Figure 6

Schematic representation of the proposed evolutionary scenario for the emergence of the retina and hypothalamus in craniates/vertebrates. The anterior neural tube of a chordate ancestor contained a periventricular photoreceptor cells (ciliated cells, at least in part glutamatergic) intermingled with neuroendocrine cells synthesizing dopamine and neuropeptides. These cells could also be connected to the CNS. These cell types are lining the anterior neural ventricle and contact the CSF. A reminiscent but derived situation is found in modern protochordates such as ascidian (inferior left part of the drawing) where photoreceptor cells line the ventricle and are adjacent to the DA cells of the sensory vesicle, which make coronets inside the ventricle. DA cells are able to modulate the motor response to light. The situation could be very similar in the amphioxus neural tube, based on current description of the photoreceptor and DA cells. In craniates/vertebrates, the optic vesicle becomes separated from the anterior hypothalamus at the end of the neurulation process and bulged out of the neural tube to reach the lateral neuroectodermal epithelium and the lens placode, leading to new morphogenetic movements and to the inversion of the retina. The retina comprised several cell types inherited from the protochordate ancestor, including at least photoreceptor cells, pigmented epithelium and amacrine DA cells.