Heart Formation

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Description

The embryo's heart is the first organ that forms in its body. At day 18 after conception the embryo, about the size of a raisin, is in the uterus, surrounded by its yolk sac and amnion. Above the head region is an area that contains two tubes that will form the heart. During the next two days these tubes fuse. The single tube formed begins bending and twisting to form a simple version of the heart over the next eight days. After two-and-one-half weeks, the embryonic heart will have changed from two simple tubes to a complex organ. On day 21 the fused heart tube has moved below the embryo's developing head region. By day 22 the tube has moved to the area that will eventually be the embryo's thoracic, or chest, cavity. Also, on around day 22, the heart begins to beat. At month 2 the embryo becomes a fetus; its heart superficially resembles that of a newborn. However, the insides of an embryo's heart and a newborn heart are different in form and function.