Penetrating the cumulus cells

In vivo -the spermatozoa arrive in waves at the oocyte that is surrounded by cumulus cells. When fertilization is carried out in a test-tube, the amount of sperm cells introduced must be carefully observed because if there are too few, no fertilization occurs.

Enzymes are set free by the acrosome reaction; the hyaluronidase dissolves the intercellular matrix between the cumulus cells, other enzymes dissolve the pellucid zone that lies around the oocyte.

Normally, the acrosome reaction of the spermatozoa takes place first when they encounter the pellucid zone. In a small percentage of the sperm cells, though, the acrosome reaction occurs spontaneously, just as when a small percentage of the cells experience capacitation immediately following ejaculation. This circumstance assures that a small amount of hyaluronidase is present from the very beginning and, when the wave of sperm cells meets the oocyte, a few of them are thus assisted in making their way to the pellucid zone. Upon arriving at the pellucid zone, these sperm cells themselves undergo an acrosome reaction and a further amount of hyaluronidase and other enzymes are released. In this way, the throng of cumulus cells is further loosened up and more and more sperm cells obtain the possibility of undergoing the acrosome reaction themselves at the pellucid zone.