Pre-Conception Attraction Complex
"In humans fertilization takes place in the ovarian (fallopian) tube. Under normal conditions the egg cell arrives in the first (proximal) part of the tube directly from the ovary In the mean time the sperm cells have completed a long journey up to there. They have been deposited in the female vagina and have swum all the way from the vagina via the uterus to the ovarian tube. Many millions of them (more than ninety percent of the number that were present in the male ejaculate) have passed away or have become out of order by all kinds of biological barriers that crossed their way (e.g. the sperm hostile properties of the cervical mucus). Anyhow there exists a reasonable chance for both gametes to meet each other.
... At the end some tens or hundreds of sperm cells will actually reach the egg cell and organize themselves in a circular or radial orientation with their heads directed towards and concentrating on the egg cell.
At this moment so-called nutritive cells, the corona radiata, still surround the egg cell. From the evidence of the in vitro fertilization procedure it is known that in the next phase a so-called pre-conception attraction complex (PCAC) is generated for several hours (see figure). Under the influence of the substances secreted by the egg cells and the nutritive cells, the sperm cells now undergo important changes. For example they lose their so-called acrosome (outside shell). Without this happening a sperm cell is not capable of fertilization at all. On the other hand the presence of sperm cells and related substances obviously evokes chemical reactions in the egg cell and her coat (zona pellucida), making her more receptive for the eventual fusion process between the two cells. So it is obvious that the mere existence of this biological attraction complex is a necessary condition for the actual process of conception. Both cells seem to exchange and settle mutually within the chemical and biological conditions for the eventual decision whether or not a sperm cell will enter (fuse), and if so, where, which one and when. In a very subtle mutual process of encounter and exchange of signals and substances both cells are prepared for the actual process of fertilization and conception.
In the context of the dynamic morphological considerations of this essay it is important to establish that now a biological entity is formed by an egg cell with some sperm cells. We are dealing with a state of activity, that is more than just a kind of passive composition and sum of two cell types. Specific interactions take place within this biological complex. It is a biologically active and interacting whole that is occurring here. Within the initial few hours that this complex exists, a conception is possible, but whether this actually happens or not depends on a large number of subtle reciprocal chemical interactions and exchanges. Eventually it might result in a fusion of the cell membrane of the egg cell with that of a sperm cell. It should be emphasized here that describing the whole process as the penetration of a sperm cell is an inaccurate description. If the circumstances and conditions at a given moment and at a given place are appropriate, only then can the fusion of cell membranes take place and the content of the sperm cell (nucleus and the small amount of cytoplasm with some important cell parts) be brought into the egg cell. The continuity of the egg cell membrane is never interrupted or broken! The very common and somewhat aggressive image of a sperm cell penetrating the egg cell is not correct! In the pre-conception attraction complex there is no question of an active partner versus a passive partner complex, nor of a penetrating versus penetrated partner, nor fertilizing versus fertilized one. Rather cell and cell qualities are equivalent as a subtle equilibrium of exchange and interaction are maintained. The morphodynamic process of fertilization rather is like the gesture one may observe so very often in the animal realm when mating behavior and mating rituals are taking place. In a nearly never ending process of exchanging signals, of attraction and repulsion, a male and female animal can circumambulate each other before copulation happens. Almost literally this animal image, this gesture, this morphodynamics becomes discernible in the phenomenon (that is also observable during in vitro fertilizations) that the whole pre-conception attraction complex (PCAC) exhibits a tendency to rotate. The linear (radial) movement of the sperm cells turns into a spherical motion."
Jaap van der Wal
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