Oocyte activation and Ca2+ oscillation-inducing abilities of mouse round/elongated spermatids and the developmental capacities of embryos from spermatid injection

To investigate differences in fertilization mechanisms and the potential clinical use of round/elongated spermatid, we conducted detailed studies of oocyte activation and Ca(2+) oscillation-inducing abilities in these immature sperm cells and compared these abilities against those of mature spermatozoa. When round spermatids from B(6)D(2)F(1) mice were injected, none of the oocytes was activated and no intracellular Ca(2+) ([Ca(2+)](i)) increases were observed. Elongated spermatids could induce activation normally in 87% of injected oocytes, but Ca(2+) oscillation could not be induced at all and most of the oocytes (94%) exhibited only several transient [Ca(2+)](i) rises (transient patterns). Because normal offspring could be obtained when embryos through elongated spermatid injection were transferred to foster mothers, it seems that a normal oscillation pattern of [Ca(2+)](i) is not essential for normal fertilization and embryo development. [Ca(2+)](i) patterns of injected oocytes changed from transient patterns to oscillation patterns while the injected immature sperm cells were maturing to spermatozoa. Dissociations were seen between the timing of appearance of oocyte activation and that of Ca(2+) oscillation-inducing abilities in maturing sperm cells. These dissociations may be due to differences in the thresholds to oocyte activation and Ca(2+) oscillation-inducing factor for inducing oocyte activation and Ca(2+) oscillation.