Confused on Fertilization

Another failed effort to show that human embryos are not human beings.

[This article was co-authored by Patrick Lee and Robert P. George.]

Fertilization in humans and other mammals produces a new member of the species in the embryonic stage of its natural development. That is to say, the entity produced by the union of spermatozoon and oocyte is a complete, though developmentally immature, organism. Unlike the gametes, it is not merely part of another organism; nor is it merely something that can be used to produce a complete organism. At fertilization, the ovum and the sperm cease to be and something new comes to be — an organism (the embryo) whose genetic constitution and epigenetic state orient and dispose it to develop in the direction of maturity as a member of the species.

Now, someone might object to this position on the ground that, after the sperm penetrates the ovum, it remains possible with modern technology to extract that sperm. So fertilization does not, someone might argue, result in the gametes ceasing to be. And if an embryo had been produced by their union, what happened to it? Did it die?

This objection, which we recently encountered when a student mentioned that one of her molecular-biology professors had referenced it in class, could be referring either to (1) the in vitro fertilization technology known as intracytoplasmic sperm injection (ICSI), where a technician uses a pipette mechanically to insert a sperm into an ovum, or (2) fertilization brought about by contact between sperm and ovum via sexual intercourse or conventional in vitro fertilization.

Consider the second alternative first. In normal fertilization, many sperm penetrate the corona radiata of the ovum (a layer of follicle cells surrounding the ovum). Then, typically only one sperm will penetrate the zona pellucida (a film of glycoproteins surrounding the oocyte) and reach the oocyte. The sperm’s membrane then fuses with the actual membrane of the oocyte. This fusion triggers changes in the oocyte (or rather, what was the oocyte) so that (a) the membrane of this new cell undergoes a rapid polarization, and (b) a calcium wave is produced throughout the new cell’s cytoplasm so that the zona pellucida hardens over approximately 30 minutes and repels penetration by sperm. These facts indicate that what is living at this point is not an ovum.

With the fusion of the sperm and the ovum, the tail of the sperm is lost, and the membrane surrounding the head of what was the sperm joins the surface membrane of the former oocyte, creating a single, continuous membrane. This allows cytoplasmic factors derived from the ovum to affect the nuclear contents derived from the sperm — for example, new types of histones begin to be associated with those chromosomes, modifying the behavior and interaction of the molecules in these chromosomes. This shows that the sperm has ceased to be.

At this point, the genetic material from the ovum (the female pronucleus) and the genetic material from the sperm (the male pronucleus) are both contained within a single new cell, are being moved toward each other, and will eventually intermingle. This is the point just after the fusion of the membranes of the sperm and the ovum, when the ovum and the sperm cease to be, and a new organism — a whole human organism — comes to be. Could the sperm still be retrieved from inside the ovum? No, because the sperm no longer exists. At best, the male pronucleus could be extracted from the zygote (the new, one-celled organism). The result would not be a sperm and an ovum, but only nuclear material from a zygote on the one hand, and a disabled embryo (or perhaps the death of the embryo) on the other.

The second alternative is that the objection could be contemplating ICSI. When the sperm is mechanically inserted into the ovum, it is not clear precisely when the ovum and the sperm unite and cease to be, or at least it is not clear to us — and we have not taken a position on the question. It seems reasonable to hold that if the sperm can be retrieved and still behave as a sperm, without the assistance of extensive manipulation, fertilization has not yet occurred. These points are in no way incompatible with the position that a new, whole, though immature, human organism is generated at fertilization — whether it occurs in vivo or in vitro.