Mostly, people think that an embryo can only (except in case of parthenogenesis) result from the sum of sperm and an egg. However, two research teams, one in Israel and another multinational collaboration have recently reported producing a synthetic mouse embryo from stem cells which lasted enough to have a brain and a beating heart.
There are three types of stem cells in early mammalian embryos: one will form the body’s tissues, while the other two assist in development. One of these stem cell groups will form the placenta and the other the yolk sac, which is where the embryo grows and gets its nutrients.
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