In order to understand the techniques of embryo manipulation, it is important to understand the early stages of reproduction. When the egg and sperm unite to form a zygote, each of the parents supply the zygote with half of the chromosomes necessary for a full set. The zygote, which is a single cell, then begins to reproduce itself by the cellular division process called mitosis, in which each chromosome is duplicated before separation so that each new cell has a full set of chromosomes. This is called the morula stage, and the new cells are called blastomeres. When enough cells have been produced (the number varies from species to species), cell differentiation begins to take place. The first differentiation appears to be when the blastocyst is formed, which is an almost hollow sphere with a cluster of cells inside; and the differentiation appears to be between the cells inside, which become the fetus, and the cells outside, which become the fetal membranes and placenta. However, the process is not entirely understood at the present time and there is some variation between species; so it is difficult to pinpoint the onset of differentiation, which some scientists believe occurs during blastomere division.
During the first stages of cell division, it is possible to separate the blastomeres with the result that each one develops into a separate embryo. Blastomeres with this capability are called totipotent. The purpose of this ability of a single blastomere to produce an entire embryo is probably to safeguard the process of embryo development against the destruction of any of the blastomeres. In theory, it should be possible to produce an entire embryo from each blastomere (and blastomeres are generally totipotent from the four to eight cell stage), but in practice it is usually only possible to produce two embryos. That is why this procedure is generally referred to as embryo splitting rather than cloning, although both terms refer to the same thing (cloning is the production of genetically identical embryos, which is a direct result of embryo splitting).
Interestingly enough, although the embryos produced from separated blastomeres usually have fewer cells than a normal embryo, the resulting offspring fall within the normal range of size for the species.
It is also possible to divide an embryo at other stages of development. For instance, the time at which embryo division is most successful is after the blastocyst has formed. Great care must be taken when dividing a blastocyst, since differentiation has already occurred to some extent, and it is necessary to halve the blastocyst very precisely.
Another interesting embryonic manipulation is the creation of chimaeras. These are formed by uniting two different gametes, so that the embryo has two distinct cell lineages. Chimaeras do not combine the genetic information of both lineages in each cell. Instead, they are a patchwork of cells containing one lineage or the other. For this reason, the offspring of chimaeras are from one distinct genotype or the other, but not from both. Thus chimaeras are not useful for creating new animal populations beyond the first generation. However, they are extremely useful in other contexts. For instance, while embryo division as described above is limited in the number of viable embryos that can be produced, chimaeras can be used to increase the number. After the blastomeres are separated, they can be combined with blastomeres of a different genetic lineage. It has been found that with the additional tissue, the survival rate of the new embryos is more favorable. For some reason only a small percentage of the resulting embryos are chimaeric; this is thought to be because only one cell lineage develops into the cells inside the blastocyst, while the other lineage forms extra-embryonic tissue. It is believed that the more advanced cells are more likely to form the inner cells.
Another application of chimaeras could be for breeding endangered species. Because of the different biochemical environments in the uterus, and the different regulatory mechanisms for fetal development, only very closely related species are able to bear each other's embryos to term. For example, when a goat is implanted with a sheep embryo or the other way around, the embryo is unable to develop properly. This problem can perhaps be surmounted by creating chimaeras in which the placenta stems from the cell lineage of the host species. The immune system of an animal attacks tissue it recognizes as "non-self," but it is possible that the mature chimaeras would be compatible with both the host species and the target species, so that it could bear either embryo to term. This has already proved to be true in studies with mice.
A further technique being developed to manipulate embryos involves the creation of uniparental embryos and same-sex matings. In the former case, the cell from a single gamete is made to go through mitosis, so that the resulting cell is completely homozygous. In the latter case, the DNA from two females (parthogenesis) or two males (androgenesis) is combined to form cells that have only female- or male-derived DNA. These zygotes cannot be developed into live animals, as genetic information from male and female derived DNA is necessary for embryonic development. However, these cells can be used to generate chimaeras. In the case of parthogenetic cells, these chimaeras produce viable gametes. The androgenetic cells do not become incorporated in the embryo; they are used to form extra-embryonic tissue, and so no gametes are recovered.
Aside from these more ambitious embryo manipulation endeavors, multiple ovulation and embryo transfer (MOET) could soon become a useful tool. MOET is the production of multiple embryos from a female with desirable traits, which are then implanted in the wombs of other females of the same species. This circumvents the disadvantages of breeding from a female line (which are that a female can only produce a limited number of offspring due to the time investment and physical rigors of pregnancy). At the present time, MOET is still too expensive for commercial application, but is being applied experimentally.
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