Few doubts that determining whether the preimplantation human embryo is a living being of our species is one of, if not the most, fundamental issues in modern bioethics. Another thing is whether that living biological being is, or is not, a human person. That, however, is another matter that we shall leave for another time.

There are numerous scientific data that appear to establish that the preimplantation embryo is a living being of our species and not a cluster of cells with no differentiated structure and without the organization that characterizes living biological beings (read HERE).

Human embryo biological entity 

While this might seem to be a debate that only affects the biological nature of that early embryo, it is a crucial bioethical issue, because if that biological entity to which we are referring were only a cluster of cells, it could be manipulated in accordance with the interests of science and of society, with no ethical responsibility. If, however, it is a biological being of our species, i.e. a living human being, it should be respected in all circumstances, since the dignity intrinsic to its nature makes it a subsidiary of that respect.

The starting point of the evolutionary biological process of the human being is cell division of the zygote, understanding as such the result of fertilization of the egg by the sperm, both human of course.

However, a fundamental fact of this first cell division is that it is asymmetric, i.e. two new cells are generated, two blastomeres, of different sizes and, more importantly, different as regards their function.

While it might be thought that this asymmetric division is random, as would happen if it were purely mechanical, in fact, this first division already has a specific teleological function. As already mentioned, each blastomere will have a certain function: one of them will give rise to the inner granular mass from which the body of the embryo will be generated, and the other to the placenta. This division is regulated by fine biological mechanisms foreign to a disorganized cell structure.

Indeed, as Cayetano González says in Investigación y Ciencia (July 2016), “An essential aspect of the development of multicellular organisms is the generation of multiple and very varied cell types from a single cell.

In certain cases, this is achieved by asymmetric cell divisions, so-called because the resulting daughter cells receive different combinations of factors that determine their cell fate, i.e. the molecules that determine the type of cell that each of them will become”.

A complex and well-programmed system of divisions

In his paper (See HERE), González makes reference to another article, by Emmanuel Derivery and colleagues (See HERE), who studied the division of the cells that organize the sensory organs of the fruit fly, Drosophila melanogaster, revealing a complex and well-programmed system of divisions that essentially consists of two phases.

First phase: Towards the end of cell division, a structure composed of microtubules is assembled in the centre of the cell and moves equally towards both sides of the plane that will split the cell in two. The endosomes (molecular vesicles) are distributed uniformly on this structure, moving in both directions along the microtubules forming it.

Second phase: Just before the cell divides, the microtubules are destabilized to one side, with the result that the endosomes will spend more time on that side and will end up accumulating in it.

As González says, “Taking into account the ubiquitous nature and high degree of evolutionary conservation of the components involved, the mechanism described herein could be operational in other species and cell types in which asymmetric distribution of a load — vesicular or another type — transported by proteins that move along an asymmetric bundle of microtubules”. This could provide important clues to understanding the functioning of fundamental biological processes in higher organisms and among them, why not in the asymmetric division of the zygote? This would undoubtedly confirm that the human embryo from the zygote phase is a living being of our species that controls its development with special biological mechanisms, which in no way could occur in random cell clusters.

(See HERE more about the embryo entity)

Justo Aznar

Bioethics Observatory – Institute of Life Sciences

Catholic University of Valencia



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