Lipid droplet formation is spatiotemporally regulated in oocytes during follicular development in mice

Communication between oocytes and the surrounding granulosa cells during follicular development is essential for complete oocyte growth. Oocytes contain lipid droplets (LDs), organelles assembled in the endoplasmic reticulum (ER) that store neutral lipids, including triglycerides and cholesterol esters. Although the LD content varies among animals, LDs stored in oocytes have been shown to play an important role in oocyte maturation and preimplantation embryonic development. However, knowledge is lacking regarding how and when LDs are initially produced in developing oocytes within follicles. In the present study, we found that LDs appeared in mouse oocytes in a specific phase during follicular development. The emergence of LDs in intrafollicular oocytes was induced within a similar time window in vitro and in vivo. Fluorescence imaging and electron microscopy revealed that LDs emerging in oocytes during the early stages of follicular growth were in close proximity to the ER. Furthermore, fatty-acid-tracking experiments have revealed that exogenous fatty acids are rapidly incorporated into oocytes, and their uptake is regulated by the interaction between oocytes and granulosa cells, likely in part through transzonal projections. In summary, our results suggest that LD synthesis observed in growing oocytes is spatiotemporally regulated and that oocyte-granulosa cell contact may be involved in LD biosynthesis during follicular development.