快猫短视频

Researchers work with the early stages of embryos as they attempt to cure genetic diseases, treat infertility and breed better farm animals. In the process they make clones and hybrids

HOW does a single cell develop into you or me? This central question remains a mystery. But on the way to an answer, researchers have discovered how to manipulate very young embryos, just a few days old.

As a result, they can now create a motley collection of mythical-sounding creatures 鈥� chimeras, clones and transgenic animals. These creatures are not only valuable research tools, but may also help to transform agriculture and medicine in the future. The famous 鈥渢est-tube babies鈥�, too, are a spin-off from research on embryos.

Most of these breakthroughs stem from what we have discovered about mouse embryos. Mice are convenient animals to study in the laboratory and, in their early development, resemble human embryos. Indeed, for the first few days they resemble the embryos of all mammals (except for the marsupials, such as kangaroos, and the egg-laying monotremes).

The key manipulations that scientists can now perform in the laboratory occur before 鈥渋mplantation鈥� 鈥� the stage at which the embryo begins to burrow into the wall of the uterus, and the placenta starts to form. After this, scientists cannot provide the right environment in the laboratory for an embryo to survive outside the womb.

Before implantation, the embryo is a self-contained entity, starting with a single cell and multiplying to about 32 cells. Some researchers argue that it is better called a 鈥減re-embryo鈥� at this stage. This is because the cells have not yet become specialised.

Those that will develop into the fetus (if development proceeds) still look identical to the cells that will form the placenta and other supporting tissues. Perhaps no fetus will develop, or perhaps two will, to form twins. It is impossible to tell at this stage. So it makes little sense to think of these cells as an 鈥渋ndividual鈥� or a 鈥減erson鈥�.

Before implantation

The first six days

When an egg is first fertilised by a sperm, the embryo is just a single, albeit large, cell. Inside are two small structures known as pronuclei. These carry the genes, inherited from the parents, which carry the vital instructions that the cell will need to start on the course towards becoming a fetus and its support system. Soon, the pronuclei will fuse to form the nucleus.

This single-celled embryo then begins to divide. In these early stages, the divisions are known as cleavage because the cells do not grow 鈥� they just split into two. The first division results in the two-celled embryo. Cleavage continues, creating embryos with four cells, eight cells, 16 cells and so on.

By now, the embryo is a solid cluster of cells, known as the morula. The identical looking cells are called blastomeres.

When it reaches the morula stage, about three days after fertilisation, the first signs of organised structure begin to appear. In all mammals that have placentas (this includes mice and people, but not marsupials and monotremes), the cells start to flatten (compaction), stick together and develop elaborate connections.

They then organise into a hollow, fluid-filled ball, called the blastocyst. Now the cells on the outside of the ball look different. Under a microscope, you will see projections on their surfaces, called microvilli.

The position of cells in the blastocyst 鈥� whether they are on the inside or the outside of the ball of cells 鈥� is crucial. It directly influences what kind of cell they will become. Those on the outside are called the trophectoderm, and will form the placenta and other supporting tissues. On the inner surface of the blastocyst is a cluster of cells called the inner cell mass. Only these inner cells will go on to form the embryo proper.

At this stage the blastocyst is ready to attach itself to the wall of the uterus. About six days after fertilisation, it hatches from the outer covering that has surrounded the cells, the zona pellucida, and burrows into the lining cells. Implantation has begun.

For a mouse, some 15 more days must elapse before a fully developed baby mouse (and its litter mates) leaves the womb. For a human, it will be almost nine months.

Transgenic animals

Carry someone else鈥檚 gene

With developments in genetic engineering, biologists have perfected techniques for introducing genes from one organism into other organisms. The result 鈥� a cow, say, carrying a human gene among its own genetic material 鈥� is called a 鈥渢ransgenic animal鈥�.

The most popular way of producing transgenic animals is to inject genes directly into one of the pronuclei in a newly fertilised egg. Researchers perform this delicate task by holding the egg in a suction tube. They then pierce the egg and pronucleus with a micropipette to inject the foreign gene.

Remarkably, this crude procedure works, some of the time at least. The foreign gene becomes permanently incorporated into the genetic material of the animal. There are problems, though. The genes, for example, do not always work properly once in place. Some mayalso damage the hosts鈥檚 genetic material, creating what are known as 鈥渋nsertional mutations鈥�.

In spite of these problems, 鈥渕icroinjection鈥� has created many transgenic mammals 鈥� including mice, rats, pigs and sheep. The first to emerge from the laboratory, in 1981, were mice carrying a gene for growth hormone. These grew up to 50 per cent larger than their contemporaries.

Another way to create transgenic animals is to take advantage of viruses (retroviruses) that naturally infect cells. These retroviruses insert their own genes into the genetic material of a cell.

Genetic engineers take the key infecting parts of the retrovirus and add to this the foreign gene of their choice. Such a 鈥渞ecombinant鈥� retrovirus, made from artificially combined bits of genes, can then carry foreign genes into the embryonic cells. Normally, researchers infect an embryo with this 鈥渆ngineered virus鈥� when it is at about the four- or eight-cell stage.

The difficulty with this technique is that a virus may not infect all the embryo鈥檚 cells. If it infects just some of them, the animal will be a 鈥渃himera鈥� 鈥� that is, its cells are not all genetically identical. If a virus misses the cells that will go on to form the sex cells (the germ line), the animal will not be able to pass the foreign gene onto its offspring.

Many researchers believe that this approach will, one day, lead to 鈥済ene therapy鈥�. The idea is that doctors could cure a person with a genetic disorder by introducing a gene to compensate for the faulty gene responsible for the disease.

