Problems encountered during Cloning

There are two major problems or limitations found in the cloning of mammals. First, following the introduction of the donor's nucleus into the egg, it must be reimplanted into a gestating surrogate mother. Most of the implanted eggs abort, forcing scientists to perform several implantations, in the hopes that at least one of the females will have normal gestation. In the case of amphibians (e.g., toads), the development of the embryo occurs outside of the adult's body, thereby facilitating the development of the fetus.

The second major challenge in animal cloning is the size of the fetus at birth. Most of the surrogate mothers have to deliver via Cesarean section. This is especially true in bovines, as the clones tend to be about twice as big as normal newborn calves. The large size of the fetus during gestation can represent a substantial risk for the surrogate mother. Additionally, clones tend to have a high incidence of birth defects, and many clones die in the first hours following birth. Common abnormalities observed in cloned animals include failures of the kidney, heart, circulatory system, liver, and lungs. In addition, the placenta of the surrogate mother does not always function properly during gestation.

The causes of the high abortion rate and abnormalities in clones are still not completely understood, but it is suspected that they are at least partially the result of the complexity of the genetic reprogramming that takes place in the genes from the donor that are inserted into the egg. If a gene is expressed inadequately or it is not expressed at a critical point in development, the result can be a developmental defect. Genetic reprogramming involves the regulation of thousands of genes in a systematic and orderly way. Any asynchrony in the expression of the genes can contribute to defects in the fetus or even result in abortion. Additionally, when cloning is done with nuclei from somatic cells, they bear any preexisting mutations that might have occurred after the cells had differentiated into specialized cells. These mutations would have otherwise been screened out in gametogenesis.

With the current knowledge and technology, mammalian cloning is still a highly unsafe and inefficient procedure. The expectation is that, as new knowledge is generated from more experience, the main limitations in cloning will be at least partially solved. This science is continuing to make progress worldwide, even in developing counties. For example, in Brazil, Embrapa-Cenargen recently pioneered the cloning of the first bovine calf from somatic cells, born in March 2001.

Tags: Bio Technology, Bio Genetics, Cloning

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