. OS, outer segment; IS, inner segment; S, synaptic spherule; Ax, axon; N, nucleus; M, myoid; E, ellipsoid; CP, calycal process; ND, nascent disc; CC, connecting cilium; DM, mature disc membrane; PM, plasma membrane. Cyan: apical membrane. NDs are open to the extracellular milieu and contiguous with the CC and PM. DMs are enclosed within and separate from the OS PM and each other. Modified from Maza et al. (2019).