Efficiency and safety of animal cloning.

Abstract

Developmental totipotency of the cell nuclei in the early embryonic stage was first clearly demonstrated by Spemann (1938). He ligated a recently fertilized newt egg so that cell division occurred in only one of two halves. When the nucleated portion cleaved to 8 to 16 cells, one of these nuclei was allowed to enter the uncleaved portion of the egg before it was completely separated from the rest. This resulted in the development of two normal larvae, indicating that the nuclei of an 8–16 cell embryo retain full developmental potential. Theoretically, 8–16 genetically identical offspring can be obtained from a single 8–16 cell embryo. Since most frontier experiments of animal cloning were carried out with the frog (; ), it was somewhat unexpected that the first animal cloned with an adult somatic cell was a mammal (sheep), not a frog. Almost all investigators thought that nuclei of adult somatic cells of the frog (and all other higher animals) were differentiated irreversibly and had lost their developmental totipotency. () surprised the world by reporting the birth of a sheep (Dolly) that was cloned with an adult mammary gland cell. We () and () soon confirmed that cloning with adult somatic cells was indeed possible, at least in the mouse and cattle. Subsequently, goats, pigs rabbits and a cat were cloned from adult somatic cells (; ; ; ). The cells used for cloning include: cumulus cells, oviductal cells and skin cells (Table 1). Table 1. Comparison of cloning success rates1 in various animals Type of Donor Cell Species Total number of reconstructed (nuclear-transferred) oocytes Number (%) of live offspring Notes Ref2 FETAL Fibroblast Mouse 3057 5(0.2) 1 Bovine 276 4(1.4) 1 died 2 1896 6(0.3) 3 Goat 285 3(1.1) 4 Pig 210 1(0.5) 5 Sheep 417 14(3.4) 11 died within 6 months 6 JUVENILE Skin cell Bovine 175 1(0.6) Died 51 days later 7 Sertoli cell Mouse 1190 7(0.6) 8 ADULT Mammary gland cell Sheep 227 1(0.4) 9 Cumulus cell Mouse 2468 31(1.3) 10 Fibroblast Bovine 440 6(1.4) 2 died 11 664 8(1.2) 12 Granulosa cell Bovine 557 10(1.8) 13 Pig 977 5(0.5) 14 1 As many investigators did not describe the original numbers ofthe oocytes used for nuclear transfer, success rates of cloning shown here are based on the number of reconstructed oocytes. 2 Ref(1) Ono et al., 2001; (2) Cibelli et al., 1998; (3) Lanza et al., 2000; (4) Baguisi et al., 1999; (5) Onishi et al., 2000; (6) McCreath et al., 2000; (7) Renard et al., 1999; (8) Ogura et al., 2000; (9) Wimmut et al., 1997; (10) Wakayama et al., 1998; (11) Kubota et al., 2000; (12) Lacham-Kaplan et al., 2000; (13) Wells et al., 1999; (14) Polejaeva et al., 2000.