4.4 The sperm cells meet the oocyte - the acrosome reaction



Penetrating the cumulus cells

Quiz

Quiz 20


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.


The hyperactivity of the spermatozoa caused by the capacitation is a decisive factor that contributes to the spermatozoa being able, with the whipping motions of their tails, to go through the mass of cumulus cells, in the beginning even without much assistance from the hyaluronidase.


Summarizing, we have here to do with a directed "attack" of many sperm cells on the structures surrounding the oocyte, with the final goal of making it possible for one single spermatozoon to unite with the oocyte.
Fig. 31 - Oocyte surrounded by
   cumulus cells
 Legenda

1
2

3
Center of the oocyte
Corona radiata (surrounds and
partly covers the oocyte)
Head of the spermatozoon

Fig. 31
Picture of an IVF (in vitro fertilization). The oocyte surrounded by cumulus cells is added to the sperm cells. The sperm cells fertilize the oocyte in the normal manner.

See in the first video (230 kB), how a spermatozoon attempts to get through the cumulus cells.
In the second video (220 kB) several sperm cells circle around the oocyte.



Video

ICSI: Insemination via intracytoplasmic sperm cell injection (3 MB)
© Prof. B. Imthurn, Unispital, Zürich



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