Embyology for RE's
Reproductive endocrinologists spend a great deal of time doing research during their fellowship, but are not actually taught about the nuts and bolts of embryology. Setting up a lab is also not discussed, as I think many programs worry that their graduates will set up a lab across the street.
Embryology boils down to recreating human physiology. It is not different than an ICU, but an ICU for embryos. The embryos need to be kept warm, fed, and have their rooms cleaned every so often.
Culture media is very similar to human plasma. I have done blood gas measurments on media to be certain it is as the label says it is. It has physiologic levels of sodium, potassium, glucose, etc. It has to, as the goal of culture media is to recreate the human fallopian tube.
Culture media also has anti-oxidants and antibiotics, which really help the culture system. Many RE's try to make their egg retrievals look like a sterile procedure, but they are anything but that. Like ENT surgery, it is pretty impossible to make any perineal or nasal procedure sterile.
Nonetheless, infection is relatively uncommon in IVF. The antibiotics work well and do not seem to be deleterious to embryo development. In addition, most embryologists add 10% or 20% protein (HSA or SSS) to their culture media. This keeps the embryos from sticking to the dishes, as well as provides a buffer for VOCs.
Incubator Environments
As with culture media, the goal of an incubator is to recreate the fallopian tube environment. In IVF, they speak of oxygen and CO2 levels in terms of percent, whereas in medicine, we speak about these levels in terms of O2 saturations or mmHg.
The fallopian tube is a relatively hypoxic environment with a normal CO2 level. 6% CO2 correlates witha PCO2 of 44 mmHg, which is normal for a healthy person. Maybe just a touch on the acidic side. 5% O2 correlates with a PO2 of around 38, which is an O2 sat of about 50%. Embryos like to be hypoxic.
A low oxygen environment will take your pregnancy rates from around 40% to 55%. That is a pretty big jump.
This does come at a cost, as low O2 incubators, also called "tri-gas" cost more. Then, they use large amounts of nitrogen (nitrogen is injected into the incbuator chamber to displace and lower the oxygen environment.) There is additional maintenance and testing equipment as well.
But, most IVF centers in the US are using low oxygen environments, to it is pretty much the standard of care in this country.
Acid-Base Physiology
This is a subject that is never a favorite of medical students, but is very important in IVF. Many people worry themselves to death that their incubators are not set exactly at 6% CO2. The issue is not what the setting is, but what is the pH. Embryos are very sensitive to changes in pH and each culture media system is designed to work at an optimal pH.
Whether it is due to altitude or the fact that you have a thermocouple CO2 sensor or an infra-red CO2 sensor that is out of calibration, the CO2 setting is merely a tool to get your pH where you want it.
The Henderson-Hasselbach equation applies to culture media just as it does to blood. Here is the modified equation.
[ H+ ] x [ HCO3- ] = k x PCO2
Basically, small changes in CO2 concentration, can produce big changes in pH. Embryos do not like any changes in pH, so it is important to get your pH right and make sure it is consistent.
There are some labs (usually at higher elevations) that have to set their CO2 settings at 7% to obtain their target pH. Other places need 5%. In the end, what matters most is the pH and is that pH giving you the results you want.
Culture media systems
Culture media has received a great deal of attention in the last few years, with much emphasis on two set media systems. Dogma has been that two step media systems are much better than single step systems, mainly because of the metabolic change that occur in embryos at day three, when the begin utilizing glucose rather than pyruvate for energy production.
Unfortunately, as frequently occurs in medicine, the two step media systems are being found to not be that superior to single step systems and people are returning to single step systems.