There is debate as to whether the acrosome reaction is necessary for sperm
incorporation after intra-cytoplasmic sperm injection (ICSI). Ultrastructural
evidence is presented to show that the acrosome reaction could occur in the
ooplasm before sperm incorporation in mature human oocytes or the acrosome
could be discarded intact before sperm incorporation in immature oocytes,
matured in vitro. Both germinal vesicle and growing
follicular oocytes showed sperm chromatin decondensation, with discarded
acrosomes close to the sites of incorporation, and were able to form male
pronuclei. This is probably the first report of microfertilization of a
growing oocyte with a reticulate nucleolus by ICSI. The acrosome reaction,
when it occurs, is preceded by acrosome swelling and is followed by
vesiculation of surface membranes exposing the inner acrosome membrane, as
observed on the surface of the zona during IVF or in the perivitelline space
after subzonal sperm injection. These sperm were probably capacitated at the
time of ICSI. There was subtle evidence of leaching of the acrosomal matrix
from intact discarded acrosomes and from partially depleted acrosomes attached
to decondensing spermheads. These sperm were probably not fully capacitated at
the time of ICSI. It is concluded that both the acrosome reaction and acrosome
deletion are possible prerequisites to sperm incorporation after ICSI.
Number of embryos for transfer following in vitro fertilisation or intra-cytoplasmic sperm injection
