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 Ca(2+) 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 Ca(2+) 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 Ca(2+) between the connecting/middle and principal pieces of bull and boar spermatozoa.