Early development
Early development

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Early development

1.3.7 Factors affecting fertilization

It is useful at this stage to summarize the main factors involved in a successful fertilization. First and foremost, fertile gametes must be made. This depends fundamentally on the health of the prospective parents. If they are diseased or undernourished, or have been exposed to high levels of radiation, then not only will they not produce healthy gametes, but they will probably not want to engage in the kinds of activity that might bring their gametes together.

DNA replication and protein synthesis are metabolically expensive processes. Our metabolism only runs if we supply it with a source of energy and the correct raw materials, and these are obtained from our food. It follows therefore that if we cannot eat enough of the ‘right’ food, i.e. that containing sufficient energy and building blocks, our metabolism will run down. Ultimately this will result in death, but before that time the body will find ways to maximize its resources by cutting down on non-essential activities, including reproduction. It is well documented that undernourished women do not have menstrual cycles, and though there is nothing as easily measured in men, it is clear that sperm production suffers too in times of hardship. Indeed, there is some concern about the observation that over the past century in the UK there has been a drop in men's average sperm count. The reason(s) for this remains a mystery, and so far there has been no adverse effect on fertility, but there is obviously a possibility that, unchecked, it could result in an increasing proportion of infertile men.

Any process that interrupts the correct production of hormones is likely, as you have seen, to have an adverse effect on gamete production. Many sex hormones are derived from the lipid cholesterol which, although it generally receives a very bad press on account of its propensity for blocking arteries, is a vital constituent of the healthy body. It, or precursors required to synthesize it, must be present in the diet. Cholesterol is a component of cell membranes, and is also vital for the production of several hormones, the steroid hormones. Figure 15, which you need not memorize, and which is included only for interest, shows the chemical structures of the main sex hormones, and how these structures are related to the structure of cholesterol. (Chemical formulae can be written out to show the relative positions of the atoms in a molecule. This gives an idea of the shape of the molecule, which is very important for its function.) Although we are not suggesting that everyone should adopt a high-cholesterol diet, it is clear that some at least is necessary for reproductive success.

Figure 15
Figure 15 Structures of cholesterol, progesterone (a progestogen), testosterone and oestradiol (an oestrogen). The cholesterol fused ring structure is shown in colour. (Note: these are skeleton representations – none of the carbon atoms and directly attached hydrogen atoms are shown. Each line denotes a single C to C bond, with the C atoms at the ends of the lines and H atoms attached to them; so a ‘free end’ represents a CH3 group, a ‘bend’ is a CH2 group and where three lines converge it means there is a CH group.)

We have looked at some of the factors affecting the production of healthy gametes, but there remains a further important criterion for fertilization.

SAQ 23

Q What is it?


A Biologically successful sexual intercourse.

Biologically successful intercourse is defined as intercourse in which the man can maintain an erection for long enough to allow sperm to enter the woman's vagina. This usually means that ejaculation has to occur, but some leakage of sperm may take place before this. The maintenance of an erection depends not just on physical factors but, very importantly, on psychological ones too. There is no requirement for female orgasm to occur, and there is certainly no substance to the old wives’ tale that ‘you can't get pregnant unless you come’; interestingly, though, there is some evidence that a woman's emotional state may affect her ability to conceive. (Unfortunately, not wanting a baby is not an effective contraceptive!). Of course, it is difficult to conduct experiments to clarify this, and for the time being this evidence remains rather circumstantial. Nevertheless, it seems clear that, in humans, the process of reproduction is more than just a physical one.


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