Identification of retinotectal projection pathway in the deep tectal laminae in the chick.

The optic tectum is a visual center of nonmammalian vertebrates that receives retinal fibers in a retinotopic manner. It has been accepted that retinal fibers project to some superficial laminae of the tectum, but do not go through lamina g of stratum griseum et fibrosum superficiale (SGFS). By a novel fiber-tracing method, we found a novel pathway of retinal fibers that run through deep laminae of the tectum. The retinal fibers that would run through the newly identified pathway first run caudally along the medial edge after invading the tectum, turn laterally, and extend toward the lateral side through the deep pathway. The deep pathway runs through stratum album centrale and stratum fibrosum periventriculare. The fibers that run through the deep pathway do not enter the stratum opticum, where the conventional retinal fibers run. As development proceeds, these fibers decrease and disappear by the adult stage. By the new method, we found that some of the conventional retinal fibers transiently run through lamina g of SGFS and invade laminae h/i. In conclusion, we found distinct but transient retinal fiber pathway in the deep tectal laminae, which have not been thought to be retinorecipient.