Ultrastructural Characteristics of the Juvenile Chum Salmon (Oncorhynchus keta) Cerebellum: Interneuron Composition, Neuro-Glial Interactions, Homeostatic Neurogenesis, and Synaptic Plasticity

The organization of the cerebellum of the juvenile chum salmon Oncorhynchus keta was studied using transmission (TEM) and scanning (SEM) electron microscopy. The study of the interneuron composition of the cerebellum showed the presence of stellate cells in the molecular layer, projection Purkinje cells, and eurydendroid cells in the ganglion layer. Large Golgi cells and granular cells were found in the granular layer. The study of the synaptic structure of the molecular layer showed the presence of synaptic contacts of electrotonic and chemical types, which are an important link in interneuronal communications. The TEM showed that numerous synaptic structures are formed in the molecular layer of juvenile chum salmon, including one of the largest synapses in the cerebellum between the parallel fibers and the dendrites of Purkinje cells. Most synaptic endings of parallel fibers of the excitatory type in juvenile chum salmon converge on the dendrites of the Purkinje cells. The TEM study of neuro-glial relationships revealed also the presence of a heterogeneous population of astrocytes and microglia in the cerebellum of juvenile chum salmon. Patterns of apoptosis and phagocytosis involving protoplasmic astrocytes were detected. The detection of protoplasmic astrocytes in the cerebellum of juvenile chum salmon contrasts with the data for zebrafish. Homeostatic growth of the cerebellum of juvenile chum salmon may follow an indeterminate growth pattern and be mediated by the presence of adult-type precursors (aNSPCs). The presence of aNSPCs of glial and non-glial types in the cerebellum of juvenile chum salmon was demonstrated by TEM and SEM. The discovery of a large population of non-glial aNSPCs in the dorsal matrix zone (DMZ) and granular layer of juvenile chum salmon and also stromal cell clusters on the surface of the cerebellar molecular layer suggests the activity of a neurogenic program in the brain of juvenile chum salmon that is mainly active during embryonic stages in other vertebrate species. The phenomenon of embryonization in the cerebellum of juvenile chum salmon is determined by the presence of non-glial aNSPCs, which provides homeostatic growth. The neotenic status observed in the cerebellum of juvenile chum salmon may be part of the species' adaptation to a changing ecological niche and identifies promising cellular and molecular targets for regenerative or anti-aging therapy.