Embryology for Beginners
When does life begin?
This is a bit like asking which comes first, the chicken or the egg? In formulating an answer it is always a good idea to start with some basic facts and that is the intention of this article. The miracle of life is just that considering the natural loss along the way – unused eggs, unwanted sperm, rejected zygotes – and all that before pregnancy even begins. Curiously, the journey from fertilisation to birth begins with a baby.
Primordial origins
When a female baby is born the ovary is already there - well two actually, one on either side of the uterus - and within the ovaries (although you can’t see them in the diagram above) are millions of primordial follicles (the fancy name for egg cells) patiently waiting until puberty before anything much happens. The term primordial is a reminder of how life began all those aeons ago and the important role these germ cells have in carrying precious DNA from one generation to the next. That is why some people like to think of life as a continuum.
Others place arbitrary points on this continuum and say life starts here or there but that’s only for their convenience or to make some point or other. The ovum or egg is alive; sperm or spermatozoa are alive. Nobody seems to mind that millions and millions of sperm die each day, their destiny unfulfilled. And in a woman’s lifetime only about 400 eggs will reach maturity, much fewer if she bears lots of children and breast feeds. The surplus eggs will just fade away at menopause. Fertilisation, implantation, quickening (when the mother first feels movement) and birth are important milestones on this continuum.
Everything changes at puberty
Within the ovary, follicles are developing continuously but, after puberty, once a month, or about every 28 days, hormonal changes result in several follicles increasing in size and maturity. Usually only one will fully mature but non-identical twins are the result of two follicles reaching maturity. The follicle will then burst spectacularly releasing the ovum from the confines of the ovary into the world beyond, well actually into the peritoneal cavity. The ovum is then enveloped by the arms (fimbria) of the Fallopian tube and wafted into the outer part of the tube. Meanwhile the cells which surrounded the ovum become the corpus luteum or yellow body which produces the hormone progesterone necessary to sustain a pregnancy, a sort of long distance support system.
Fertilise or perish
The ovum must be fertilised within 24 hours or it will literally disappear down the tube into oblivion. If sperm are in the area they will naturally be attracted to the ovum. To get there they will have had to swim a perilously long marathon with many falling beside the way. Only the most active will reach the finishing tape which is the ovum. And then, only one can penetrate the special layers covering the ovum. When that happens, a chemical barrier is released by the ovum to prevent any other sperm from entering.
Fertilisation does not happen in an instant. Sometimes people talk about the ‘moment’ of fertilisation but it isn’t over in a moment and is actually quite a process which takes the best part of 24 hours. Amazing as it is, nobody knows it is happening (unless it happens in a laboratory) and there are no tests to identify it (yet).
Sometimes people use the word ‘conception’ to mean exactly the same thing as fertilisation. Others use it to mean implantation which happens more than a week later. In the 1977 report of the Royal Commission on Contraception, Sterilisation and Abortion the term was used both ways which was a little confusing. You just have to know what definition people are using otherwise you end up talking at cross purposes.
Boy or girl?
At fertilisation changes take place in the nucleus of both the ovum and the sperm. The nuclei literally fall to pieces and then 23 chromosomes from the female nucleus pair up with 23 chromosomes from the male nucleus. Some of the chromosomes are given special names or numbers or letters of the aIphabet. With the paired sex chromosomes, if there are two X chromosomes, one from the female nucleus and one from the male nucleus then the resulting conceptus will be female (XX). If there is one X chromosome from the female and a Y chromosome from the male then the resulting conceptus will be male (XY). That’s just the way it is!
Name change
The fertilised ovum is now called a zygote. This comes from a Greek word meaning to yoke together (nothing to do with yolk). If you prefer you can call it a pre-embryo. On the second day after ovulation, this single-celled zygote travels down the Fallopian tube by the action of the cilia (hair like structures) which line the walls of the tube. Left to its own devices the fertilised ovum or zygote has no means of locomotion.
Division and multiplication
The next big event on Day 2 is the division into a two-celled zygote, while staying exactly the same size. Next day a further division results in a four-celled zygote and so on, at intervals of about 15 hours, until there is a little ball of cells called a morula, so called because of the resemblance to a mulberry. The first four days of life are extremely perilous! One in 10 zygotes or pre-embryos is discarded at this stage and this is regarded as being biologically useful as the discarded ones may be defective in some way. Even among those who say that all life must be protected from fertilisation no-one is campaigning to save these lives.
On the fifth day nearing the end of the Fallopian tube there is another transformation. The cells begin to separate leaving a space between the two areas. The pre-embryo is now called a blastula or blastocyst after a Greek word meaning to sprout. The two parts are quite distinct. One part will eventually develop into an embryo and in the diagram above you can see it developing layers, which will become the first tissues. The other larger part will develop into the life support system. Eventually (at birth) this part will be discarded as the placenta.
Two distinct parts! And yet there are still some who believe that a unique being is created at fertilisation! It is even more complicated than that. Identical twins can result if there is a division usually before the 13th day. Try telling twins they are not unique! Sometimes the supporting cells can go completely berserk and destroy the embryo. There is a name for this - a hydatidiform mole. Hydatidiform is another Greek word meaning watery vesicles which pretty much describes what it is. With this condition there are no ifs and buts about saving a human life. It must be removed.
