4.4 The sperm cells meet the oocyte - the acrosome reaction



The contact with the pellucid zone


When the sperm cells encounter the pellucid zone they bind themselves there. Following the binding of the sperm cells to the pellucid zone the acrosome reaction is then induced through ZP3 (zona protein 3).



Commentary

The changes that occur on the cell membrane of the spermatozoon while going through capacitation are also decisive for the success of the acrosome reaction. Both the binding to the pellucid zone as well as the subsequent acrosome reaction depend on the function of the cell membrane.



The acrosome reaction


During the acrosome reaction the contents of the acrosome are released outwardly. The cell membrane of the spermatozoon fuses with the outer membrane of the acrosome. The contents of the acrosome flow out through the resulting pores.

Quiz

Quiz 21


Fig. 32 - Acrosomal reacting spermatozoon  Legenda

1
2
3
4
5
6
A
B
C
Pores
Emerging of the acrosomal contents
Inner acrosomal membrane
Acrosomal content (enzyme)
Outer acrosomal membrane
Cell membrane
Head
Neck
Mid-piece

Fig. 32
For the acrosome reaction the cell membrane fuses with the outer acrosomal membrane



A prerequisite for the success of the acrosome reaction is the previous binding of the spermatozoon to the pellucid zone.



That the zona-binding represents a decisive step in the fertilization cascade can perhaps be seen in the fact that the zona-binding is species specific; the subsequent binding of the oocyte membrane onto the oolemma, on the other hand, is not.

When the acrosome reaction has been completed, the spermatozoon is now covered at its upper end only by the former inner membrane of the acrosome. For the further progress of the fertilization this is decisive because structures are thereby uncovered which are necessary for contact with the oocyte. One consequence of this is that changes appear, especially in the post-acrosomal membrane area of the spermatozoon.



Fig. 33 - Acrosomal reacted spermatozoon  Legenda

7
A
B
C
Membrane residues dropping behind
Head
Neck
Mid-piece

Fig. 33
The acrosome reaction is completed. The residues of the vesicle membrane are partly still held together by a sticky molecular bonding. Through the lively forwards movements of the sperm cells these are quickly left behind, however.



Fig. 34 - Acrosomal reacted spermatozoon  Legenda

8
A
B
C
Post-acrosomal membrane region
Head
Neck
Mid-piece

Fig. 34
The forward part of the sperm cell head is now covered only by the former inner membrane of the acrosome. For binding to the oocyte the now active post-acrosomal region is decisive.



The penetration of the pellucid zone


The enzymes that are released in the immediate vicinity of the pellucid zone by the acrosome reaction dissolve it locally and thus create a way through it for the sperm cells. A number of enzymes that have been released are involved. The best known are the already mentioned hyaluronidase and acrosin, whereby the acrosin makes it possible for the spermatozoa to get through the pellucid zone.



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