Module
4
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Fertilization

The termination of the second meiosis of the oocyte

At the moment of impregnation one encounters the following condition in the secondary oocyte, which was created shortly before ovulation:
The mitotic spindle, arrested in the metaphase of the second meiosis, becomes active again due to the impregnation. From the docking and impregnating spermatozoon a signal for a resumption and termination of the second oocyte meiosis is triggered. One speaks now of an impregnated oocyte.

Fig. 40 - Secondary oocyte
at the moment of impregnation
media/module4/d5f_ohpostimprag.gif

  1. Mitotic spindle with chromatids
  2. 1rst polar body
  3. Pellucid zone
  4. Perivitelline space
  5. Cell membrane of the spermatozoon
    (Remainder as appendage)
  6. Kinocilium
  7. Nucleus (compact) of the spermatozoon
  8. Proximal centrosome of the spermatozoon

Fig. 41 - Secondary oocyte
impregnated oocyte
media/module4/d5g_sekimpeizel.jpg

2
1rst polar body

Legend
Fig. 40

The radially oriented mitotic spindle, anchored near the first polar body has become active and is captured here in the anaphase.

Fig. 41

Immediately after impregnation no trace of it can be ascertained in the secondary oocyte. The oocyte is distinguished by the presence of the first polar body. (Sometimes it is slightly segmented as seen here in expectancy of the coming division.

Vidéo

Video about the first polar body.

In the two hours following impregnation the mitotic spindle pulls the chromatids, situated around the equator, apart so that in the oocyte only 1n1C remains. A new formation of the 2nd polar body takes up the other 1n1C. The remaining first polar body bound by a thin cytoplasma bridge receives the same signal and also divides.

The termination of the second meiosis implies the division of the secondary oocyte (1n,2C) into a mature oocyte (1n,1C) by the expulsion of the 2nd polar body (1n,1C) into the perivitelline space.
Fig. 42 - 2 hours after impregnation
media/module4/d5h_2hpolkorper.gif

  1. 1rst polar body
  2. Nucleus (slightly unpacked) of the spermatozoon
  3. Proximal centrosome of the spermatozoon
  4. 2nd polar body (being formed)
  5. Remainder of the mitotic spindle with maternal chromosomes 1n,1C

Fig. 43 - Mature oocyte
media/module4/d5i_reifeoozyte.jpg

1
1rst polar body
4
2nd polar body (being formed)

Legend
Fig. 42

The 2nd polar body is being formed. Beside it the first polar body is visible while dividing (more info with video). The DNA of the spermatozoon nucleus has begun to unpack itself.

Fig. 43

Three polar bodies can now be recognized: The 2nd polar body and the first polar body that has divided.

In what follows the structures of the mitotic spindle dissolve. Mitotic spindles formed in the future will be formed from the centrosome introduced by the father.