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

Currently, the problem of climate change on Earth is becoming increasingly urgent. These changes are the reason for the increasingly pronounced adaptive differences in different species of fish. A significant gap in ultrastructural data on the organization of the salmon cerebellum was the main motivation for this study’s microscopic and ultrastructural analyses using transmission and scanning electron microscopy of the cerebellum of juvenile chum salmon Oncorhynchus keta. 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. Most synaptic endings of parallel fibers of the excitatory type in juvenile chum salmon converge onto dendrites of Purkinje cells. Transmission electron microscopy (TEM) study of neuro–glial relationships also revealed 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 presence of protoplasmic astrocytes in the cerebellum of juvenile chum salmon contrasts with data reported for zebrafish. The conducted studies allow us to conclude that the homeostatic growth of the cerebellum of juvenile chum salmon can occur according to an uncertain pattern and be mediated by the presence of adult-type neural stem/progenitor cells (aNSPCs). The presence of aNSPCs of glial and non-glial types in the cerebellum of juvenile chum salmon was demonstrated by TEM and scanning electron microscopy (SEM). The discovery of a large population of non-glial aNSPCs in the dorsal matrix zone (DMZ) and granular layer of juvenile chum salmon, as well as 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 contribute to homeostatic growth.