Embryo 36 Day Old External and Internal Anatomy
Computer Generated Image from Micro-MRI, actual size of embryo = 6.0 mm. These images offer several perspectives of the internal and external development of an embryo at the beginning of six weeks. The age is calculated from the day of fertilization. The upper left image depicts a left anterolateral view of the external anatomy of the embryo. The liver region is semi-transparent so as to display the vasculature. The upper right image reveals the a right anterior view of the internal structures of the same embryo. The lower left image illustrates limb development as seen from a superior right view. The lower right image reveals the inner structural development from the same perpective. The cardiovascular system continues to develop at a rapid rate during this phase. The heart 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 upper and lower right hand images depict 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. These two images also offer 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 (as seen in the upper left, upper right and lower right images). 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 mothers blood. Once the fetal circulatory system is formed, few vascular changes occur until birth and the umbilical vessels close. In the eye region (as represented by a red dot surrounded by a green ring the two upper and right lower images) 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 in the two right hand images. The brain is also undergoing rapid differentiation as the irregularly shaped vesicles continue to form. The prominent uppermost bump in these images represent the future cerebellum. Continual development in the brain will bring about three major components, the forebrain, midbrain, and hindbrain. During this phase of development the limbs buds become visible as outpocketings from the body walls (as seen in the lower left-hand image). Six week old embryos, the distal portions of the limb buds become flattened to form the handplates and footplates. Fingers and toes will develop when a process called cell death separates the these structures into five distinct parts.