The Balbiani body is a non-membrane-bound organelle found in primordial oocytes that can contain mitochondria, the Golgi complex, endoplasmic reticulum and RNA. It has been suggested to play a role in oocyte dormancy, but its function remains unknown. Since primordial oocytes remain dormant in the ovary for long periods of time, studying their cellular biology is key to understanding the mechanisms of oocyte viability. Now, Elvan Böke and colleagues (Dhandapani et al., 2022) develop an elegant and detailed live-imaging comparative study of Xenopus, mouse and human primordial oocytes. Using specific fluorescent markers for different organelles, the authors show that primordial oocytes of all three species contain active lysosomes, Golgi and mitochondria. Moreover, the cytoplasmic distribution of these organelles is similar in Xenopus and humans, and mitochondria are clustered in a Balbiani body; interestingly, mouse oocytes do not show this cytoplasmic structure. Further investigation shows a ring-shaped Golgi complex in mouse oocytes that disassembles following oocyte activation but is not linked to oocyte dormancy. Overall, the authors present exciting new findings suggesting that mouse primordial oocytes do not contain a Balbiani body and highlight the differences in cytoplasmic organisation between different model systems used to study oocyte dormancy.
