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The polyspermy block

As soon as a spermatozoon has docked, further sperm cells must be hindered from also doing the same. In the oolemma as well as in the pellucid zone the appropriate changes occur.

The docking triggers a rapid wave of depolarization in the oolemma, leading to changes in the membrane surface.

The depolarization wave then also causes small cortical vesicles, found on the inside of the oolemma, to empty out their contents into the perivitelline space. The pellucid zone is "hardened" thereby. It no longer allows sperm cells to pass through unhindered. From this time on, and over the next few days, the pellucid zone provides excellent protection for the developing zygote.

Fig. 38 - Polyspermy block

  1. Pellucid zone
  2. Perivitelline space
  3. Cortical vesicle
  4. Oolemma

Fig. 38

The contents of the cortical vesicle that are released in the perivitelline space leads to a hardening of the pellucid zone.

The entry of the spermatozoon into the oocyte (impregnation)

After the spermatozoon has docked onto the oolemma, a coalescence of the two membranes takes place. This makes it possible for the structures lying inside the spermatozoon to enter the cytoplasma of the oocyte. One calls this process the impregnation of the oocyte. Among other things the nucleus with the highly concentrated DNA, the centrosome that lies across the nucleus in the neck region and the mid piece with the mitochondria and the kinocilium (tail) are transferred.

The genetic material, lying in the nucleus and coming from the father, is unpacked and is used for building the paternal pronucleus. In what follows, the centrosome plays an important role in the convergence of the two pronuclei. Later - after the subsequent division - it will also be responsible for building the first division spindle of the new creature. All centrosomes in the bodily cells of a human originate from that of the father.
Other sperm components transferred to the oocyte cytoplasm, like the kinocilium, are dissolved. Effective processes also exist for eliminating sperm mitochondria from the cytoplasm of the oocyte.
Thus, all mitochondria in the bodily cells of an individual normally derive from the mother alone.

Fig. 39 - Impregnation of the oocyte

  1. Oolemma
  2. Cell membrane of the spermatozoon
  3. Kinocilium
  4. Nucleus (compact) of the spermatozoon
  5. Centrosome of the spermatozoon

Fig. 39

The membrane of the sperm cell head and mid piece coalesces with the oolemma and all of the sperm cell contents enter into the oocyte. The tail membrane remains as an appendage.

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In rare occasions, the mechanisms for eliminating sperm mitochondria are defective, allowing them to survive and prevail in certain tissues. Diseases caused by mutated mitochondrial DNA derived from the father have even been described.