Twins and chimeras

From flexible embryos

The cells of a very young embryo are open-minded about their future. They have not yet begun the complicated process of developing into particular types of cell, such as nerve or muscle, in a particular place. This lengthy process, known as differentiation, finally produces a cow or a human with feet on the ground and a head on its shoulders.

As a result of their undifferentiated state, embryos only a few days old are surprisingly robust when chopped in two, or manipulated in other ways.

At the two-cell stage, divide an embryo into two and you can produce identical twins. Even a much older embryo, which has reached the blastocyst stage, will form twins if you cut evenly through the inner cell mass. Bisecting embryos to produce twins is now routine for cattle breeders working in 鈥渆mbryo transfer鈥� 鈥� they can transfer both halves to mother cows, to produce two identical calves.

Similarly, researchers can test one half of the embryo to determine its sex or to test for the presence of genetic defects. They then return the other half to the mother, or even transfer it to a surrogate mother.

The flexibility of the early embryo even allows animals of other species to be surrogate mothers. A horse, say, can give birth to a zebra.

To ensure that the pregnancy takes, researchers perform a bit of transplant surgery on embryos from both horse and zebra. They swap the outer, placenta- forming cells (the trophectoderm, called the trophoblast in older embryos) so that the inner, embryo-forming cells from the zebra are encased in trophoblast from a horse. The horse surrogate mother then accepts the developing zebra embryo as if it were of her own species.

This embryonic surgery has great advantages for breeders of endangered species. One female of a rare species can produce many embryos at once, which domestic animals can then carry to term.

Researchers find that they can also take samples from eight-cell embryos of mice, without damaging them. So they can remove one cell (blastomere) and test it for genetic defects. In primates, too, researchers have clipped away some trophectoderm from the blastocyst and tested the cells biochemically. The rest of the embryo, transferred to a receptive female, developed quite normally.

Finally there are 鈥渃himeras鈥�. Normally, all the cells in an animal鈥檚 body are genetically identical; but not in a chimera.

Creating chimeras is surprisingly easy. Mix cells from a four-cell sheep embryo with cells from an eight-cell goat embryo. They will reorganise themselves and form an animal that is sheep and goat.

It is difficult to predict how many cells will be sheep and how many goat with this procedure, so it has few practical applications.

Clones

Are identical copies

Clones are individuals that are genetically identical. In theory, it should be possible to produce identical copies of yourself: take one of your body cells, remove the nucleus (which contains the genes) and implant this into a one-cell embryo which has had its own nucleus removed. Then your genes would control the development of another you.

Unfortunately, it does not work. Nuclei from the body cells of adult mammals just cannot induce a denucleated embryonic cell to develop. Apparently, crucial genes that control development are switched off in adult cells.

You can, however, produce several genetically identical individuals by splitting up the embryo at an early stage (see 鈥淭wins and chimeras鈥�). At the eight-cell stage, for instance, each cell can reform a whole embryo. Or you can transplant a nucleus from another embryo. But these approaches produce only a finite number of clones from the embryo.

Test-tube babies

And pregnant men

Our newfound ability to manipulate young embryos in the laboratory has led to a revolution in the treatment of infertility. Test-tube babies, surrogate mothers and talk of male pregnancies are the result.

The process that leads to a test-tube baby is straightforward. A woman receives a course of hormone drugs to stimulate her ovary to produce more than its normal one mature egg in a monthly cycle. Doctors harvest these eggs, usually by inserting a tube through the woman鈥檚 abdomen to find the fluid- filled follicles that contain the ripe eggs. They then suck them out with a catheter.

The next stage is where the 鈥渢est-tube鈥� comes in. Egg and sperm meet in a dish in the laboratory. In vitro fertilisation (IVF) is the term for this procedure 鈥� in vitro literally means 鈥渋n glass鈥�. Special nutritive substances in the dish keep the embryo alive while the pronuclei fuse and cleavage starts.

Next comes ET 鈥� embryo transfer. A doctor transfers the young embryo now probably a few cells 鈥� into a woman鈥檚 uterus. If the woman has not herself contributed the egg, and so is not the 鈥済enetic鈥� mother, she could be called a surrogate mother.

The success of in vitro fertilisation and embryo transfer is not high 鈥� only about 10 per cent of the women who go through a cycle of treatment become pregnant. No one is quite sure what the success rate is for eggs fertilised normally, but probably about 50 per cent result in a birth.

A man could become pregnant through embryo transfer, in theory. An embryo can implant in the abdominal cavity, outside the uterus. This sometimes happens in women, producing what is known as an ectopic pregnancy. Unfortunately, such pregnancies, if not detected and terminated, nearly always kill the mother. The embryo obstructs the gut and other organs to cause severe bleeding.

Even if we could overcome this problem, a man who wanted to become pregnant and give birth (inevitably by Caesarian section) would also have to be castrated or given drugs to counterbalance the hormones produced by testes and to mimic the balance of hormones in women.

The man might well find that, as a result of such treatment, he developed feminine characteristics, such as breasts and a higher voice. He might also become infertile.

Such wild flights of fancy aside, research on young mammalian embryos has created many new possibilities for agriculture and medicine. It is also essential if we are to create ways of preventing and treating infertility, genetic diseases and birth defects.

Further reading

鈥淲hy study early human development鈥�, an article by Anne McLaren (快猫短视频, 24 April 1986, p 49) gives an overview of research on human embryos.

Also helpful for the nonspecialist are two books in the series 鈥淩eproduction in Mammals鈥�, edited by C. R. Austin and R. V. Short and published by Cambridge University Press.

These are Embryonic and Fetal Development, Vol 2, second edition 1982, and Manipulating Reproduction, Vol 5, second edition, 1986.