Some time ago I claimed that the question about the beginning of life doesn’t belong to the field of biology, but should be a domain of philosophy rather than natural sciences. I believe I need to elaborate on this thought. Disputes over abortion, contraception, in vitro fertilization and basically all subjects aiming at peeking into one’s underwear are filled with same old vague – and technically speaking – meaningless terms: the moment of conception, the starting point of a human life, the moment of fertilization and so on. All of them are as woolly as sheep. Why? Well, y’all come close, let’s have a talk about the birds and the bees!
The first and most important thing:
Fertilization is not a moment. It’s a relatively lengthy and complex process.
What is taught at school, and what comes to our mind when we think about fertilization is more like fractions of a second – there comes a spermatozoon (sperm cell) , there is an egg, boom!, they join and that’s all there is to it. Suddenly the zygote is formed. In reality, fertilization encompasses many subsequent phases and doesn’t happen quick as a flash.
Here we go.
The sperm cells race
To begin with, the most common mistake in understanding fertilization is the sperm cells’ race towards the oocyte. We believe that, immediately after an intercourse, a gruelling race takes place in female genital tract, and the only thing that’s on competitors’ mind is to reach the oocyte as quick as possible. Well… just like with any other simplification, there’s a grain of truth in it, as well as a handful of lie. And it’s all because of a phenomenon called…
Capacitation is an extended sequence of biochemical changes that each sperm cell has to undergo in order to gain fertilizing ability. To launch the process, spermatozoa anchor themselves to the inner wall of female genital tract, and let certain enzymes, proteins and lipoproteins modify parts of their acrosomal membrane (i.e. part of the membrane covering their acrosome – a sack containing all the substances necessary to pass through the oocytal membrane). Therefore, only damaged sperm cells rush towards the oocyte immediately after entering female reproductive tract. It’s knocking-off time for all the rest. It takes them few hours to gain the ability to fertilize an egg. It has been proven that first sperm cells reach female gamete in about an hour after an intercourse. Fertilization, however, can take place even a couple of days after the coitus, which proves that first come, first served is not necessarily applicable to sperm.
Sperm cell vs. oocyte’s bodyguards
Okay, let’s assume that our spermatozoon underwent capacitation successfully, went on a journey inside the female genital tract, and finally reached the right spot in the Fallopian tubes. It sees the oocyte. Now what? Does it just… go in? Is this the magical moment of fertilization?
Hold your horses – we’re not there yet. There’s still a long way ahead of him because the oocyte is not stupid and is heavily guarded. The oocyte has produced the so called zona pellucida, and as if that were not enough, there’s also a squad of cells known as the corona radiata. The follicle cells (which form the zona pellucida) are joined together with a substance rich in hyaluronic acid (that’s right, it’s not only used to plump wrinkles). But that’s all nothing for the sperm cell as he has the ability to secrete hyaluronidase – an enzyme that digests a patch through the corona radiata.
It’s close to the oocyte now:
But there’s still the zona pellucida to force. It’s a thick glycoprotein coat the most important component of which is zona pellucida glycoprotein 3 (ZP3). It’s a sperm binding protein that induces the acrosome reaction. Acrosin is secreted from the acrosome and then bites through zona pellucida, however, the exact course of this action is still surrounded by a veil of mystery. Nevertheless, without going into much detail, the sperm cell may now enter the oocyte. Mission completed! Okay, so this is the fertilization, isn’t it?
Well… Not exactly. Once the spermatozoon has entered the female gamete, the oocyte has to complete meiosis (which, by the way, had started when the oocyte’s owner was a foetus in her mummy’s belly – a act constantly boggling my mind). The spermatozoon has to wait a moment. It looks, more or less, like this:
As a result of a chain-reaction triggered off by the sperm cell present in cytoplasm, the oocyte accomplishes the second meiotic division, releasing the second polar body. From that moment on, we can speak of the egg cell (ovum).
All right, so do we have this fertilization yet?
Take it easy. What we have is simply a spermatozoon in the ovum’s cytoplasm. For the whole male genetic material to fit into a structure as small as a sperm cell, it has to get condensed and packed more tightly than in the case of an ordinary somatic cell nucleus. Proteins responsible for genetic material condensation in a sperm cell must, therefore, be replaced with histones. It usually happens when the oocyte is about to finish the meiotic division. When both genetic materials – the one from mummy and the one from daddy are ready to fuse, we call them pronuclei. They migrate towards the oocyte centre where the final fusion takes place – chromosomes find and pair with their homologs, everything gets sorted out and ultimately copied so that the zygote can divide.
Which means that we sure have the fertilization now, that’s the moment new life has started, hurrah!
Hold your water! There’s one more minute nuance to it. The last one, I promise.
Embrionic genome activation
In fact, a newly formed embryo relies entirely on maternal genetic material when it comes to the whole complex metabolic-regulatory-transcription-and-replication-machinery. This dependency occurs during the first divisions. In humans the activation of embryonic genome takes place between the stadium of 4 to 8 cells. Technically speaking, within first few hours after fertilization, an embryo is nothing more than an extension of its mother’s organism, which at this point in time is being indwelled by a partially alien DNA. The embryo wouldn’t be able to divide nor live, if it weren’t for its mother’s genetic material. The tiny lump of cells contains DNA with all the information on how to build a new human being, still the genome is totally inactive, incapable of transcribing any genes at all, not to mention sustaining its own life.
So – when does life begin, which nanosecond can be marked as the moment of fertilization, and how to call an embryo which whole existence depends on the correct functioning of its mother’s genetic material? I don’t wish to resolve this dispute here. Moreover, I’m afraid there’s not one person in this world who could, without a single doubt nor hesitation, state when – from a scientific point of view – does life begin.
Heaven only knows.