4 A new life
There is a common belief that life begins at the moment of conception, i.e. when a sperm fuses with an egg. This is a step forward from past years, when life was alleged to start at the time of ‘quickening’, i.e. when a woman could feel her fetus moving inside her. However, both these opinions suffer from an underlying falsehood: that life ‘begins’ at all. Life is a continuum; gametes are produced by living parents, and fuse to produce new living individuals, but unfused gametes are nonetheless alive and capable of metabolic activities. So the question is not ‘when does life begin?’, but rather ‘When does a new individual come into being?’ The answer to this highly controversial question could, in theory, be at any time from the moment of penetration of the egg membrane by the sperm up to the time of birth, and, indeed, the Roman Catholic Church debated for centuries the time when the soul enters the new being (see above). As with many contentious issues, people's opinions will be drawn not just from biology, but from psychology and sociology too. However, for now we shall concentrate on a biological answer to the question.
Common (female) experience suggests that a new individual is alive, and certainly kicking, from around 17 weeks of gestation. Heartbeats and other spontaneous movements can be detected by ultrasound scans several weeks earlier than this. And, if life is defined in terms of metabolic activity, a new individual is alive by two weeks of gestation, when pregnancy tests can detect a substance, chorionic gonadotropin, made by the trophoblast cells as part of placental activities, so the conceptus is metabolically active at this stage at least.
So let us now address the question of the development of individuality.
Q What characteristics distinguish one individual from another?
A Almost any answer is correct here! You might have mentioned eye colour, fingerprints, or handedness, for example. But a more molecule-orientated answer might have mentioned enzymes, or genes (an individual's inheritance).
As you saw above, a new individual, resulting from the fusion of two gametes, is characterized by having two sets of genes, and therefore two alleles of each gene. Some of the alleles will have come from the mother, and others from the father.
Q In view of this, how could you distinguish a new individual from its mother?
A By identifying an allele that had been inherited only from the father.
It is technically quite tricky to carry out this kind of test in humans. It is not possible to start with laboratory breeding stocks with well characterized alleles as geneticists can do in some other species! So recourse has been made to studying traits that could only ever have come from the father. One such trait is the sex of the embryo. Sex, as you may know, is determined by the chromosomes: among the 46 human chromosomes are two that are called sex chromosomes. There are two sorts of sex chromosomes: X chromosomes, which are among the largest, and Y chromosomes, which are among the smallest. Every female has two X chromosomes in each of her cells; every male has one X chromosome and one Y chromosome. The sex chromosomes behave as a pair during meiosis, lying together at the equator, then separating to opposite poles of the dividing cell, even though they are physically ill-matched in terms of size (see Figure 8, left).
Q Which sex chromosomes will be present in the gametes of men and women?
A Eggs will always contain one X chromosome among their 23. Sperm contain either an X chromosome or a Y chromosome. (If you are at all unsure about this, go back to the diagram of meiosis, Figure 9.)
This means that an X-containing egg may be fertilized by either an X-bearing sperm or a Y-bearing sperm. In the former case the embryo will have two X chromosomes and be female; in the latter case the embryo will have an X and a Y and be male.
The sex chromosomes do not only carry genes determining sex; most of their length is taken up with genes concerned with quite different characteristics such as blood group. In fact, the only part of the Y chromosome that is essential for maleness is a tiny area called Sry (formerly tdf, testis-determining factor). Another gene on the Y chromosome codes for a protein that is a normal part of the (male) cell membrane, and it is possible to distinguish the presence of this protein on the surface of pre-implantation embryos.
Q What does this tell us?
A If the protein is there, it must have been made by the embryo's cells, as the mother cannot make it – she has no Y chromosome. So this means that the embryo's own genes are active, and it is beginning to become a male individual.
The earliest that this protein has been detected on embryos is the 4-cell stage. This and other evidence tells us that the embryo's genes have probably been ‘switched on’ by the 4-cell stage, i.e. about 2 days after fertilization. This is long before most mothers have any inkling that they might be pregnant. As we have mentioned before, more than 50% of conceptions will fail, and the majority of these failures are at very early stages. A significant proportion is probably due to an inability of the embryo's genes to become functional. In these cases, implantation will not occur, and menstruation will take place at the normal time, washing the failed embryo out, and leaving the woman with no sign that she was ever pregnant.
Strictly speaking, then, the question we asked at the beginning of this section – When does a new individual come into being? – has been answered from a biological perspective. The embryo begins to function independently at the 4-cell stage, but it is certainly not capable of independent existence at this stage. It still uses cytoplasmic components from the mother, although these are progressively metabolized and by the blastocyst stage, few, if any, remain. Once the embryonic genes have been switched on, early development seems to proceed automatically, with little reference to the outside world; this is why early embryos can be successfully grown in the laboratory. The embryo, moving slowly down the Fallopian tube into the womb, is constantly bathed in various secretions, but all that it appears to need is a suitable temperature (371°C), an appropriate level of acidity, an energy source, raw materials for synthesis as well as the right mixture of ions. Large glycoprotein molecules, present in the tubal secretions, seem to help stabilize the cell membranes, which are fragile because the cells are so big.
A few days later, however, the picture is very different. The embryo begins to burrow into the wall of the womb, and a placenta is formed. The placenta is a large and important organ, and it is made jointly by the embryo and its mother. By the time the placenta begins to form, about two weeks have passed since fertilization. Only if a woman has very regular 28-day menstrual cycles, and her period has not appeared on time, will she have any inkling that she may be pregnant. We shall go on to look at this aspect of reproduction now.