Teleost fish exhibit the most pronounced and widespread adult neurogenesis.
Recently, functional development and the fate of newborn neurons have been
reported in the optic tectum (OT) of fish. To determine the role of neurogenesis in
the OT, this study used histological, immunohistochemical, and electron microscopic
investigations on 18 adult Molly fish specimens (Poecilia sphenops). The OT of the
Molly fish was a bilateral lobed structure located in the dorsal part of the mesencephalon. It exhibited a laminated structure made up of alternating fiber and cellular layers,
which were organized into six main layers. The stratum opticum (SO) was supplied by
optic nerve fibers, in which the neuropil was the main component. Radial bipolar neurons that possessed bundles of microtubules were observed in the stratum fibrosum
et griseum superficiale (SFGS). Furthermore, oligodendrocytes with their processes
wrapped around the nerve fibers could be observed. The stratum album centrale
(SAC) consisted mainly of the axons of the stratum griseum centrale (SGC) and the
large tectal, pyriform, and horizontal neurons. The neuronal cells of the SO and large
tectal cells of the SAC expressed autophagy-related protein-5 (APG5). Interleukin-1β
(IL-1β) was expressed in both neurons and glia cells of SGC. Additionally, inducible
nitric oxide synthase (iNOS) was expressed in the neuropil of the SAC synaptic layer
and granule cells of the stratum periventriculare (SPV). Also, transforming growth factor beta (TGF-β), SRY-box transcription factor 9 (SOX9), and myostatin were clearly
expressed in the proliferative neurons. In all strata, S100 protein and Oligodendrocyte
Lineage Transcription Factor 2 (Olig2) were expressed by microglia, oligodendrocytes,
and astrocytes. In conclusion, it was possible to identify different varieties of neurons
in the optic tectum, each with a distinct role. The existence of astrocytes, proliferative
neurons, and stem cells highlights the regenerative capacity of OT.