Ultrastructural Aspects of Oocyte Maturation in Dogs, with Comparative Insights from Cats: Current Evidence and Research Perspectives.

Reproductive success in dogs and cats remains limited despite advances in assisted reproductive technologies, highlighting the need for a clearer understanding of the ultrastructural processes governing oocyte maturation. This review critically examines current knowledge on the ultrastructural and cytoplasmic mechanisms underlying oocyte maturation in dogs, with particular emphasis on cytoskeletal dynamics, organelle redistribution, mitochondrial activity, and lipid metabolism. Comparative insights from other mammalian species, especially the cat, are integrated to highlight fundamental differences in the timing and regulation of oocyte competence. The evidence suggests that inadequate cytoplasmic maturation, not meiotic progression, is the principal bottleneck affecting fertilization and early embryonic development in dogs. We conclude that future improvements in canine fertility and reproductive technologies will require a shift from predominantly hormonal and nuclear-focused approaches toward strategies targeting oocyte cytoplasmic quality and ultrastructural maturation.