During the fertilization process, a series of interactions take place between the sperm and the oocyte (egg), with the starting point of the process being the adhesion of the sperm to the membrane of the egg and the subsequent fusion of both cells, a process that involves molecular mechanisms that have been widely studied (read more HERE).

Among the proteins that play some role in fertilization is CD9, which was the first essential protein identified on the surface of the oocyte. Another two have been identified more recently: IZUMO I and JUNO, which plays a fundamental role in the fusion of sperm and egg, proven in mice (see HERE ). These three proteins are involved in the recognition of the gametes and in their adhesion, but do not induce membrane fusion.

The role of protein CD9 in the fertilization process has been confirmed, because if mice are produced in which this protein has been suppressed, their fertility is significantly reduced as the membrane of the eggs loses its ability to fuse to a large extent. In effect, CD9 facilitates adhesion of the sperm to the eggs by binding these to sperm receptors that bind them permanently with the oocytes.

IZUMO I is also important for this process, since mice lacking this protein are infertile, because their sperm cannot fuse with the eggs.

Similarly, JUNO plays an important role in fertilization, since it has been demonstrated that female mice missing this protein are healthy but infertile, that they produce mature oocytes, but that they cannot bind to the sperm to be fertilized.

The proteins CD9, IZUMO I and JUNO are also present on human gametes. However, knowing if JUNO is expressed on human oocytes and plays an important role in human fertilization has not yet been determined.

New experiments show that this result is also true for humans

Experiments have now been published (see HERE) showing that JUNO is expressed on the plasma membrane of human oocytes, and that its inhibition using monoclonal antibodies completely blocks the fusion process with sperm.

These results suggest that what is already known about the role of JUNO in fertilization in mice is also true for humans.

Are there any ethical difficulties in these experiments?

From a bioethical point of view, one important aspect to consider is whether these experiments were carried out in human embryos that were subsequently manipulated and even destroyed, which would not be acceptable. However, the authors specify that, since French bioethics laws prohibit the creation of human embryos for experimental purposes, the fertilization experiments were conducted in such a way that favors polyspermy , thus preventing the development of viable embryos.

If this is so, the ethical difficulties of these experiments could be resolved. However, the paper itself reports that in some cases, the fertilization was not polyspermy, and that the authors were even able to obtain a single embryo, which might cause the embryo produced in that case to be viable.

As always, this is a bioethical dilemma that is difficult to resolve when trying to reconcile experiments that are undoubtedly positive for the advance of science but which present objective bioethical questions.

Justo Aznar

Bioethics Observatory