After a sperm fertilises an egg, a zygote is formed which then has the potential to develop into a complete embryo. At this point, the fertilised egg is regarded as a totipotent stem cell, which can potentially create any cell required for embryonic development. The egg then undergoes several cell division that produce identical totipotent cells, therefore having the ability to develop into a foetus. These totipotent cells then undergo several rounds of cell division themselves and begin to specialise to form a blastocyst. A blastocyst is a ball of cells consisting of a hollow outer layer of cells termed the trophoblast, within which is a cluster of cells called the inner cell mass. The trophoblast will eventually form the placenta and other tissues that are needed for the support and development of a foetus. The inner cell mass are the ones that form the human body tissues and are termed pluripotent. Thus this means that they are able to diverse widely, although with limitations with relation to foetal development. One of the three different kinds of pluripotent cells is embryonic stem cells. These are isolated from the inner cell mass of the blastocyst and as mentioned, can specialise into different cell types.
The pluripotent cells themselves undergo another round of specialisation and become multipotent cells, which are committed to give rise to cells with a particular function, for example, blood stem cells are developed into red blood cells, white blood cells and platelets while skin stem cells give rise to different types of skin cells. As a result, tissues and organs can be developed from totipotent cells. However, this method will not work in the case of therapeutic cloning, which is defined as – ‘Medical and scientific applications of cloning technology which do not result in the production of genetically identical foetuses or babies. These techniques may be undertaken to advance fundamental research and therefore not all such applications will lead to immediate therapeutic utility.’ Also, there is the problem of rejection from the patient. To solve this, stem cells genetically identical to the host can be used in somatic-cell nuclear transfer, where the donated egg cell need not be fertilised with a sperm.
Instead, the nucleus of the donated egg cell is destroyed by ultraviolet radiation. Then, a nucleus from a somatic cell replaces it, itself being reprogrammed. The now-diploid egg is allowed to develop in culture to the blastocyst stage, when embryonic stem cells can be harvested and collected and then grown in culture, with their specialisations controlled in order to develop the certain tissue or organ. When they have acquired the desired properties, they are then implanted into the patient with no fear of rejection.
