Most living acrosome-reacted spermatozoa do not fuse with the oocyte when inserted into the perivitelline space

To evaluate the frequency with which acrosome-reacted spermatozoa fuse with the oocyte after subzonal insemination (SUZI).

The acrosome reaction frequency determined in sperm suspensions used for SUZI was related with the frequency of sperm-oocyte fusion after the microinsemination procedure. In some experiments, the acrosome reaction frequency was compared in sperm suspensions aspirated into the microinjection needle and in those leaving the needle.

Private hospital, public research center, and university-based laboratory.

Patients involved in a program of micromanipulation-assisted fertilization and healthy sperm donors.

IVF-ET.

Frequency of the acrosome reaction, frequency of sperm-oocyte fusion.

The procedure of sperm aspiration to the microinjection needle followed by active swim-out of motile spermatozoa did not change the overall percentage of acrosome-reacted spermatozoa even though the subpopulation of acrosome-reacted spermatozoa leaving the needle was enriched in recently reacted cells. The frequency of sperm-oocyte fusion was considerably lower than the frequency of the acrosome reaction in both patient and donor sperm samples, irrespective of the number of spermatozoa inserted per egg.

A spontaneous acrosome reaction does not necessarily predispose a spermatozoon to fuse with the oocyte after SUZI. It remains to be elucidated whether this relative inefficiency of acrosome-reacted spermatozoa is related to technical aspects of SUZI or whether, in a more general sense, the acrosomal exocytosis and the development of sperm-oocyte fusion capacity are distinctly regulated, though closely related, phenomena.