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somatic cell nuclear transfer is usually referred to as cloning

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Somatic cell nuclear transfer

Somatic cell nuclear transfer can create clones for both reproductive and therapeutic purposes. The diagram depicts the removal of the donor nucleus for schematic purposes; in practice the whole donor cell is transferred.

In genetics and developmental biology, somatic cell nuclear transfer (SCNT) is a laboratory strategy for creating a viable embryo from a body cell and an egg cell. The technique consists of taking an enucleated oocyte (egg cell) and implanting a donor nucleus from a somatic (body) cell. It is used in both therapeutic and reproductive cloning. In 1996, Dolly the sheep became famous for being the first successful case of the reproductive cloning of a mammal.[1] In January 2018, a team of scientists in Shanghai announced the successful cloning of two female crab-eating macaques (named Zhong Zhong and Hua Hua) from foetal nuclei.[2]

"Therapeutic cloning" refers to the potential use of SCNT in regenerative medicine; this approach has been championed as an answer to the many issues concerning embryonic stem cells (ESCs) and the destruction of viable embryos for medical use, though questions remain on how homologous the two cell types truly are.

Contents

1 Introduction 2 Process 3 Applications

3.1 Stem cell research

3.2 Reproductive cloning

3.3 Interspecies nuclear transfer

4 Limitations 5 Controversy

6 Policies regarding human SCNT

Introduction

Somatic cell nuclear transfer is a technique for cloning in which the nucleus of a somatic cell is transferred to the cytoplasm of an enucleated egg. After the somatic cell transfers, the cytoplasmic factors affect the nucleus to become a zygote. The blastocyst stage is developed by the egg to help create embryonic stem cells from the inner cell mass of the blastocyst.[3] The first animal to be developed by this technique was Dolly, the sheep, in 1996.[4]

Process

The process of somatic cell nuclear transfer involves two different cells. The first being a female gamete, known as the ovum (egg/oocyte). In human SCNT experiments, these eggs are obtained through consenting donors, utilizing ovarian stimulation. The second being a somatic cell, referring to the cells of the human body. Skin cells, fat cells, and liver cells are only a few examples. The genetic material of the donor egg cell is removed and discarded, leaving it 'deprogrammed.' What is left is a somatic cell and an enucleated egg cell. These are then fused by inserting the somatic cell into the 'empty' ovum.[5] After being inserted into the egg, the somatic cell nucleus is reprogrammed by its host egg cell. The ovum, now containing the somatic cell's nucleus, is stimulated with a shock and will begin to divide. The egg is now viable and capable of producing an adult organism containing all necessary genetic information from just one parent. Development will ensue normally and after many mitotic divisions, the single cell forms a blastocyst (an early stage embryo with about 100 cells) with an identical genome to the original organism (i.e. a clone).[6] Stem cells can then be obtained by the destruction of this clone embryo for use in therapeutic cloning or in the case of reproductive cloning the clone embryo is implanted into a host mother for further development and brought to term.

Applications

Stem cell research

Somatic cell nuclear transplantation has become a focus of study in stem cell research. The aim of carrying out this procedure is to obtain pluripotent cells from a cloned embryo. These cells genetically matched the donor organism from which they came. This gives them the ability to create patient specific pluripotent cells, which could then be used in therapies or disease research.[7]

Embryonic stem cells are undifferentiated cells of an embryo. These cells are deemed to have a pluripotent potential because they have the ability to give rise to all of the tissues found in an adult organism. This ability allows stem cells to create any cell type, which could then be transplanted to replace damaged or destroyed cells. Controversy surrounds human ESC work due to the destruction of viable human embryos, leading scientists to seek alternative methods of obtaining pluripotent stem cells, SCNT is one such method.

A potential use of stem cells genetically matched to a patient would be to create cell lines that have genes linked to a patient's particular disease. By doing so, an model could be created, would be useful for studying that particular disease, potentially discovering its pathophysiology, and discovering therapies.[8] For example, if a person with Parkinson's disease donated his or her somatic cells, the stem cells resulting from SCNT would have genes that contribute to Parkinson's disease. The disease specific stem cell lines could then be studied in order to better understand the condition.[9]

Another application of SCNT stem cell research is using the patient specific stem cell lines to generate tissues or even organs for transplant into the specific patient.[10] The resulting cells would be genetically identical to the somatic cell donor, thus avoiding any complications from immune system rejection.[9][11]

Source : en.wikipedia.org

Cloning animals by somatic cell nuclear transfer – biological factors

Cloning by nuclear transfer using mammalian somatic cells has enormous potential application. However, somatic cloning has been inefficient in all species in which live clones have been produced. High abortion and fetal mortality rates are commonly observed. ...

Reprod Biol Endocrinol. 2003; 1: 98.

Published online 2003 Nov 13. doi: 10.1186/1477-7827-1-98

PMCID: PMC521203 PMID: 14614770

Cloning animals by somatic cell nuclear transfer – biological factors

X Cindy Tian,1 Chikara Kubota,2 Brian Enright,1 and Xiangzhong Yang1,3

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Abstract

Cloning by nuclear transfer using mammalian somatic cells has enormous potential application. However, somatic cloning has been inefficient in all species in which live clones have been produced. High abortion and fetal mortality rates are commonly observed. These developmental defects have been attributed to incomplete reprogramming of the somatic nuclei by the cloning process. Various strategies have been used to improve the efficiency of nuclear transfer, however, significant breakthroughs are yet to happen. In this review we will discuss studies conducted, in our laboratories and those of others, to gain a better understanding of nuclear reprogramming. Because cattle are a species widely used for nuclear transfer studies, and more laboratories have succeeded in cloning cattle than any other specie, this review will be focused on somatic cell cloning of cattle.

