When we listen to statements on early embryo development such as how a human embryo is not a human being, or that it is only a cluster of cells, or its biological significance is questioned or other similar phrases that try to minimize or take away importance from the early stages of human life, we must remember something as basic as that after fertilisation, the human zygote, resulting from the fusion of a human ovum (egg) with a human sperm, which receives human genes, is the first biological reality of a human life.

Th early embryonic development is as a clock of life starting

Once the zygote has been formed, the clock of life is started, and the integral elements of that simple cell are triggered to begin a dynamic process that is unstoppable (unless external factors intervene), so that the life that has just been established develops. Developmental Genetics is a modern and attractive branch of Genetics that tries to explain how a complete organism is organised from one cell. It reveals to us the genetic causes of the morphological transformation observed throughout the development of living beings, applicable equally for an insect, a plant or a human being. In a first approach, we know that allmulticellular beings start from a single cell, which by successive divisions and copies of the genetic information that the zygote already possesses, will give way to the different stages of development, growing not only in number of cells, but also in complexity. At first, the cells that make up the embryo seem to be the same, but this is not so; from practically the first cell division there is already a division of roles, and the two resulting cells will follow different paths in development: one will give rise to the outer protective structures and the other to the embryo itself. From the first cell division and henceforth with ever increasing contrast, various territories in what is the same embryo are demarcated. Thus, each cell, due to the position that it occupies in the embryo, will use the part of the genetic program that applies to it. Advances in Developmental Genetics and analysis of the genes involved at each time and place in the embryo have shown that, as concernsbody organisation “decisions”, all multicellular species have two types of genes: the “structural genes”, which are those directly responsible for the morphological structures, since when they are activated they give rise to the types of proteins that determine the specific function of each cell; and the “regulatory genes”, which are those that give the instructions for expression, so that the structural genes are activated at the right time and place.The system functions like an orchestra in which the conductor (the regulatory genes) decides when each member of the orchestra (each structural gene) should become operational, so that the score (the developmental program) is executed harmoniously, and little by little the concerto is completed.
In embryonic development, nothing is by chance or improvised, in the same way as the performance of a good Mozart sonata or Beethoven symphony is not.

There is a program that must go on in the early embryonic

There is a script, a score, a program that must go on developing step by step in a coordinated, gradual and continuous manner. Thus, the three properties that characterise embryo development specifically, from the first cell division are:“coordination”, “continuity” and “gradualness”. The embryo, being always the same, continues to grow in complexity, but it should not be regarded as a simple jumble of cells that are all alike; rather it should be considered as an integrated and organised whole, in which each part fulfils its function, fully interacting with the other parts. Once fertilisation is complete, the embryo is not a potential individual, but a human individual who has commenced his existence, an individual in whom all the necessary and sufficient conditions are met to independently and continuously reach the full potential for which he is genetically equipped. Embryo development takes place in a coordinated manner, due to a program of genetic activities that is perfectly regulated in time and space. The development is continuous, since it goes from one step to the next without interruption. It is gradual, as it happens step by step and, as time passes and the organism grows, different pathways of specialisation of the cells that comprise itareestablished. Having said that, a question arises, already answered by our understanding of Molecular Genetics, and thanks to the latest discoveries of the human genome:What determines that in some cells some genes are activated, while different ones are activated in other cells, since all of them contain identical copies of the genome that was constituted after fertilisation? Knowledge of how only the genes whose turn it is to be activated are switched on at each time and place in the embryo, under the baton of the regulation systems, is one of the brightest chapters in modern genetics. Each regulatory gene, in accordance with signals and stimuli from the inner cell environment itself, in turn dependent on its position in the embryo as a whole and the time of development, will send signals to the structural genes to activate or silence them. Thus, the different cells acquire different specialities.

Deciphering the Human Genomemolecular-machine

The next logical question would be: So, how are the orders sent from some genes to others? The answer to this question is one of the main breakthroughs derived from deciphering the Human Genome, and in particular the ENCODE program (Encyclopaedia of DNA elements). The regulatory genes send the order to act to the structural genes through signalling proteins, transcription factors or RNA molecules, which go to the activator, promoter or repressor regions, next to the genes that are to be transcribed or stopped from transcribing, when and where applicable.
However, it is very important to stress that the embryo is an integral whole at each moment, and that the development program runs in perfect coordination and interdependence of some parts and others at the cellular and molecular levels, andthat during the development a cascade of continuous signals, cell-cell messaging, and messaging from the external and internal environment of each cell occurs in the place in the embryo that it occupies. For this reason, we insist that from the first cell division, the embryo has a heterogeneous but structurally ordered organisation. It is not a mere cluster or accumulation of cells, a term that according to the Spanish Real Academia Espanola (RAE) dictionary is applied to the action and effect of joining things without order. In embryonic development, each part fulfils its role and everything is governed under perfect order.
The word that best defines this integrated whole is “organism”.What develops in a surprisingly ordered, coordinated, continuous and gradual way is an organism, a human being in its early stages of development. The same organism that after the first eight weeks will reveal a human form that only requires the time necessary to complete the formation of its tissues, organs and systems. Faced with this reality that science has revealed to us, who has the right to sever this unstoppable process of development of a human life?
Pope Emeritus Benedict XVI recalled in his homily at a vigil for “all nascent human life”, in St. Peter’s on 27 November 2010, that “with the ancient Christian writer Tertullian, we can say: “he who will be a man is already one. There is no reason not to consider him a person from conception”. In complete agreementand accordingly, if he who will be a human reality is already one, there are no reasons for the instrumentalisation of embryonic human life nor for abortion. Laws must extend to protecting human life from the very beginning, from fertilisation (Cívica, 24-II-2015. Nicolás Jouve, Dean of Genetics. University of Alcalá de Henares, Madrid, Spain).

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