Fertilisation is the dynamic and temporal process by which an individual is formed from materials supplied by its parents. Fertilisation is a process with epigenetic temporal dynamics: the genetic information inherited is fed back by the process itself (by an interaction of the intracellular medium components) over the hours that this takes. The resulting zygote is more than the mere sum, or fusion, of the gametes.
Fertilization begins with species-specific recognition and mutual activation of the mature paternal and maternal gametes in the appropriate medium. At the site at which the sperm reaches the ovum, there is the release of calcium ions which diffuse like a wave towards the opposite side. The concentration gradient is perpendicular to the axis between the animal and vegetal poles of the ovum and goes along the dorsal-ventral axis of the body in the initial state.
Before recognition, the gametes are in a state of repression of genetic activity, and each one has to able to unlock the repression of the other. The chromosomes of the two gametes are prepared and organised in such a way that the zygote will attain its own genetic information. The processes that take place are regulated by the calcium ion levels reached in the corresponding zone. The DNA that forms each and every one of the chromosomes has chemical markers (a pattern of chemical modification due to the insertion of a methyl group in one of the four bases [cytosine] of the two strands that make up the DNA), markers that are different in the genetic material of the paternal and maternal inheritance. During the time of this process, the DNA of both parents chemically changes the pattern itself (parental imprint) until it becomes the pattern of the new individual. Only after these changes does the expression of the child’s own genome begin. A few hours after sperm-ovum fusion, DNA synthesis begins in both pronuclei. The paternal pronucleus attracts the maternal one; they are blended and arranged in one unit, moving towards the center of the zygote that is being formed. As they approach each other, the nuclear membranes dissolve and the chromosomes intermingle, fuse and are aligned (according to a plane determined by the pole inherited from the ovum and the point of entry of the sperm), ready for the first cell division of the zygote. The various components inside the cell are arranged in an asymmetric distribution, following the calcium ion concentration gradient. Furthermore, fragments of different types of sperm and ovum membranes are fused to give the zygote’s unique membrane by modifying the chemical composition of its components.
The cell, with the zygote phenotype, presents a cell organisation that constitutes its own entity, different from the that of the gametes or the biological starting materials. It differs from any other cell as it has polarity and asymmetry, since its components are rearranged along the axes that will establish, after time, the body structure, thus showing that it was constituted by a process of self-organisation of the biological material resulting from the fusion of the paternal and maternal gametes. The signal that directs the constitution and structure of the zygote is the change in the calcium level in the cytoplasm of the cell. With this, new information originates – the epigenetic information- which will be transferred thereafter in a harmoniously coordinated manner, both spatially and temporally. Its genome has the characteristic status typical of the initiation of a program of individual life. The zygote is the only totipotential single cell entity capable of developing into an organism following the life path generated. It is precisely the zygote state, as it has polarised asymmetric organisation of its components, which allows it to grow as an organism: a differential and ordered growth in which the cell multiplications are accompanied by cell differentiation. The zygote has more genetic information than the genome resulting from the mere fusion of the pronuclei of the gametes of its parents. In this sense, it can be stated that it is a true living being of its species; this reality is not to be confused with that of a living cell in an environment that allows it to grow, nor with a cluster of living cells. The zygote is, therefore, a living being with characteristics typical of “time zero” of life. It is an entity with inchoate body axes, not a simple cell. Its genome has been formed and activated at fertilisation. A spark has been lit, an enactment of the expression of the information from the genes, which are the new individual’s inheritance (Natalia López Moratalla. Extract from Persona y Bioética, 14; 120-140, 2014).