4.6 The fertilization is complete.
The formation of the zygote



Approach of the pronuclei

Quiz

Quiz 27


The paternal and the maternal pronuclei move towards each other with the help of microtubules, which begin to be formed immediately after impregnation, i.e., by the penetration of the spermatozoon. They grow in a star-like pattern out of the paternal centrosome directly beside the forming paternal pronucleus (= formation of an aster made of dozens of microtubules). The microtubular proteins themselves arise from the cytoplasma of the oocyte. Fig. 46 - 6 hours after impregnation  Legenda

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5
Paternal pronucleus
Maternal pronucleus
Paternal centrosome
"Inner bodies"
Maternal astral microtubule

Fig. 46
Schematic diagram of an impregnated oocyte: An aster has formed out of what was originally the paternal centrosome (more info). In the pronuclei the so-called «inner bodies» can be seen, which are lined up as can typically be observed.



While the microtubules of the aster pull the pronuclei together in the center of the oocyte, the synthesis of the DNA is taking place in the pronuclei. This duplication takes roughly 12 hours. The pronuclei grow in size in this time. Fig. 47 - 18 hours after impregnation  Legenda

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Paternal pronucleus
Maternal pronucleus
Duplicated paternal centrosome
"Inner bodies"

Fig. 47
The maternal and the paternal pronuclei have grown to their maximal size. They have approached each other closely and the centrosome has divided in preparation for the upcoming cell division.
Video (255 kB)



Commentary

How can one distinguish the paternal pronucleus from the maternal one? Two features can serve as aids: The maternal pronucleus is nearer to the polar bodies. The paternal pronucleus is slightly larger. Since the observation angle when looking at an oocyte with a microscope is not always optimal, it can easily happen that it is not possible to determine these associations unambiguously.

Fig. 48 - 20 hours after impregnation  Legenda

Fig. 48
Impregnated oocyte 20 hours after an assisted fertilization. The cumulus cells are routinely peeled away. The pronuclei have maximally converged, but they are not clearly recognizable (video 190 kB).This is of importance for clinical practice (more info).



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