Figure 1.

(A) Timeline of human early placental development: Adapted from Lifemap Discovery (Edgar et al., 2013), “At the beginning of week 3 primary stem villi, consisting of a cytotrophoblast core covered by a syncytial layer, appear. Extraembryonic mesodermal cells or cytotrophoblast penetrate the core of the primary villi and grow in the direction of the decidua to form secondary stem villi and by the end of week 3 the mesodermal cells differentiate into blood cells and small blood vessels, forming the villous capillary system, and creating tertiary villi. By week 4, capillaries in the tertiary villi contact capillaries developing in the mesoderm of the chorionic plate and in the connecting stalk, eventually contact the intraembryonic circulatory system, and connect the placenta and the embryo. Thus, in week 4, when the heart begins to beat, the placental villous system is able to supply the embryo with oxygen and nutrients.” (B) Timeline of human embryonic heart development: Adapted from Lifemap Discovery (Edgar et al., 2013). “The human heart develops on day 18 or 19 following fertilization. In response to induction signals from the underlying endoderm, the mesoderm in the cardiogenic area forms the cardiogenic cords. A hollow center forms within the cords, giving rise to the endocardial tubes. With lateral folding of the embryo, the paired endocardial tubes approach each other and fuse into a single tube called the primitive heart tube. The primitive heart tube develops into five distinct unpaired regions and begins to pump blood”.