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Fig. 1 | Evolution: Education and Outreach

Fig. 1

From: The Origin of the Vertebrate Eye

Fig. 1

Scenario for evolution of the vertebrate eye. Left Ciliary and rhabdomeric photoreceptors. a Prior to the divergence of protostomes and deuterostomes, more than 580 Mya, the primitive bilateral organisms possessed both ciliary and rhabdomeric photoreceptors. b At an early stage during deuterostome evolution, it seems likely that the rhabdomeric photoreceptors received synaptic input from the ciliary photoreceptors, which then no longer projected centrally. c Subsequently, the rhabdomeric photoreceptors lost their photoreceptive structures and became simply projection neurons (retinal ganglion cell, RGC). The ciliary photoreceptors required contact with RPE cells in order to re-synthesize the correct isomer of vitamin A needed by the visual pigment, opsin; this was achieved by the inward folding of the eye vesicle to form an eye-cup (see f in middle column). Middle Presumed evolution of the neural tube, eye vesicle, eye-cup, and lens, based on the embryological development of the nervous system of extant vertebrates. a–d Early in the evolution of bilateral animals, an inward folding of neural tissue occurred, which created the neural tube in chordates (by 550 Mya); the orange shading represents the lips of the neural folds. Light-sensitive cells were located in the regions that bulged laterally to form the “eye vesicles” (dashed box in e). Contact between the expanding neural tube and the outer layer of the animal (the surface ectoderm) triggered an in-folding of the optic vesicle to form the invaginated eye-cup (f), in which the retina is apposed to the RPE. We propose that the eye-cup stage illustrated in f had evolved prior to the divergence of myxinoids (as represented by extant hagfish) from our own line. Subsequently, in the line that led to both jawless and jawed vertebrates, the outer layer (ectoderm) thickened and gave rise to the formation of a lens (g); this stage had arisen by the time of the last common ancestor that we shared with lampreys, about 500 Mya. Right Approximate time-line for the progressions sketched on the left and in the middle, with times indicated in millions of years ago (Mya) [Middle panel from Lamb et al. (2007) Fig. 4; first published in Nature Reviews Neuroscience 8, 2007 © Nature Publishing Group, a division of Macmillan Publishers Limited]

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