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Lessons from Amphioxus Bauplan About Origin of Cranial Nerves of Vertebrates That Innervates Extrinsic Eye Muscles

Amphioxus is the living chordate closest to the ancestral form of vertebrates, and in a key position to reveal essential aspects of the evolution of the brain Bauplan of vertebrates. The dorsal neural cord of this species at the larval stage is characterized by a small cerebral vesicle at its anterior end and a large posterior region. The latter is comparable in some aspects to the hindbrain and spinal cord regions of vertebrates. The rostral end of the cerebral vesicle contains a median pigment spot and associated rows of photoreceptor and other nerve cells; this complex is known as “the frontal eye.” However, this is not a complete eye in the sense that it has neither eye muscles nor lens (only a primitive retina‐like tissue). Cranial nerves III, IV, and VI take part in the motor control of eye muscles in all vertebrates. Using a recent model that postulates distinct molecularly characterized hypothalamo‐prethalamic and mesodiencephalic domains in the early cerebral vesicle of amphioxus, we analyze here possible scenarios for the origin from the common ancestor of cephalochordates and vertebrates of the cranial nerves related with extrinsic eye muscle innervations. Anat Rec, 302:452–462, 2019. © 2018 Wiley Periodicals, Inc.