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In human embryology, weeks 6 through 8 are characterized by the growth and differentiation of tissues into organs. This process is known as organogenesis and occurs from weeks 3 through 8, the embryonic period. During week 3, gastrulation occurs, establishing 3 distinct cell layers: the mesoderm, endoderm, and ectoderm. These are the primary germ cell layers from which organs arise during organogenesis.[1] In particular: The endoderm forms the organs of the gastrointestinal and respiratory systems, as well as the thymus, parathyroid, bladder, and urethra. The ectoderm is responsible for developing the skin and skin appendages, the nervous system, and portions of sensory organs. The mesoderm forms the circulatory system, blood, lymphatic system, bone, cartilage, muscles, and many internal organs. For example, the kidney, spleen, ureters, and adrenal cortex derive from mesoderm.[2] At the end of week 8, organ systems have developed and are ready for further maturation. By week 9, the fetal period begins, which involves the growth and differentiation of anatomical structures and lasts until birth.

As previously stated, cardiac outflow separation occurs around week 6. The truncus arteriosus becomes divided into an ascending aorta and a pulmonary artery via spiraling of the aorticopulmonary septum. Failure of spiraling results in a congenital heart defect known as transposition of the great arteries. In this condition, the ascending aorta and pulmonary trunk are flipped, creating a right-to-left shunt. The aorta arises from the right ventricle, and the pulmonary trunk arises from the left ventricle. As a result, physiologic changes occur in the ventricles. The high pressure of the aorta causes right ventricular hypertrophy. The low pressure of the pulmonary artery causes left ventricular atrophy. This condition is incompatible with life unless a shunt is present, which allows the mixing of oxygenated and deoxygenated blood.[11]