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PERIODICUM BIOLOGORUM UDC 57:61 VOL. 111, No 3, 349–357, 2009 CODEN PDBIAD ISSN 0031-5362 Essay Facts and doubts about the beginning of human life Human embryo: a critical approach to bioethical reason A catholic perspective FERTILIZATION IN HUMANS AND FRANCESC ABEL THE ONTOLOGICAL AND ETHICAL STATUTE Institut Borja De Bioètica OF THE HUMAN EMBRYO Santa Rosa, 39-57 3a. plta. shall try to make explicit the catholic position on the topic heading / 08950 – Esplugues (Barcelona) this part of my presentation going through three statements dealing E-mail: [email protected] I with the most important problems posed in moral philosophy and the- ology, the discoveries and advances made in recent decades, in particu- lar, those referring to the processes of cell growth, differentiation, devel- Key words: Human Embryo, Fertilization, opment and death, so that the question arises of whether the principle of Ontological statute, Cell cycle, Stem cells, Bioethical reasoning, Catholic discurse, the inviolability of human life requires greater precision, both at the begin- Arguments & counterarguments, Animation, ning and at the end of human biological life. Magisterium, Anencephalic fetus, Dignity of human beings, The Borja Institute Statement One: Calling realities in earlier stages of a human embryo such as zygote, morula or blastocyst is debatable. Let us accept that, for the purposes of explaining to the general public, it is appropriate to unify terminology, without stooping to impractical hair-splitting. By this I do not mean to start – an otherwise sterile – argument on the possible evaluations of speaking of preimplantation embryo or pre-embryo. We can say that the human embryo is the structure that develops from a human zygote, through a series of divisions, differentiated into tissues and organs. The human zygote is the founding cell of an organ- ism, resulting from the meeting of two specialized cells called gametes, one from the mother (egg) and one from the father (sperm). Each of these has been duly prepared over a long process (meiosis), which leads, as the most relevant and visibly noticeable fact of the last phase of ripen- ing, to the systematic reduction of the number of chromosomes from 46 to 23. Curiously, these gametes cannot survive separately for very long, only a few hours, but following syngamy,their fusion gives rise to a new human life different from that of its parents, that lasts for years. Fertil- ization takes place in the upper third of the Fallopian tube and first af- fects the cytoplasm of the gamete cells and then their nuclei, thus mark- ing the two most relevant aspects of gamete fusion. Until relatively recently, the following statement was, for most peo- ple, fairly obvious: From the time that the ovum is fertilized, a new life is begun which is neither that of the father nor of the mother; it is rather the Received February 20, 2008. life of a new human being with his own growth. It would never be F. Abel Facts and doubts about the beginning of human life made human if it were not human already. To this is migration of the pronuclei to the centre of the perpetual evidence... modern genetic science brings oocyte. valuable confirmation. It has demonstrated that, from From a morphological point of view, fertilisa- the first instant, the program is fixed as to what this tion is not complete until the first embryonic divi- living being will be: a man, this individual-man with sion has taken place, since in humans, as in other his characteristic aspects already well determined. Right mammal species, the two pronuclei lie very close to from fertilization is begun the adventure of human each other but fusion does not occur. With the start life, and each of its great capacities requires time... to of the first embryonic divisions (mitosis) the mem- find its place and to be in a position to act. (9) branes of the pronuclei disappear and the two chro- This formulation appears questionable today, at least mosomic complements locate on the same meta- in the form. Later, we shall see whether it also affects the phasic plate. It is considered, then, that in humans content. there is no syngamy, understanding syngamy to be the fusion of the male and female pronuclei, there From the point of view of human development, mod- is only metaphasic synchronization. ern genetics states that penetration of the egg by the sperm does not take place in a moment, but that it is a The cell cycle is defined as a series of distinct process during which the sperm becomes prepared to fer- phases: G1, S, G2, and Mitosis (G=gap, S=syn- tilize the egg. It was believed formerly that shortly after thesis). The length of the cell cycle in human zy- fertilization, what we call syngamy took place, (fusion of gotes is 20–33 hours post-insemination, according the male and female pronuclei, and with it the appear- to the authors. ance of a new genome). Today, we can state that, strictly G1 – Completion of meiosis: extrusion of the speaking, there is no syngamy or fusion in humans. 