In brief: Ovulation, the female reproductive process whereby fertilizable oocytes are released from a mature ovarian follicle, is widely seen in all animals that sexually reproduce. This paper reveals that, despite the remarkably diverse styles of ovulation among animal taxa, most ovulate via follicle rupture and suggests that the rupture process may operate via similar cellular mechanisms across animals.
Abstract: Ovulation is the process by which a fertilizable oocyte is extruded from the interior of the follicle. Herein, we conducted a literature survey to explore the ovulation patterns of 11 sexually reproducing species belonging to 10 animal phyla. These results indicate a large variety of ovulation patterns. Further comparative biological and evolutionary considerations of these results led us to conclude that most female animals ovulate via follicle rupture. We propose that in all animals that ovulate by follicle rupture, two cellular events may be critically involved in the process: i) the disintegration of cell junctional systems leading to intracellular cytoskeleton rearrangement in the follicle cells and ii) the degradation of extracellular matrix (ECM) proteins filled between follicle cells. These events may result in follicular cell deformation and increased motility, both of which are necessary for the formation of a path through which an oocyte escapes from the follicle. In addition to the requirement of ECM degradation for disintegrating cell junctions, intensive ECM protein degradation at the apical region of the follicle probably became increasingly important in late-evolving animals, such as vertebrates, in which a thick follicle wall containing a large abundance of ECM proteins is formed. We also considered hypothetical scenarios for the evolution of ovulation in these animals. Furthermore, this article discusses future problems that need to be solved for a more comprehensive understanding of ovulation in the animal kingdom.
