Fertilization

Graffiti art (LEFT) in the waiting room of the Frauenklik, Berlin. Courtesy of Prof.H.Kentenich.

Fertilization is an active process which requires an interaction between the sperm and the eggs. In the first instance the sperm must reach the eggs. In conception via intercourse the sperm is deposited against the cervix and after liquefaction of the semen the sperm swim through the cervical canal to begin their journey towards the egg. Only a few of the most active sperm will complete the journey up the uterus and fallopian tubes towards the egg, and usually only one will penetrate the zona pellucida to fertilize the egg. Multi fertilization is rare but does occur in about 1% of fertilizations.

 

 

The first action of the sperm is to remove the cumulus cells which surround the egg at ovulation or after egg recovery/pickup (in IVF). The oocyte (egg) shown on the right has a large number of cumulus cells (the corona) surrounding the egg in the centre. As a result of the action of the sperm, and during fertilization, these cells fall away from the egg.

In IVF they are often dissected away by the embryologist.



 The cumulus cells are thought to be responsible for the capacitation of the sperm which must take place before fertilization can take place. In IVF the culture medium in which the sperm are suspended result in capacitation of the sperm. Note that in the animated sequence (LEFT) the swimming action of the sperm rotates the egg.

In the diagram shown on the right the first polar body of the egg is clearly seen at 11 O'clock below a well defined zona pellucida (still capture from a video-sequence) in the CD-Rom "20 Years of Assisted Reproduction" - see link in the home page to CD-Roms and books).

Animation from video of ICSI Where sperm numbers or quality are so low that fertilisation by conventional IVF fails microinjection (ICSI) might be indicated. A single sperm picked up with a very sharp glass pipette is pushed directly into the egg. Several thousand of babies have been born worldwide and many couples have benefited from this technology, which uses some of the techniques applicable to cloning. Questions are still being addressed about the development of children born as a result of the technology.

However the process of fertilisation takes place - as a result of spontaneous conception through intercourse, artificial insemination, IVF, ICSI etc. - the sequences of cleavage and implantation are the same.

There have been suggestions that implantation can be enhanced by a process known as "assisted hatching" may help. A weakness, or hole, is made in the zona pellucida surrounding the embryo to help the embryo hatch from its 'shell' before implantation.