Human oocyte activation after intracytoplasmic sperm injection

Oocyte activation is a series of events triggered by the fertilizing spermatozoon and necessary for the beginning of the embryonic development. Calcium plays a pivotal role in this process. Here we used confocal laser scanning microscopy to examine the changes in the concentration of intracellular free calcium ([Ca2+]i) in human oocytes after intracytoplasmic sperm injection (ICSI). The first considerable but short (< 2 min) increase in [Ca2+]i was detected immediately after the penetration of the microinjection needle into the ooplasm. This rise by itself did not provoke oocyte activation and was also obtained after the injection of medium without spermatozoa. After a lag period of 4-12 h, oocytes that were subsequently activated initiated a second period of [Ca2+]i changes. These changes were sperm-dependent and followed one of two alternative patterns, a non-oscillatory one and an oscillatory one. The non-oscillatory pattern resembled the changes described previously during parthenogenetic activation of mammalian oocytes. The oscillatory pattern was similar to the changes accompanying normal fertilization in different mammalian species. It is concluded that the initial [Ca2+]i rise provoked by the ICSI procedure is not responsible for oocyte activation, and that a release of a sperm factor(s) is required to initiate this process.