Approach of the pronuclei
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.
- Paternal pronucleus
- Maternal pronucleus
- Paternal centrosome
- «Inner bodies»
- Maternal astral microtubule
Schematic diagram of an impregnated oocyte: An aster (more info) has formed out of what was originally the paternal centrosome . 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.
- Paternal pronucleus
- Maternal pronucleus
- Duplicated paternal centrosome
- «Inner bodies»
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.
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.
