Module
4
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Fertilization

The docking mechanism of the spermatozoon onto the oocyte (the key-lock principle)

The final penetration of the spermatozoon into the oocyte can only take place after a complex sequence of processes have occurred for both the spermatozoon and the oocyte. See preceding chapter.

At the molecular level the recognition of the spermatozoon and its attachment to the oolemma, the oocyte membrane, functions via a key-lock principle.

Keys and locks are receptor proteins, inserted into the cell membranes of spermatozoon and oocyte and having great mutual affinity.

In order for the docking to be successful, the corresponding membrane location has to first be uncovered, which is achieved by means of the complete and error-free course of the acrosome reaction.

The course of the acrosome reaction is the precondition for the membrane coalescence between spermatozoon and oocyte.
Fig. 35 - Acrosomally reacted spermatozoon in perivitelline space
media/module4/d5a_Acro4.gif

  1. Post-acrosomal region
  2. Oolemma with microvilli
  3. Perivitelline space
  4. Pellucid zone

Legend
Fig. 35

When the acrosomally reacted spermatozoon encounters the oolemma following the penetration of the pellucid zone and touches it along its length, the reciprocal membrane receptors bind with each other.

Fig. 36 - Docking of the spermatozoon
media/module4/d5b_Acro5.gif

  1. Post-acrosomal region
  2. Oolemma with microvilli
  3. Perivitelline space
  4. Pellucid zone

Legend
Fig. 36

The post-acrosomal membrane region of the spermatozoon has bound itself with the oolemma.

Fig. 37 - Invagination of the spermatozoon
media/module4/d5c_Acro6.gif

5
Cortical vesicle at the surface of the oocyte

Legend
Fig. 37

The "contents" of the sperm cell are completely taken into the cytoplasma of the oocyte.

Through the acrosome reaction known amounts of enzymes are released that contribute to the weakening and local dissolving of the pellucid zone. Via this means and with the forceful motions of its flagellum, the spermatozoon penetrates the pellucid zone and finally gets into the perivitelline space. When it there touches the oolemma along its length through the strong movements of its flagellum, a mutual recognition of the docking receptor proteins happens.

Due to the docking of the spermatozoon onto the oocyte a cascade of events are triggered.

The docking triggers a cascade of events with the following goals:

  • Polyspermy block: The penetration of further sperm cells should be hindered
  • Hardening of the pellucid zone as a mechanical protection of the embryo
  • Entry of the spermatozoon into the oocyte
  • Termination of the 2nd meiosis of the oocyte with expulsion of the 2nd polar body
  • Preparation at the molecular level of the oocyte for unpacking the paternal DNA