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Does CAT help to improve IVF pregnancy rates ?

Posted Jun 20 2010 5:07am

IVF patients are always on the lookout for innovations which will improve their chances of success . For example, many IVF patients ask us whether we do IMSI or CAT. These are techniques which have been aggressively promoted in the press, but do not really help the patient at all. I have written about IMSI in an earlier post. Let me discuss CAT ( cumulus-aided transfer) in this post.

IVF doctors have always been frustrated by the fact that though we are quite good at growing embryos in the lab, most of these embryos do not become babies. Embryo implantation is an inefficient process, and trying to ensure that every embryo we transfer becomes a baby is the "holy grail" for all IVF specialists , which is why we use techniques such as blastocyst transfer and laser assisted hatching, to try to facilitate the implantation process .

One logical way to increase success rates is to try to improve lab quality conditions, so that in vitro conditions match in vivo conditions as closely as possible. So what are differences between growing embryos in a plastic dish in the IVF lab and in the human body ?

In the body, the embryo is bathed in nutrients provided by the cells lining the fallopian tube until it reaches the uterus. In the lab, we grow embryos in plastic dishes containing culture medium, which contains a mixture of chemicals which are cleverly designed to support the growth of embryos.

In the past, in order to help embryos to grow in vitro, doctors would add the patient's serum to the culture medium. The hope was that this serum contained ( unidentified) biological growth factors, which would help the embryos to grow well. However, we learned that along with growth factors, the serum also contained embryo-toxic factors, which could actually inhibit the development of the embryo, and we stopped adding serum to the culture medium. Meanwhile, as we learned more about the biochemistry of the early embryo, manufacturers tweaked the chemical composition of the culture medium ( using a mix and match of amino acids ) , so that it because more embryo friendly, and could support the growth of embryos more efficiently.

A lot of research in the early days involved co-culture , and you can find lots of references to this if you do a Medline search for IVF co-culture. Basically, this involved culturing the embryo on a bed of "feeder cells", so that these cells could support the growth of the embryo. Ideally, the feeder cells should be the cells lining the fallopian tube, but these were very difficult to grow, which is why doctors tried using endometrial cells for co-culture; as well cumulus cells ( which were collected from the follicular fluid at the time of egg collection). Using cumulus cells for co-culture is a very simple technique and seems very appealing. This is what was christened CAT or cumulus aided transfer. Unfortunately, the success rates with CAT are no better, which is why the technique never caught on and few IVF clinics now offer this option. After all, every clinic wants to improve their success rates, and if such a simple technique worked, then everyone would use it.

So why don't we ( and the vast majority of IVF clinics all over the world) offer CAT?

On closer analysis, it's quite easy to see why CAT does not help. For one thing, in real life, the cumulus cells do not nurture the embryo in vitro. After fertilisation, the cumulus cells disperse, so that the embryo is floating free in the fallopian tube and is not surrounded by cumulus cells in vivo . Also, transferring the cumulus cells along with the embryo into the uterus makes little logical sense because cumulus cells belong in the ovarian follicle - not in the uterus ! In the uterus, the cumulus cells are "foreign" and are unlikely to help in embryo implantation !

Caveat emptor - let the patient beware ! More is not always better - and often established techniques are much better than newer ones !
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