Although I am writing about co-culture in IVF, we are not currently doing any co-culture. With recent improvements in IVF culture media and techniques, our IVF pregnancy rates (without co-culture) have improved dramatically. Therefore, we no longer see a need for co-culture. In vitro fertilization with co-culture has been utilized in animal in vitro embryo culture systems for over 30 years and more recently in some clinical human in vitro fertilization laboratories as well. The basic concept involves growing embryos in a culture medium on top of a proliferating monolayer of cells such as fallopian tube cells or cells from the lining of the uterus called endometrial cells. The idea is that these cells, which are sometimes referred to as "feeder" cells or "helper" cells, will stimulate development of the embryos by removing toxins from the medium, adding growth factors, or some other beneficial effect. Some studies have demonstrated improved pregnancy rates and delivery rates with utilization of co-culture for human in vitro fertilization.
Why don't all IVF centers use co-culture? There are several reasons that co-culture is not currently more widely used for human IVF:
1. Co-culture involves a lot of tedious work in the laboratory which leads to additional expense.
2. Most IVF labs are not experienced with culture of cells other than eggs, sperm, and embryos. Although culturing cells from the endometrium or fallopian tube is not extremely difficult, it does involve learning some new techniques.
3. There is no universal agreement that co-culture is necessary to provide optimal pregnancy rates from human in vitro fertilization.
4. Another issue is that depending on the source of cells used for the co-culture there may be concerns about transmission of infectious diseases such as viruses from the cell line to the developing embryos. To date there have been no reported cases of viral transmission to a human fetus. Non-autologous cell lines should be screened for infectious diseases prior to use in human embryo coculture.
Coculture is usually not applied universally to all cases in an IVF program. It is usually reserved for use in the "poor prognosis" patients. Studies have suggested that these patients can benefit the most from IVF with co-culture. Examples of poor prognosis patients include women over 40, women with previous IVF failures, women with elevated FSH (follicle stimulating hormone) levels, and women who respond poorly to ovarian stimulation with gonadotropins.
In general there are two schools of thought in this area. One school says that co-culture can be of a benefit for some patients undergoing in vitro fertilization. The philosophy here is that we do not need to know the exact mechanism of the benefit of co-culture, or exactly how standard in vitro culture systems are deficient - what we want is to help the couple get their baby. More like experienced hunches & claims rather than solid evidence based medicine. The other school says co-culture is a crutch that masks the real problem which is sub-optimal in vitro embryo culture systems. These people would prefer to use very pure and carefully defined media in order to maximize the culture environment. They believe that this can yield an equally high pregnancy rate without the use of co-culture. This is what we strongly believe in. I believe that the heart of an IVF clinic are good solid culture systems.
Clinical in vitro fertilization programs that are utilizing co-culture for their human IVF generally use one of three cell types. However, there are numerous other cell lines that have been successfully utilized as well. The cell lines most often used are fallopian tube cells which can be from either human or animal origin, endometrial cells from the lining of the uterus, or Vero cells which are from an immortalized cell line derived from African Green Monkey kidney cells. Cumulus cells from around the egg with or without granulosa cells from the walls of the ovarian follicles where the eggs develop can also be used for co-culture.
Most commonly the eggs and sperm are mixed together on the day of egg retrieval without the co-culture cells. The next morning, after identification of the fertilized eggs (called zygotes), these embryos are then transferred on to the co-culture cells which have been prepared several days in advance. The embryos are then cultured with the helper cells until the time of embryo transfer. This is usually two more days of culture.
Another potential application of co-culture for human in vitro fertilization programs is that of culturing embryos to the blastocyst stage and then performing blastocyst transfer. This allows selection of embryos that have been able to survive through the early cleavage stages of the first five days after fertilization. It is generally very difficult to get good numbers of high quality blastocysts when culturing in defined medium (no co-culture). This technique can allow transfer of fewer embryos while still maintaining an excellent pregnancy rate. For example, some programs have cultured embryos to blastocyst stage and had very good pregnancy rates resulting from transfer of only two blastocysts. This would greatly reduce the risk of high order multiple pregnancy that is seen in some programs transferring higher numbers of embryos.
Further research is needed in order to define exactly which patients would be benefited by co-culture. Also, the co-culture technique itself may be able to be further modified such that in vitro embryonic development is even better than what can be achieved with current technology. For example, many aspects of the co-culture technique could be altered, such as using a different cell line, a different medium, smaller droplets for culture, changing the medium more frequently, or other changes. By varying the usual co-culture techniques, we might obtain a further improvement in embryonic development over what is currently possible.
A practical problem with research in this area is that studies using variations on standard techniques are relatively easy to perform using animal embryos, but studies using human embryos are problematic to set up and implement. Results from co-culture studies done with animal embryos will not necessarily be applicable to IVF with human embryos.
Much has been learned about co-culture both for animal in vitro embryo culture and for in vitro culture in the human as well. Studies continue to attempt to discover exactly how co-culture improves embryonic development. If the cells make certain products that stimulate development of healthier embryos, these products might be able to be produced commercially and added to conventional culture media.
It is possible that pure and exactly defined chemical media might someday be so improved as compared to what is now in use that co-culture would not offer any increase in pregnancy rates, even for poor prognosis patients. However, we do not appear to be at that point today. Further co-culture research is needed before we make tall claims about this being a panacea & a potential Nobel prize winner.
