Neuronal organization of the optic tectum in the hagfish, Eptatretus burgeri: a Golgi study.

The neuronal organization of the optic tectum (OT) was studied in the hagfish using the rapid Golgi method. The OT shows laminar structure. Beginning from the ventricular surface, the following four concentric strata are discernible: the stratum ependymale, stratum periventriculare, stratum cellulare et fibrosum, and stratum marginale. The stratum ependymale consists of several rows of ependymal cells and neuroblasts lining the mesencephalic ventricle. The stratum periventriculare contains medium-sized and small neurons whose dendrites extend mainly superficially. The stratum cellulare et fibrosum occupies a wide area and consists of densely packed neurons and fibers. Fibers in this stratum are derived mainly from the bulbar lemniscus and run ventrodorsally in several bundles, among which numerous neurons are embedded. Neurons in the stratum cellulare et fibrosum are divided into large, medium-sized and small neurons whose dendrites are arranged in a network rather than being oriented in any particular direction. Some of these dendrites extend contralaterally through the commissure of the OT. The neurons in the stratum marginale are divided into medium-sized and small neurons whose dendrites extend mainly tangentially. The axons of neurons in the stratum periventriculare and those of a few neurons in the stratum cellulare et fibrosum extend rostromedially and can be traced into the stratum periventriculare. On the other hand, the axons of neurons in the stratum marginale and stratum cellulare et fibrosum run rostrally, turn ventrally and join fiber bundles running dorsoventrally.