Crucial first cell cycle

Whether you believe that life begins at conception or not, the life of each and every one of us began at the pronuclear stage, when two mortal cells came together and transcended into the continuum of immortality of humans as a species. There are many critical stages in the development of a human being but the first cell cycle stands out in its significance, ever more so as we advance our knowledge about it. In this brief article I will discuss the three most crucial events of the first cell cycle, and how they may be affected by embryology practitioner now, and into the future.

First, during natural fertilization, the development is set into motion by calcium ion waves elicited by a factor in the sperm (1), most likely PLC zeta (2), located initially in the head of a spermatozoon and subsequently migrating into the pronucleus. Experimental evidence suggests that full term development is also possible following parthenogenetic activation (3). Those studies, and Ozlil’s work (4) in particular, clearly demonstrate that the duration and amplitude of calcium oscillations affect embryonic developments after implantation. On the other hand, from clinical experience with sharing of donor eggs, we have learned that embryonic development is affected by a fertilizing spermatozoon. Therefore, it is plausible that even though the activation itself is all or nothing phenomenon, the amount of PLC zeta carried by a given sperm cell could affect the duration and amplitude of oscillations and thus produce an effect similar to that observed in Ozil’s experiments. In the same way that intracytoplasmic sperm injection turned out to be more effective than natural fertilization, we may one day find that artificial activation is more efficient and allows more control over the activation process than natural activation.

The second crucial event involves the telomeres. There is accumulating evidence of the importance of the length of the telomeres for the life span, probability of cancer, and the length of reproductive life. Because it has been well established that the length of the telomeres shortens with every cell cycle due to inability of telomerase to replicate in 5’-3’ direction (5), it is obvious that at some point during reproduction length of the telemeres must be restored. However, until recently, the stage and mechanism of this restoration remained a complete mystery. It was not until 2007 that this puzzle was solved. It turned out that telomere length abruptly increases during pronuclei stage by a telomerase independent mechanism through sister chromatid exchange (6). This is the only opportunity to reset the length of the telomeres in an individual’s lifetime. It is possible that a better understanding of how telomere length is restored may allow us to extend and improve life span. Only the embryology practitioner will be in a position to accomplish that. Furthermore, we must realize that we may already be unknowingly affecting this process, and will have to wait for a few more decades to see if our in-vitro interventions are affecting life span.

The repair of DNA damage is the third critical event happening during the first cell cycle. By the time of fertilization, both gametes, particularly the oocyte, have accumulated some damage. Of particular concern is DNA damage by by-products of oxidative phosphorylation, which may result in as many as 1 million individual molecular lesions per day. Data suggest that, in the oocyte, the telomeric region of the chromosomes is particularly vulnerable, leading to chromosomal non-disjunctions, the majority of which take place during meiosis I (7). In the sperm, no regional preference for chromosomal damage has been clearly demonstrated, while there is overwhelming evidence that sperm cells accumulate very significant damage to their DNA, particularly in cases with abnormal sperm parameters. Unlike the oocyte, the spermatozoon has virtually no mechanism to repair DNA damage.

Thus, one of the critical missions of the first cell cycle is to repair the DNA from both gametes. The accuracy requirements for this repair are much higher than for any other cell, as it will serve as the template for all other cells. Reparative DNA synthesis must precede DNA replication. If the damage was extensive, it may increase the length of the first cell cycle. In any other cell, this increase would not matter much, because of the checkpoint that prevents premature activation of p34 kinase, which does not begin phosphorylation of histones before the DNA is replicated. However, during the first cell cycle cell, division is not as tightly coupled with chromosomal replication as it is in any other cell. In fact, experiments show that the enucleated zygote will undergo cell division, albeit disorganized, around the time when it would normally take place. Furthermore, an experiment with okadaic acid (OA) clearly demonstrated uncoupling between completion of DNA replication and nuclear envelope break down (8).

The exact mechanism for that is unknown. One hypothesis states that OA specifically activates H1 kinase, although it is more plausible that it simply gives advantage to H1 kinase, removing a counterbalancing influence of a phosphatase. However, whatever the mechanism, remarkably it only forces the zygote into premature chromosome condensation, but does not affect 2-cell or any later-stage embryos. These and other observations suggest that the normal progress of the first cell cycle relies, at least in part, on a general balance of phosporylation/dephosphorylation, and that shifting it one way or the other will delay or accelerate its pace. One of the factors that has a powerful impact on this balance is the pH of the culture medium, which is in turn affected by CO2 concentration, specific for each culture medium. Therefore, ‘playing” with pH gives us an unique opportunity to control the first cell cycle in a multitude of ways. Beyond the cell cycle, it is likely to also impact DNA methylation, responsible for gene imprinting, which was shown recently may differ drastically between in-vitro and in-vivo generated embryos (9). The significance of this is to be understood in the future. In summary, from the moment of fertilization, our future existence is molded by epigenetic pressures which come in the variety of forms. Therefore, embryology practitioners are to a large extent, responsible for establishing a strong foundation for the future of the human being that may be born as a result of their efforts.

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