Cytoarchitecture, fiber connections, and ultrastructure of the nucleus pretectalis superficialis pars magnocellularis (PSm) in carp.

The cytoarchitecture, fiber connections, and ultrastructure of the nucleus pretectalis superficialis pars magnocellularis (PSm) were studied in cypriniform teleosts (Cyprinus carpio). The PSm is an oval nucleus in the pretectum. Medium-sized cells and synaptic glomeruli are the main components of the nucleus. A lesser number of small cells are also present. Most of the medium-sized cells form one or two cell layers on the periphery of the nucleus, and some cells are scattered among synaptic glomeruli in the nucleus. Cell bodies in the peripheral cell layer are pyriform and sprout a thick dendrite directed inward. The dendrite gives off fine dendritic branches, which are postsynaptic elements in synaptic glomeruli. The PSm projects to the ipsilateral corpus mamillare (CM) and sends collaterals to the ipsilateral nucleus lateralis valvulae (NLV). Axons of the PSm neurons have terminals with many varicosities in the CM, and collaterals in the NLV have cup-shaped terminals around the cell bodies of the NLV neurons. Following horseradish peroxidase (HRP) injections into the PSm, HRP-labeled cells are found ipsilaterally in the optic tectum, the nucleus tractus rotundus of Schnitzlein, and the nucleus ruber of Goldstein. The tecto-PSm projections are topographically organized. The rostral optic tectum projects mainly to the rostral portion of the PSm, and the caudal tectum projects to the caudal portion of the PSm. The ventral tectum sends fibers mainly to the ventral part of the PSm. The dorsomedial tectum projects to the medial part of the PSm, and the dorsolateral tectum projects to the lateral part of the PSm. Tectal projection neurons to the PSm are of only one type. The tectal cell body is pyriform and is situated in the superficial part of the ipsilateral stratum periventriculare (SPV). The tectal neurons have a long perpendicular dendrite, which branches out in the stratum opticum (SO). An axon emerges from the branching site in the SO. Judging from the dendritic branching pattern of the tectal projection neurons, we concluded that the PSm receives visual information from the optic tectum.