2nd polar body and start of pronuclei formation. Average duration: 8 hours post-insemination (3–10 Fertilization in humans: cell cycle (2) hpi). Concomitantly to fusion of the spermatozoon with S – Replication of DNA. Phase of great nuclear the oocyte, the process of meiosis is reactivated (anaphase activity, in which the zygote is more likely to be II and telophase II) extruding the 2nd polar body and damaged. Average duration: 8–14 hours post-in- thus re-establishing the diploid character (2). The chro- semination up to 10–18 hpi. matids that remain at the inner pole of the oocyte, fuse G2 – Latent phase. Average duration: 4–6 hours. with each other and begin a migration towards the center of the oocyte at the same time as they decondense and be- Mitosis: Non-reductional division that ensures gin to be enveloped in small fragments of membrane, fi- the diploid nature of the two daughter cells. It be- nally constituting the female pronucleus. gins with the dissolution of the pronuclei and con- densation of the DNA in the form of chromosomes. * Meiosis: a process of cell division of germ cells Next these duplicate guaranteeing that each daugh- to form gametes (oocytes and spermatozoa). It is a ter cell has a copy of each chromosome and there- reductional division. Two successive divisions take fore that they are all, in principle, chromosomally place, but only one duplication of the chromo- equal. Average duration: 3 hours (22–31 hpi). somes. From a diploid cell, theoretically 4 different haploid cells are formed. In the case of spermato- Cell division into 2 cells occurs at 25–33 hpi. genesis, 4 spermatozoa are obtained from one sper- matogonium, while in oogenesis, a single viable Gametic embryos and somatic embryos gamete (oocyte) is obtained from one oogonium. Wewill call an oocyte (gamete from the mother) ferti- In the meantime, the male pronucleus begins to lised by a spermatozoon (a male gamete) a gametic em- form near the fertilisation cone. This process is far bryo, to use the current terminology. more complex than the formation of the female Today, however, we can have non-gametic human em- pronucleus and is regulated by factors of the oocyc- bryos, obtained through the »in vitro« transfer of the diploid te itself. If penetration takes place when the female nucleus of a differentiated (adult) cell to the cytoplasm of gamete is not sufficiently mature or in adequate an enucleated oocyte. Using the current terminology culture conditions, the regulatory factors may be we’ll call it a »somatic embryo«. missing or have a defective constitution that leads The question until recently received a clear answer: to the non-formation of the male pronucleus. human life starts at the moment of fertilization. Today, Following formation, the female and male pro- when on the one hand there is no such moment, and, on nuclei are morphologically indistinguishable, except the other, we have the option of obtaining somatic em- for the greater proximity of the female pronucleus bryos, we find ourselves perplexed by the question of to the 2nd polar body and the male’s proximity to whether we really know what we are talking about when the remains of the sperm tail structure (visible only we speak of a human embryo. Can we define first, with through the study of ultrafine sections under a sufficient clarity, when we have a human embryo? And, transmission electron microscope). The next step if we agree on an anatomical-physiological criterion of 350 Period biol, Vol 111, No 3, 2009. Facts and doubts about the beginning of human life F. Abel considering an embryo human from the first cell division In cloning by nuclear transfer there is no differen- up to 14 days after the start of the fertilization process tial gene imprinting, which guarantees correct gene (pre-embryonic stage), and from day 15 to the end of the expression and normal development of the embryo. 8th week after fertilization (embryonic stage), is it our Genetic reprogramming, by methylation, occurs ab- moral duty to ensure its »complete« proctectability? What normally in most cloned beings, which probably con- are we really saying when we say the human embryo tributes to the lack of efficiency of cloning. Malforma- must be respected as a person from the first moment of its tions or defects in clones suggest a deregulation of gene existence? expression, a deregulation that may be effectively asso- I think it would be worthwhile focusing on the scien- ciated with a problem of methylation.

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