MadSci Network: Molecular Biology |
In the experiments that led to Dolly, 277 attempts at nuclear transplantation were made. These zygotes were all implanted into ewe's oviducts and allowed to develop. After a few days, the transplants were collected in order to monitor their development. The researchers only recovered 247 implants. Further, only 29 zygotes actually developed into morulas. These 29 morulas were then implanted into 13 ewe's (one, two, or three at a time; two being the most common natural condition.) From these 29 morulas there was only one completed pregnancy and birth - Dolly. 277 Zygotes -> 247 implants recovered = Loss of 30 due to small size + human error. 247 implanted zygotes -> 29 morulas = Loss of 218 due to unknown problems with the micro-scale aspect of the procedure. Although these losses are not further specified, they represent problems with injecting the nucleus through the enucleated oocyte's protective coat (zona pellucida), and with the general handling of the oocytes and nuclei. 29 morulas -> 1 live birth = Loss of 28 due predominantly to a lack of synchronization between the cloned morula and the ewe into which the morula is implanted. If the right balance of hormones is not acheived, the ewe will not "adopt" an embryo and the embryo will not implant in the ewe's uterus. Just like natural impregnation, implantation is hormone- cycle stage specific. Unfortunately, difinitive molecular explanations are not available. These "answers" are actually reverse-engineered possible explanations. Sources: 1. Sheep cloned by nuclear transfer from a cultured cell line. Nature 1996 Mar 7;380(6569):64-6 Campbell KH, McWhir J, Ritchie WA, Wilmut I 2. http://www.synapses.co .uk/science/clone.html Hope this helps, Chris Neale.
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