Flagellar hyperactivation of bull and boar spermatozoa

Abstract Background In mammals, flagellar hyperactivation is indispensable to sperm fertilization with oocytes in vivo, although there are species differences in regulatory mechanisms for this event. In this study, I reviewed researches regarding hyperactivation of bull and boar spermatozoa, in comparison with those of spermatozoa from other species. Methods Recent publications regarding sperm hyperactivation were collected and summarized. Results (Main findings) In bull and boar spermatozoa, there are two types of hyperactivation “full‐type hyperactivation and nonfull‐type hyperactivation” which are equivalent to anti‐hock hyperactivation and pro‐hock hyperactivation of mouse spermatozoa, respectively, on the basis of the flagellar parts exhibiting asymmetrical beating. Full‐type hyperactivation is initiated in response to a rapid increase of cytoplasmic Ca2+ in the connecting/middle and principal pieces by the mobilization of this divalent ion from extracellular space and internal store through cation channels. Regulatory molecules for the increase of cytoplasmic Ca2+ in the connecting/middle pieces are probably different from those in the principal pieces. Conclusion I have proposed a hypothesis on the regulation of full‐type hyperactivation by the distinct signaling cascades leading to the increase in cytoplasmic Ca2+ between the connecting/middle and principal pieces of bull and boar spermatozoa.