Pituitary–ovary–spleen axis in ovulation

Leukocytes are rapidly recruited to the preovulatory ovary and play a crucial role as facilitators of ovulation and luteal formation. In this article, recent findings on leukocyte trafficking to the ovary, as well as the physiological role of leukocytes in the ovary, will be summarized and discussed. We then explore the novel hypothesis that the hypothalamus–pituitary–ovary (HPO) axis might include the spleen as a reservoir of leukocytes by summarizing recent reports on this topic, both in the fields of immunology and reproductive biology.