An attempt to search for the common cellular mechanism of ovulation across all metazoans: A review

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 eleven 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: 1) the disintegration of cell junctional systems that lead to intracellular cytoskeleton rearrangement in the follicle cells and 2) the degradation of extracellular matrix (ECM) proteins filling 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 oocytes escape 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 the future problems that need to be solved for a more comprehensive understanding of ovulation in the animal kingdom.