Eye development and retinal differentiation in an altricial fish species, the senegalese sole (Solea senegalensis, Kaup 1858).

We describe the major events in the retinogenesis in an altricial fish species, the Senegalese sole. The major developmental events in the sole retina occurred early after hatching (posthatching day 0, P0). Thus, (1) plexiform layers became recognizable at P1. (2) Proliferative activity disappeared from the central retina at P1, and, as development progressed, became restricted to cells located in the circumferential germinal zone, and to sparse cells dispersed throughout the inner nuclear layer and the outer nuclear layer. (3) Apoptotic cells were sparsely observed, randomly localized in all three nuclear layers of the early posthatching retina from P0 to P4. (4) The first synaptic vesicles were detected at P0 in early postmitotic ganglion cells. However, their appearance in the plexiform layers was delayed until P2. (5) The neurochemical development of most major retinal cell classes occurred between P0 and P5. Thus, although Isl1 immunoreactive ganglion cells were the first to become postmitotic in the vitreal surface of the central retina at P0, the first glutamine synthetase-expressing Müller cells appeared in the central retina by P5. The onset of expression for other retinal markers, such as rod opsin, calretinin, parvalbumin, a-tyrosine hydroxylase, and a-protein kinase C, occurred between P2 and P4. Our results suggest that the most relevant processes involved in Senegalese sole retinogenesis occur during the prolarval and early larval stages (P0–P5). Furthermore, we conclude that altricial fish species may constitute a convenient model organism to address the relationship between the structural and functional development of sensory organs with the acquisition of behavioral repertoires.