Evaluating fertilisation

An embryologist will examine the oocytes under a microscope 18-20 hours after insemination to verify if fertilisation has occurred or not. The presence of two pronuclei inside the oocyte and two polar bodies in the perivitelline space means that correct fertilisation has been achieved. One pronucleus corresponds to the female pronucleus and the other to the male. Each pronucleus stores the genetic material characteristic of the partner from which it comes, and the combination of these two shall determine the genetic makeup of the new embryo.


Fertilised oocyte (zygote) at 2PN+2pb stage

Some inseminated oocytes may present signs of incorrect fertilisation. When this occurs, the resulting embryos are not viable.

In some cases, none of the oocytes inseminated are fertilised. This fertilisation failure may be due to the male factor, the oocyte or a combination of these. But there are also occasions in which the cause of the fertilisation failure is unknown. When none of the inseminated oocytes have been fertilised, the case is studied to determine an alternative course of action.

 Non-viable zygote at 3PN stage
Non-viable zygote at 3PN stage

Correctly fertilised oocytes are deposited in culture dishes containing a special medium for the development of the embryo and stored at 37º under appropriate humidity and gaseous conditions for one or two days before being transferred. During this time, embryo development is monitored and those failing to show signs of correct cell division are disregarded.

Not all fertilised oocytes develop correctly. Very asymmetric cell division, the presence of fragmentation and/or multi-nucleation are indicative of a poor prognosis. If, due to past experience, it is considered appropriate to conduct a more detailed assessment of the embryo development, the in vitro culture period may be prolonged to the blastocyst stage. In these cases, the embryo transfer takes place 5-7 days after the follicular puncture.

Embryo at 4 cell stage (D+2)
Embryo at 4 cell stage (D+2)  

 

Embryo at 8 cell stage (D+3)
Embryo at 8 cell stage (D+3)


 Blastocyst (D+5)
Blastocyst (D+5)

 

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