Comparative analysis of vertebrates reveals that mouse primordial oocytes do not contain a Balbiani body

Oocytes spend the majority of their lifetime in a primordial state. The cellular and molecular biology of primordial oocytes is largely unexplored; yet, it is necessary to study them to understand the mechanisms through which oocytes maintain cellular fitness for decades, and why they eventually fail with age. Here, we develop enabling methods for live-imaging-based comparative characterization of Xenopus, mouse and human primordial oocytes. We show that primordial oocytes in all three vertebrate species contain active mitochondria, Golgi and lysosomes. We further demonstrate that human and Xenopus oocytes have a Balbiani body characterized by a dense accumulation of mitochondria in their cytoplasm. However, despite previous reports, we did not find a Balbiani body in mouse oocytes. Instead, we demonstrate that what was previously used as a marker for the Balbiani body in mouse primordial oocytes is in fact a ring-shaped Golgi that is not functionally associated with oocyte dormancy. This study provides the first insights into the organization of the cytoplasm in mammalian primordial oocytes, and clarifies the relative advantages and limitations of choosing different model organisms for studying oocyte dormancy.

Automated summary

Oocytes spend the majority of their lifetime in a primordial state, and understanding their cellular and molecular biology is crucial for studying cellular fitness and aging. This study developed methods for comparative characterization of Xenopus, mouse, and human primordial oocytes using live imaging. The results showed that all three species have active mitochondria, Golgi, and lysosomes in their primordial oocytes. Additionally, human and Xenopus oocytes have a Balbiani body characterized by dense accumulation of mitochondria, while mouse oocytes do not. This study provides insights into the organization of the cytoplasm in mammalian primordial oocytes and clarifies the advantages and limitations of different model organisms for studying oocyte dormancy.