Embryo 36 Day Old Cardiovascular System
Computer Generated Image from Micro-MRI, actual size of embryo = 6.0 mm. This image offers a left-sided perspective of the internal organ development of an embryo at the beginning of six weeks. The age is calculated from the day of fertilization. The most prominent organs displayed are those of the cardiovascular system which continue to develop at a rapid rate during this phase. The heart (seen here as a bright red rounded structure on top of a dark red bulge) is the first functional organ to develop in the human embryo. It begins its existence as two simple tubes that quickly fuse to form a single chamber or heart tube that is busily pumping blood by the 23nd day. At around the 25th day, it exhibits four slightly bulged areas that represent the earliest heart chambers called sinus venosus, atrium, ventricle, bulbous cordis. During the next three weeks of development, the heart tube undergoes dramatic contortions so as to change its structure to become a four-chambered organ capable of acting as a double pump. This image depicts the heart at the 36th day of development. The tubes have undergone the aforementioned changes and the heart is divided into its four definitive chambers. They will assume their adult positions in just one more month. This image also offers a clear depiction of the vessels that convey blood to and from the heart and brain. By the fourth week of development, the heart is pumping blood through the rudimentary vascular system. The blood largely bypasses the liver (seen here as the dark red bulge posterior to the heart). The umbilical vessels (posterior and anterior to the heart and liver) convey blood between the fetal circulation and the placenta where gas and nutrient exchanges occur with the mother's blood. Once the fetal circulatory system is formed, few vascular changes occur until birth and the umbilical vessels closed. In the eye region (as represented by a red dot surrounded by a green ring) the lens is almost completely closed and are starting to undergo retinal pigmentation. The growing nerve endings around the spinal cord are indicated in white. The brain is also undergoing rapid differentiation as the irregularly shaped vesicles continue to form. The prominent uppermost bump in this image represents the future cerebellum. Continual development in the brain will bring about three major components, the forebrain, midbrain, and hindbrain.