Ultrastructure of gametes after intracytoplasmic sperm injection

The ultrastructure of 18 metaphase-II oocytes after intracytoplasmic sperm injection (ICSI) was analysed from 30 min to 8 h following the microinjection. Three control metaphase-II oocytes were not injected. The 21 oocytes were embedded individually and examined in serial pole-to-pole sections. Seventeen oocytes had been matured in vitro from the germinal vesicle stage and showed ultrastructural alterations due to long-term culture. In microinjected oocytes, membrane-bound vacuoles and oolemma inclusions were found at the injection site. The ICSI oocytes showed evidence of plasma-membrane damage and increased oocyte exchange processes with residual and multivesiculated bodies. Gamete membrane fusion was not observed. Acrosome reaction was observed within the ooplasm 15 min after ICSI. Sperm elements were not incorporated into any oocyte vacuole. Onset of oocyte activation was associated with sperm chromatin decondensation. Cortical granules were released at 60 min and two pronuclei were formed at 6 h after ICSI. The male and female pronucleus formation was asynchronous. The findings suggest specific ICSI-related morphological features of early fertilization.