Cilia take the egg on a magic carpet ride.

A critical step in fertilization of eutherian mammals is the “pickup” of oocytes (eggs) by the oviduct (fallopian tube) from the surface of the ovary. In the ovary, each oocyte matures within a follicle, in which it is surrounded by supportive granulosa cells. As the time of ovulation nears, the granulosa cells close to the oocyte transform into cumulus cells, which begin secreting a hydrated elastic extracellular matrix (1). At ovulation, the oocyte plus its cumulus cells and their matrix—the cumulus–oocyte complex (COC)—is released from the ovary and must be pulled into the oviduct, vaulting over a gap between the surface of the ovary and the open end of the oviduct, the infundibulum. Failure of COC pickup can lead to failure of sperm to fertilize the oocyte or, worse, to ectopic implantation of an embryo outside of the uterus. In PNAS, Yuan et al. (2) demonstrate that the carpet of tiny motile cilia that coats the lining of the infundibulum is crucial to successful oocyte pickup and transport into the oviduct. Yuan et al. (2) created a strain of female mice whose oviducts were largely devoid of cilia. They did this by simultaneously knocking out two clusters of microRNAs (miRs) that, together, are necessary to generate cilia on the epithelial cells of the oviduct; tissue/time-targeted knockout narrowed down the effects of the miRs to the adult oviduct epithelium. Knockout females were infertile, even though they produced normal amounts of oocytes and reproductive hormones. Their infertility resulted, instead, from a failure of oocyte pickup: Yuan et al. (2) observed that oocytes did not enter the oviducts of the knockout mice but instead accumulated in the bursal cavity outside of the oviduct after they had been released from the ovary. Although some knockout female mice retained a reduced number of motile cilia … [↵][1]1To whom correspondence may be addressed. Email: sss7{at}cornell.edu. [1]: #xref-corresp-1-1