Keywords: nuclear transfer, donor cell types, donor age, serum starvation, cell passage

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Introduction

Somatic cell cloning (cloning or nuclear transfer) is a technique in which the nucleus (DNA) of a somatic cell is transferred into an enucleated metaphase-II oocyte for the generation of a new individual, genetically identical to the somatic cell donor (Figure ​

(Figure1).

1). The success of cloning an entire animal, Dolly, from a differentiated adult mammary epithelial cell [1] has created a revolution in science. It demonstrated that genes inactivated during tissue differentiation can be completely re-activated by a process called nuclear reprogramming: the reversion of a differentiated nucleus back to a totipotent status. Somatic cloning may be used to generate multiple copies of genetically elite farm animals, to produce transgenic animals for pharmaceutical protein production or xeno-transplantation [2-5], or to preserve endangered species. With optimization, it also promises enormous biomedical potential for therapeutic cloning and allo-transplantation [6]. In addition to its practical applications, cloning has become an essential tool for studying gene function [7], genomic imprinting [8], genomic re-programming [9-12], regulation of development, genetic diseases, and gene therapy, as well as many other topics.

Figure 1

Schematic diagram of the somatic cloning process. Cells are collected from donor (a) and cultured in vitro (b). A matured oocyte (c) is then enucleated (d) and a donor cell is transferred into the enucleated oocyte (e). The somatic cell and the oocyte is then fused (f) and the embryos is allowed to develop to a blastocyst in vitro (g). The blastocyst can then be transferred to a recipient (h) and cloned animals are born after completion of gestation (i).

One of the most difficult challenges faced, however, is cloning's low efficiency and high incidence of developmental abnormalities [13-19]. Currently, the efficiency for nuclear transfer is between 0–10%, i.e., 0–10 live births after transfer of 100 cloned embryos. Developmental defects, including abnormalities in cloned fetuses and placentas, in addition to high rates of pregnancy loss and neonatal death have been encountered by every research team studying somatic cloning. It has been proposed that low cloning efficiency may be largely attributed to the incomplete reprogramming of epigenetic signals [20-23].

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Factors affecting nuclear reprogramming

Various strategies have been employed to modify donor cells and the nuclear transfer procedure in attempts to improve the efficiency of nuclear transfer. Most of these efforts are focused on donor cells. These include: a) synchrony of the cell cycle stage of donor cells [24-26], as well as synchrony between donor cells and recipient oocytes [27,28]; b) using somatic cells from donors of various ages [29-33], tissue origins [26,34-39], passages [16,40,41] and culture conditions [42]; c) transfer of stem cells with low levels of epigenetic marks [43-48]; and d) modifying epigenetic marks of donor cells with drugs [49-51]. Although the efficiency of nuclear transfer has been dramatically improved from the initial success rate of one live clone born from 277 embryo transfers [1], none of the aforementioned efforts abolished the common problems associated with nuclear transfer. These observations suggest that further studies on nuclear reprogramming are needed in order to understand the underlying mechanisms of reprogramming and significantly improve the ability of the differentiated somatic nuclei to be reprogrammed. In the following section, we will discuss several strategies used to improve nuclear transfer efficiencies.

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Serum starvation of donor cells

Serum starvation was used in the creation of Dolly and was believed essential to the success of nuclear transfer [1]. Serum starvation induces quiescence of cultured cells, and arrests them at the cell cycle stage of G0. Most laboratories that have succeeded with nuclear transfer have utilized a serum starvation treatment. However, there is a debate as to whether inducing quiescence is required for successful nuclear transfer. Cibelli et al. [52] proposed that G0 was unnecessary and that calves could be produced from cycling cells. In his study, actively dividing bovine fibroblasts were used for nuclear transfer and four calves were born from 28 embryos transferred to 11 recipients. Because 56% of cycling cells in that study were in G1 stage, it is likely that all cloned animals produced in this study were from donor cells at G1 stage. Cells at G2, S or M would not be expected to generate cloned animals in this study because they are incompatible with the recipient oocytes used. This study demonstrated that cells at G1 stage can produce live cloned animals and G0 induction is not essential.

Source : www.ncbi.nlm.nih.gov

Cloning and Stem Cells Flashcards

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Cloning and Stem Cells

Somatic Cell Nuclear Transfer (SCNT)

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the main technique used in cloning

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Reproductive Cloning

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cloning to produce a full-term individual

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Terms in this set (55)

Somatic Cell Nuclear Transfer (SCNT)

the main technique used in cloning

Reproductive Cloning

cloning to produce a full-term individual

Therapeutic Cloning

cloning for some medical benefit

Stem Cell

an undifferentiated cell from which specialized cells develop

Totipotent

a stem cell that can become any type of cell

Pluripotent

a stem cell that can become most types of cells in the body

Multipotent

a stem cell that can differentiate and become only a few types of cells

Differentiate

the process in which a stem cell becomes a specialized cell

Specialized

a cell that has a particular function in the body

IVF

process in which embryos are created outside the womb

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