Retinotopy of visual projections to the optic tectum and pretectum in larval sea lamprey

The sea lamprey has a complex life cycle with very different larval and adult stages. The eyes of larvae are subcutaneous, lack a differentiated lens and probably work only as an ocellus-like photoreceptor organ, while the well-developed adult eyes are capable of forming images. The larval retina differs greatly from the adult retina and presents a central region with differentiated photoreceptors and a lateral, largely undifferentiated part that grows in the second half of larval life. In the present study, we examined the retinotopy of projections from larval ganglion cells to the optic tectum and pretectum in sea lamprey by using retrograde tract-tracing techniques. In most regions of the tectum, application of the tracer neurobiotin (NB) resulted in labelled ganglion cells in the lateral retina, mostly in the contralateral eye. Ganglion cells of the lateral retina showed a very simple dendritic tree, possibly because of the lack of differentiation of most retinal layers in this region. The retinotectal projection is already retinotopically organized in larvae and follows a pattern similar to that observed in adult lampreys and other vertebrates. Application of NB to the central region of the tectum also led to labelling of a few ganglion cells in the central retina, which were clearly more complex than those in the lateral region, as they had dendrites that branched both in the outer and inner plexiform layers. Application of NB to the medial pretectum led to labelling of ganglion cells in the contralateral central retina. Occasional cells were also labelled in the lateral retina. The differential organization of larval retinal projections to the pretectum and tectum suggests a different role for these projections, which is consistent with the different involvement of these centres in visual behaviour, as determined in adult lampreys. The observations in larvae also reveal very different developmental timetables for these putative functions.