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The evolution of culture media began from a basic salt solution designed by Ringer based on constituents of serum to the sequential media, catering to the needs of embryo metabolism, until the current trend of a single medium from day 1 to blastocyst. What exactly do we need to know before we decide the best for culture? To begin with, fertilization under natural circumstances takes place in the ampullary portion of the fallopian tube. Hence the constituents of the oviductal fluid play a vital role in metabolism and cell division. Basically there are 3 important components which may be classified as electrolytes (sodium, potassium, chloride and bicarbonate), non electrolytes (glucose, pyruvate, lactate and amino acids) and macro-globulins (albumin and immunoglobulin G). While the former act to maintain the pH and osmolarity, the latter (non electrolytes) enable cleavage of embryos. Of these lactate and pyruvate play an important role in days 1-3 and glucose takes over thereafter. The albumin provides the source of amino acids and the immunoglobulins act against microbial contamination. The commercially available media can only mimic as close as possible the components of the oviductal fluid, as the exact concentrations of its constituents cannot be accurately ascertained
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Popular medias have based the concentrations of the vital ingredients like lactate, pyruvate, glucose and amino acids on two principles “ Back to nature” and “Let the embryos choose”, justifying sequential culture. On the other hand, the switching of embryos from one medium to another may cause an osmotic shock and deprive them of any autocrine or paracrine factors that they may have secreted in to the surrounding media during culture. The challenge to this thought came in the form of a single medium devised on the basis of sequential simplex optimization, whereby the embryos studied were cultured in a “Start” media in which the concentration of essential constituents were varied and optimized to overcome the two-cell block. The results obtained from culturing embryos in sequential and single medium have been comparable according to studies. On one hand, although it seems less cumbersome there is always the question of the integrity of gene expression in cultured embryos which ultimately determines its normalcy and ability to implant and form a healthy baby. Hence this would be an ongoing food for our thoughts as we continue to decipher the appetite of embryos in need of the ultimate and optimum culture environment.
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1. Michael C.Summers and John D.Biggers. Chemically defined media and the culture of mammalian preimplantation embryos : historical perspective and current issues. Human Reproduction Update, Vol. 9, No. 6, 2003, pp. 557-582.
2. John D Biggers and Catherine Racowsky. The development of fertilized human ova to the blastocyst stage in KSOMAA medium : is a two-step protocol necessary? Reproductive BioMedicine Online. Vol. 5, No.2, July 2002, pp. 133-140
3. John D. Biggers. History of KSOM, A single medium for Embryo culture. Fertility World Volume 3. www.ivfonline.com |
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