Oocyte transport against fluid flow to the fertilization site in mice: contributions of cilia beating and peristalsis†.

The transport of ovulated oocytes to the site of fertilization involves two main processes, the initial collection of the oocyte by the fimbria and its subsequent transport through the upper ampulla. These are crucial events preceding fertilization. Around ovulation, the oviduct exhibits active fluid secretion, peristaltic movements, and ciliary beating, all of which are believed to be involved in oocyte transport. However, their specific contributions require further clarification. In this study, we investigated how these three factors influence oocyte transport to the fertilization site in the oviduct in vivo. The oviduct of anesthetized mice was installed in a fluid-circulating chamber. By introducing fixed and stained cumulus-oocyte complexes (COCs) into the fimbria and injecting a small amount of ink into the oviduct lumen, we monitored oocyte transport and fluid dynamics. Interestingly, while oviduct fluid flowed toward the fimbria, the COC moved in the opposite direction to reach the site of fertilization. Inhibiting ciliary beating disrupted both the collection (or "pickup") and transport of the oocyte, whereas inhibiting peristalsis had no immediate impact on these processes. However, extended inhibition of peristalsis resulted in impaired oocyte transport. Under these conditions, fluid accumulated, and the oviduct lumen expanded, disrupting the intimate contact between the COC and the cilia. These findings indicate that ciliary beating, rather than fluid flow or peristalsis, propels the COC against the fluid flow toward the fertilization site. In addition, peristalsis maintains the luminal conditions required for effective transmission of ciliary propulsion to the COC.