Defining which biological entity can be considered a human embryo is an outstanding bioethical issue, since it can be manipulated depending on whether it is recognised as so or not. This affects a great many biomedical practices, especially in the ethical assessment of the use of embryonic stem cells.
Three biological entities in question
There are essentially three biological entities to which the condition of human embryo may be attributed: those obtained by natural fertilisation, those derived from human cloning, and the “parthenotes”, which are generated by activating the division of a non-fertilised ovum.
The requirement in order for each of the aforementioned biological entities to be considered as a human embryo, is that a living human being can be developed from it.
That this is the case with those obtained by natural fertilisation is beyond doubt. There is a larger question with respect to those obtained by cloning (somatic nuclear transfer), since although they have the complete genetic makeup characteristic of human beings, it has not hitherto been possible to confirm whether living human beings can be produced from them. This has,however, been achieved in various types of mammals (we need only remember Dolly the sheep) and non-human primates, such as those achieved by Mitalipov’s team in Oregon (USA). It even appeared that in April 2013, this same team managed to produce human blastocysts, from which they were able to derive functioning human embryo stem cell lines, although for obvious ethical reasons, the blastocysts obtained were not transferred to a woman to see if a pregnancy and birth of a child could be achieved. In any case, after Mitalipov’s experiments it appears, at least due to a prudent ethical decision, that the blastocysts obtained could be considered as human embryos, and as such should be treated with the respect that their potential human dignity merits.
The issue of “parthenotes”, human ova asexually activated to their division into cells is more complex. According to Nicolas Jouve, dean of Genetics at the University of Alcalá (Cívica 27-IX-2014), an unfertilised egg in which cell division is stimulated until it reaches the blastocyst stage should not be considered an embryo. The fact that it proliferates and resembles a human embryo does not confer it with the biological nature of a human being in an embryonic state. Since it has not been fertilised, the entity, which has been termed “parthenote”, lacks the genetic information necessary to develop, as it has only half of the chromosomes and genes necessary for development. There are no human beings with 23 chromosomes – developed human beings have 46 (23 paternal and 23 maternal). An embryo with half its chromosomes (more so if it is from a single parent) is in principle, biologically non-viable. On the other hand, when talking about a human embryo, we are talking about higher beings with sexual reproduction in which parthenogenesis does not exist as an alternative. Thus far has been stated by Jouve.
In essence, we believe that the naturally conceived embryo is undoubtedly a human embryo. The embryo obtained by cloning, while we cannot be absolutely certain, on a principle of prudency must be considered as such, and finally, we believe that “parthenotes” cannot be considered human embryos.