Not too fast, not too slow
The speed of travel down the tube is extremely important. Too fast and the pre-embryo will not be mature enough to settle into the lining of the uterus. Too slow and it may settle into the Fallopian tube which is not designed to carry a pregnancy. This can result in a medical emergency. Ectopic pregnancies, growing where they shouldn’t be, happen in about one in every hundred pregnancies. This is another exception to the claim that all life must be protected.
Implantation into the lining of the uterus (endometrium) can start as early as the fifth day but may not start until the seventh day or even later. That is still quite normal. The process of embedding takes three or four days to complete and it is a very tricky stage. The loss of pre-embryos at implantation is even greater than the loss in the first four days. Conditions have to be just right. About four out of 10 will not properly implant. Even after this stage there continues to be some natural loss and if it happens in the first six weeks it is often referred to as early pregnancy loss.
Pregnancy, at last
Doctors and lawyers define pregnancy as beginning at implantation but nobody says whether this means the start of the process or the end of the process and it has never been tested in a court of law! Unlike fertilisation which is undetectable, the cells which settle into the uterus begin to produce a hormone called human chorionic gonadotrophin or HCG which is the basis of pregnancy tests.
The progression from primordial follicle, to mature ovum, to fertilised ovum, to zygote, to morula, to blastocyst takes about 14 days. These stages are pre-embryonic. From 2-8 weeks of pregnancy the correct term is embryo. Not everyone keeps to that definition. Some people refer to all stages of early pregnancy as embryonic. It doesn’t matter too much as long as everyone understands how the word is being used in a particular context.
Here’s another curious thing. Because fertilisation cannot be identified, for practical purposes pregnancies are dated, not from that point, but from a point that is much more easily recognised, the beginning of the last normal period, which comes about 2 weeks before ovulation and fertilisation. So an embryo which is really only 2-8 weeks old is referred to by doctors as a 4-10 weeks pregnancy counting from the last normal menstrual period. At this early stage the embryo can be seen by means of an ultrasound scan but you need special training to recognise what you are seeing.
Another name change
After eight weeks the term embryo is replaced by the term fetus. In New Zealand this is the generally accepted modern spelling but you will often see it spelt as foetus. It remains a fetus until the day of birth when it becomes a child with legal status. To quote from Section 159 of the Crimes Act “A child becomes a human being when it has completely proceeded in a living state from the body of its mother, whether it has breathed or not, whether it has an independent circulation or not, and whether the navel string is severed or not.” Some anti-abortion groups are attempting to give legal status to the child before birth but this is a minority viewpoint.
Understanding these facts is important because they will inevitably come up in discussions and debates on the rights of the child, the right to life, abortion issues, scanning, and even on the use of contraception, especially emergency contraception and intrauterine devices (IUDs).
Miscarriage and abortion
According to New Zealand law (Section 182A of the Crimes Act) a miscarriage is the destruction or death of an embryo or fetus after implantation. In law the terms miscarriage and abortion have the same meaning but because the word abortion is associated with induced abortion (and the stigma surrounding that) many people are more comfortable calling a spontaneous abortion a miscarriage and save the term abortion for induced abortions. Even for induced abortions some people prefer the term ‘termination of pregnancy’. There are no strict rules about the use of these terms.
Emergency contraception (which used to be called the ‘morning after’ pill) acts before implantation so it is definitely not a mini-abortion as some claim. IUDs act by preventing implantation so they are true contraceptives, not abortion agents (abortifacients). Mifepristone (it used to be called RU486) acts after implantation so it is a true abortion pill and is used for early medical abortions.
Right to life
Does a one-week-old blastocyst, an eight-week embryo or a 12-week fetus have a right to life? Ideally yes, which is why so much emphasis is put on the prevention of unwanted pregnancies, but not absolutely. We have already seen above a number of exceptions. There is the natural wastage which means that overall about 50% of fertilised ova never realise their potential, there are the 1% ectopic pregnancies, the rare cases of hydatidiform mole and the much more common occurrence of natural abortions or miscarriages, clinically recognised in about 10% of pregnancies.
Competing rights
So far the mother has hardly been mentioned but the competing rights of the mother must be taken into consideration. Many people consider that forcing an unwilling mother to continue a pregnancy is not in the best interests of the mother or the child. Historically the original reason for abortion laws was not to protect the fetus or to protect public morality, but to protect women from what were then dangerous procedures.
Special categories
It is a giant leap from asserting that biologically a fetus is a human being to concluding that abortion is murder. Taking a human life is not always considered murder. Legally we distinguish between murder and manslaughter, such as killing in self defence. For killing the enemy at war we are awarded medals.
Infanticide and suicide are also special categories. Abortion rightly belongs in a special category. Abortion does not reflect a loss of reverence for life. Reverence for life includes concern for the quality of life for the children born and consideration for the rights and well-being of women unwillingly pregnant.
Everyone has their own values, their own limits and in a democracy allowance should be made for this difference of opinion. After all, one of the founders of the Society for the Protection of the Unborn Child, took his own life by suicide.
A final word
When there are conflicting and equally sincerely held different viewpoints, one particular viewpoint should not be allowed to dominate to the extent that that viewpoint is imposed upon others by law.
Written by Margaret Sparrow
20 May 2009