Scientists called for a key 14-day time limit for growing human embryos in the lab to be relaxed as they outlined new ethical guidelines for the fast-changing world of stem cell research.<br /> <br /> In its first update since 2016, the International Society for Stem Cell Research (ISSCR) said the decades-old rule had little scientific merit, although they stopped short of giving the green light for researchers to breach the time constraint, which is baked into law in some countries. For years scientists have had to infer the exact changes those human embryos go through in early development. But new methods of culturing them in a dish -- and of creating embryo-like models using stem cells -- have enabled scientists to track this process right up to the edge of the two-week limit, when the embryos must be destroyed. <br /> <br /> It is soon after 14 days that the first signs of the central nervous system appear and the beginnings of tissues are formed. Scientists think it is during this time that problems occur that cause recurring miscarriages and congenital abnormalities, like those of the heart and spine.<br /> <br /> The ISSCR update covers a wide spectrum of stem cell research, including studies into the implantation of human cells in animal hosts, gene editing and "organoids" -- tissue cultures derived from stem cells to replicate organs. One major development in stem cell science is the production of early-stage human embryo models -- made using stem cells derived from tissue, like skin cells.<br /><br />بواسطة د. سرى عبد الخالق عوض