Development of the human superior colliculus and the retinocollicular projection.

We have used carbocyanine dye tracing from the brachium of the superior colliculus in conjunction with Nissl staining and immunohistochemistry to GAP-43 and calretinin to study the development of retinal projections to the superior colliculus in 17 human embryos and fetuses aged from 8 to 28 weeks. Lamination of the superior colliculus begins to emerge by 11 weeks, and by 16 weeks all seven layers of the mature superior colliculus are visible. Fibres immunoreactive to GAP-43 were seen at 13 weeks in the most superficial layers. By 19 weeks, GAP-43 immunoreactivity was present in the stratum opticum as well as the deeper fibres layers, indicating the development of fibre pathways following those laminae. Carbocyanine dye tracing of retinocollicular projections showed extensive rostrocaudally running unbranched fibres in the superficial superior colliculus at 12 weeks. Shortly after this (13 weeks), retinocollicular fibres penetrate the deeper collicular layers and branching becomes apparent. We also saw occasional retrogradely labelled somata following DiI insertion into the superior brachium. Our findings indicate that development of the human superior colliculus and its connections is largely complete by 20 weeks. This would suggest that functional capacity of the human superior colliculus should also be mature by the middle of gestation.