What are an embryo’s nutritional requirements prior to transfer to the mother? 0During in vitro fertilisation processes, for the first few days of their lives, human embryos have to develop outside the mother’s body in special incubators.  Temperature and pH conditions need to be optimum and embryos also need to have access to all the necessary ingredients in order to feed and, in doing so, meet their energy requirements. Culture media are used for this.

The very first culture media were used experimentally on mouse embryos around the mid-20th century. They were simple, aqueous media, if we are to give them their chemical definition, and they could, therefore, be reproduced and manufactured in any laboratory. It was not until 1984 that Yves Menezo designed a specific medium for human embryos. That is, the B2 medium. It was special in that it was supplemented with amino acids from serum albumin. A host of studies carried out on laboratory animals culminated in 1997 with the development of specific or sequential media containing pyruvate and glucose as sources of energy and that facilitated embryo development up until the blastocyst stage (fifth/sixth day of embryo development).

How is a culture medium designed?

As well as water (60% of the composition), first of all, a defined quantity of ingredients need to be selected. On the whole, these are ingredients that are common to all the body’s fluids.  Some are fluids found specifically in the oviduct and uterus whilst others are chemical ingredients that cannot be found in natural form. Secondly, the specific concentration of each needs to be determined. Culture media are currently defined as complex since they contain over 12 ingredients although the base in all cases are standard, well-known saline solutions such as the balanced Earle solution or the Krebs-Ringer bicarbonate solution.

As well as pyruvate and glucose as sources of energy, the inclusion of amino acids (essential and non-essential ones) was key to the well-being and optimum development of embryos. This is down to the multiple functions that amino acids have in culture media: they are a source of energy, biosynthetic precursors, intracellular pH buffers and chelating agents for heavy metals.

Whilst the ingredients mentioned above cover the nutritional needs of human embryos, vitamins such as C, E, B2, B5 and B6 are also commonly added. They play an antioxidant role and stop and/or decrease the production of free radicals. Last of all, the use of antibiotics such as gentamicin or streptomycin is important and they are included in the composition of any culture medium with the aim of stopping contaminants from growing.

Culture media need to undergo strict quality controls during manufacture and endotoxin tests, osmolality measurements, prior tests using mice embryos (mouse embryo assay, MEA) as well as end sterility certification are essential. Storage at between 2 and 8⁰C and not breaking the cold chain until it is time to put the medium to use are also essential so that the properties are guaranteed.

Dr Jorge TenDirector of the Reproduction Biology Unit at Instituto Bernabeu

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What are an embryo’s nutritional requirements prior to transfer to the mother?
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