Early human development

I would like to publicly congratulate Rev. Professor Emmanuel Agius for his article on syngamy (The Sunday Times, April 10). His philosophical arguments clearly bear out the scientific evidence available to date on this issue.

When Norman Ford first wrote When Did I Begin, he clearly pointed out that the really valid argument that may be used in identifying the point of human individuation during embryological development is not the argument of potential, but that of ontological development of the human being.

This means looking back at the time-fractions preceding a present moment and going back to trace the beginning of the existing individual, as much as possible. Ford himself was actually misled by the argument on twinning from the same monovular embryo. Had it not been for this, which can be explained by realising that a new individual arises when a new pluripotent cell separates from the developing cell mass,(1) Ford himself could have come to an entirely different conclusion. His conclusion could have invariably been genetic union of the two pronuclei to form the zygote, that is about 20 hours after penetration of the ovum by the sperm cell. Beyond that point, it is difficult to trace the ontogenic continuation of the same human individual for both anatomical and functional reasons.

Anatomically, when the sperm enters the ovum, it sets off a reaction to seal off the ovum from the outside environment and creates a number of ionic changes in the ovum which leads to certain changes in the female pronucleus. The sperm itself loses its tail and becomes known as the male pronucleus!

It is interesting to note that two or three hours after penetration, the genetic information in the female pronucleus changes substantially in a haphazard fashion, as the ovum passes through its so-called second meiotic division with sufficient crossing over and changes between chromosomes thereby ensuring that the result of this, is a genetically different female pronucleus than the one just before.

The excess chromosomes after this division are thrown out into the now newly called ootid, as the second polar body, which usually comes to sit just inside the outer border of the ootid. There is no way that the genetic information inside the now present female pronucleus is the same as that previous to the second polar body formation, so there is no way that ontological development may be traced beyond this particular point in time!

According to the different genetic result obtained within the second meiotic division, the new zygote will be able to specifically read for protein synthesis, which is the material basis for a new body. This reason alone should alert one to the fact that so it is not possible to have a human individual before this point.

At penetration, the cell is a human cell with the potential to become a zygote or embryo. If there is a human individual at penetration, it definitely is not the same one that exists after the second meiotic division! So do we get two individuals out of the same ovum? I think not.

Another point of interest is that the original female pronucleus after penetration and before the second polar body formation is diploid not haploid, and therefore prior to this point in human development, the total complement of genes between the male and female pronuclei, is triploid, that is containing three sets of chromosomes which is not exactly normal.

Interestingly, once the second polar body is extruded, this may itself be fertilised by a second sperm cell to produce monovular twins which are however genetically non-identical.(2)

After the formation of the ootid with the two pronuclei, male and female, each with a haploid (one half) set of chromosomes, the two come together, the nuclear membrane disappears and the new complete set of chromosomes of the zygote meet for the first time.

As Professor Jerome Lejeune (geneticist, first president of the Pontifical Academy for Life) remarked, this is the human embryo with a new combined set of diploid (one whole) human chromosomes. Therefore this must mark the beginning of a complete individual with consequent connected ontogeny.

The information from both the male and the female pronuclei is necessary to form the zygote, otherwise it will not develop into a foetus, but change into a shapeless hydatidiform mole, which sometimes happens when one of the two pronuclei dies out.

For those who maintain individuation from penetration, this would mean that there would first have been a specific individual, who then was changed at the second meiotic division into another genetically different second individual, who then disappeared completely when the hydatidiform mole formed. Rather preposterous, I would think! The reality is that with the formation of the hydatidiform mole, the individual never did exist and never will, because development is disordered!

We therefore know that the only really substantial change in human development, is when the male and female pronuclei each carrying 23 single chromosomes, transform themselves into a completely different entity with 46 chromosomes, the human zygote. Beyond this stage substantial change does not occur.(3)

Some are saying that the full complement of genes exists at penetration so there is a human individual here. They are alluding to the fact that since the full complement of genes exist in two separate pronuclei, that constitutes the full human genome, but again I ask when is this, prior to or after the second meiotic division? Or are there two individuals?

Some claim that there is an individual in the pronuclear stage because some new protein is being laid down and the two pronuclei have actually started some transcription of their DNA (transcription is the copying of the DNA information in the gene, onto RNA templates called m-RNA).

First of all, for the formation of protein as the physical basis of the new individual, transcription is not enough, but it must be followed by another process called translation, where another form of RNA called t-RNA, must bind to the m-RNA produced previously and bring the protein precursor units together, which then join and form the protein! This takes place outside the nucleus on another type of RNA called r-RNA which acts as an anchor and bottle opener at the same time.

At the pronuclear stage, transcription is spurious and haphazard and is not coupled to translation.(4) It is also non-directed. If one adds some DNA to a test tube and pours in the precursors of RNA, these will start to be formed spontaneously after some time. Does this mean that there is human life inside the test tube? This transcription is non-directional and passive. There is no human life in the test-tube.

Directional transcription together with the translation of the new body proteins only occurs after pronuclear fusion to form the zygotic nucleus. This is then activated at the four-cell stage of morular development at what is called Zygotic Genome Activation or ZGA. It is here that the zygotic genome directs the development of the new individual!(5) In fact up to this stage, the new cells only grow in number, not in size, as the existing cytoplasm is shared between the ever increasing number of dividing cells and it is only after ZGA that the cell volume increases in size too.

It is important to add that initial short segments of m-RNA called are needed from certain specific chromosomal transcriptions to read off the information for new transcription from other different chromosomes within the same nucleus, so it seems that all the chromosomes should be present together in one nucleus for the whole genome to be read properly.

This still leaves one unanswered question. The detection of new proteins in the pronuclear ootid. It seems that these new proteins are transcribed and translated by 'new' m-RNA which lay dormant inside the original ovum and sperm and which were produced by the mother and the father, so-called maternal and paternal m-RNA.(6)

It is a fact that fertilization and early zygotic development up to the four-cell stage of the embryo is under maternal m-RNA control from the ovum. If one removes both pronuclei of a developing ootid (enucleation), this will continue to develop spontaneously up to the four-cell stage of the morula or embryo and then stop suddenly.

Therefore it shows that the elements for fertilisation and very directed early embryological development already lie dormant within the ovum and they are then superceded by the directed control of the new embryonic genome after pronuclear fusion at ZGA. This process is called maternal to embryonic transition of developmental control, or MET for short.(7)

To my mind all this leaves no doubt that integrating the anatomical and functional imperatives of embryological development, the point of individuation lies with pronuclear fusion.

As somebody wrote previously, when one goes into these matters with questions, one comes out with answers, but when one enters with preconceived answers, one comes out with many questions!

References

1. Kennedy Institute of Ethics Journal, June 1999, Johns Hopkins University Press, Maryland, p. 138.

2. Scott F. Gilbert, Non-identical Monozygotic Twins, www.devbio.com/printer.php?ch=11&id=111

3. German, L.J., Fundamental Ethical Issues in Assisted Procreation, paper.

4. Schultz, R.M., The Molecular Foundations of the Maternal to Zygotic Transition in the Preimplantation Embryo. Human Reproduction Update, Vol.8 pg.323-331, 2002.

5. Ibid.

6. Scott F. Gilbert, Early Research Docuting Stored Maternal mRNAs, www.devbio.com/printer.php?ch=5&id=52

7. Allan King, W., Maternal to Embryonic Transition of Developmental Control, www.begc. crbr.ulaval.ca/themes/themes3.htm

Dr Michael Asciak, MD, M.Phil., MP (Nationalist) is chairman of the Bioethics Consultative Committee

michael.asciak@gov.mt