Fertilisation
The next morning at 8 o'clock, the oocytes are examined again to see if fertilization has occurred correctly. The oocytes are checked for small nuclei, called pronuclei (see arrow). One pronucleus is from the egg, the other from the sperm (it is shortly before syngamy takes place). If fertilization was correct, one can see two small pronuclei in the morning after oocyte retrieval.
All oocytes are cultures separately and photographed every morning to see which develops best. Although the oocytes are described with letter codes, a picture tells more than you can write down on a table. Besides
the number of pronuclei, the embryologist looks at how the pronuclei are located inside the oocyte in relation to each
other. Furthermore, she is looking at their size and how many small nucleoli there are inside
each pronucleus. She also estimates how the small nucleoli are located to each other inside the pronuclei. Finally, she looks for presence of two polar bodies and evaluates the thickness of the zona pellucida (*).
In the afternoon at 13:45, the embryologist looks whether the eggs have begun to divide for the first time (see photograph #2). If it has already happened at that time, it is noted as early cleavage, which is a good prognostic sign. Eggs with early cleavage are also investigated for presence of nuclei in both cells and whether the cells are equal in size. All oocytes with early cleavage are photographed once more.