Scientists from the Chinese Academy of Sciences celebrated the second “birthday” of the rhesus macaque ReTro, which is a cloned organism. This is a unique achievement, an exception, as cloning primates almost always fails because of the many complexities involved. 113 embryos were used to create ReTro, but only 11 of them proved suitable for implantation into surrogate mothers, and there were only two pregnancies in the end. One was a miscarriage, the second was ReTro.
The movement of biological materials in cloning is too complex and unpredictable a process, especially in the case of primates. Each success, like ReTro’s, is a consequence of overcoming another fundamental problem. In his case, it was a defect in the development of the placenta when placing an egg with clone material into the uterus of a surrogate mother.
While working on the problem, Chinese scientists replaced the trophoblast layer from which the placenta is formed with a man-made analog. To do this, they took material from another embryo obtained by in vitro fertilization. It was more “healthy” than the original, allowing the fetus to develop normally.