Enucleated eggs

Instructions for the behaviour of every cell in the bodies of worms, flies and humans will soon reside in public databases for all to read. A complete set of these instructions, packaged as chromosomes, is inherited by most cells in our body. Because of this, many if not most somatic nuclei in mammals are totipotent that is, they are capable of programming all of mammalian development when injected into enucleated eggs (Wilmut et al 1997). Dolly s creation had dramatic practical consequences but its feasibility was never improbable on theoretical grounds. How cells inherit two complete packages of the genome at each cell division is one of the most fundamental questions in biology (Fig. la). 

Nuclear transplantation technique. The nucleus of an unfertilized egg is mechanically removed with a micropipette (.left), and a nucleus from a blastula cell is removed and microinjected into the enucleated egg. 

In 1962, J. Gurdon took this type of experiment one step further by showing that adult frogs could be developed by injecting enucleated eggs with nuclei from the intestinal epithelium of tadpoles. The success frequency was much lower than when nuclei from earlier stages of development were used. Nevertheless, such results showed the influence of the cytoplasm on nuclear expression they also demonstrated that in certain cases nuclei already tentatively com-... 

In fact, if the nucleus isolated from the cell of one animal is injected into the enucleated egg from another animal of the same species and the egg is implanted in a foster mother, the resulting offspring is a clone of the animal from which the nucleus was derived (Fig. 17.13). Clones of sheep and pigs have been produced, and similar techniques could be used to clone humans. Obviously, these experiments raise many ethical questions that will be difficult to answer. 

HATTORi M, HASHiMOTO H, BUBENSHCHiKOVA E and WAKAMATSu Y (2011) Nuclear transfer of embryonic cell nuclei to non-enucleated eggs in zebrafish, Danio rerio. Int... 

NiWA K, LADYGINA T, KiNOSHiTA M, OZATO K and WAKAMATSu Y (1999) Transplantation of blastula nuclei to non-enucleated eggs in the medaka, Oryzias latipes. Dev Growth Differ, 41,163-172. 

PEI D s, SUN Y H, CHEN s p, WANG Y p, HU w and ZHU z Y (2007a) Identification of differentially expressed genes from the cross-snbfamily cloned embryos derived from zebrafish nuclei and rare minnow enucleated eggs. Theriogenology, 68, 1282-1291. 

Reproductive Cloning. To clone an animal, mature e s are isolated from females of the animal species that is to be cloned. The egg is enucleated by piercing it with a microscopically narrow (0.0002-inch-wide) glass tube that is used to vacuum out the egg nucleus. The enucleated egg is fused with a cell from the body of the animal to be cloned and activated with either chemicals or an electric current. This procedure is called somatic cell nuclear transplantation (SGNT). 

FIGURE 8.4 Alternate methods for deriving pluripotent stem cells. In addition to conventional derivation of ESCs from the ICM of blastocyst stage embryos, ESCs have also been derived from a single blastomere derived from eight-cell morulas, by injection of a somatic cell nucleus into an enucleated egg, or by treating somatic cells with defined factors for reprogramming the differentiation state. 

Smith, L. D., and Ecker, R. E. (1965). Protein synthesis in enucleated eggs of Rana pipiens. Science 150, 777-779. 

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