During their growth, oocytes, the female reproductive cells in mammals, accumulate the resources necessary for future fertilization and pre-implantation development. A recently discovered cluster in mouse oocytes appears precisely when this growth accelerates. Initial observations of this structure have provided a better understanding of oocyte development and could, in the long term, help determine which oocytes are capable of being fertilized and have good future developmental potential.
Before it can be fertilized, the oocyte must grow and accumulate reserves needed for the early stages of embryonic development. Assessing oocyte quality after growth remains a challenge, particularly during in vitro fertilization. "But the size of the oocytes is not enough to know if they are of good quality," emphasizes Noemi Zollo, a biologist at the Collège de France.
By studying smaller cells than expected, Noemi Zollo noticed a condensed cluster near the nucleus, revealed by an RNA marker. This structure, never before described, was named the 'Zollo body' by Marie-Hélène Verlhac, one of the researcher's thesis supervisors.
Mass spectrometry analysis revealed that the 'Zollo body' gathers RNAs, proteins, and several types of organelles. Some RNAs carry instructions for producing proteins involved in the growth of the oocyte and follicle. The structure appears when the oocyte reaches about 60 micrometers, just before a striking acceleration in its growth.
The hypothesis is that the 'Zollo body' actively participates in oocyte growth by promoting the production of proteins and organelles necessary for its preparation for fertilization. To test this hypothesis, the team blocked the activity of a protein, DDX5, identified in the cluster. "After inhibiting DDX5, the cluster came apart, and the growth of the oocyte and its follicle completely stopped!" reports Noemi Zollo, proving the active role of the assembly.
Research is ongoing to verify the presence of the 'Zollo body' in humans and to better understand the mechanisms that maintain this assembly. The goal is to understand how a reproductive cell prepares to enable the beginning of new life.
