Medaka Oocytes Rotate Within the Ovarian Follicles During Oogenesis

Abstract

The purpose of the current investigation was to ascertain whether medaka oocytes rotate within the follicle. Isolated medaka follicles were incubated in modified L15 Medium for 3 hr at 26°C. During incubation, movement of oocytes within follicles held on slides under a microscope was recorded by a video cassette recorder. Within the follicle, the surface of which was marked with carbon particles, the movement of the intrafollicular oocyte was traced by dislocation of its attaching and non-attaching filaments on the chorion. Pre-vitellogenic oocytes exhibited rotation around the predetermined animal-vegetal axis, accompanied by rotation at a slightly oblique angle to the axis. The velocity of oocyte rotation was about 40-48 ,urn hr-' and was similar among oocytes of different stages between the pre-vitellogenic and early vitellogenic phases of oogenesis. Rotation was inhibited by cytochalasin B treatment. Also, it was not observed in oocytes surrounded only by the granulosa cell layer when the thecal cell layer and the basement membrane were removed from the follicle. In oocytes with a thick chorion, rotation was also inhibited by impaling the oocytes with a glass needle at a right angle to the animal-vegetal axis of the oocyte. These results provide evidence that growing medaka oocytes rotate primarily around their animal-vegetal axis and at a slightly oblique angle to the axis. That the rotation of medaka oocytes may depend upon the movement of the granulosa and the thecal cells within the follicles was discussed.