Embryo 36 Day Old Brain and Cardiovascular System: Computer Generated Image from Micro-MRI, actual size of embryo = 6.0 mm. This image offers a right-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. Of the 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 somewhat shadowed red rounded structure in the medial aspect of the embryo) is the first functional organ to develop in the human embryo. The heart 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. By the fourth week of development, the heart is pumping blood through the rudimentary vascular system. 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 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.
Prenatal development is the process in which an embryo and later fetus develops during gestation. Prenatal development starts with fertilization the first stage in embryogenesis which continues in fetal development until birth.
In human pregnancy, prenatal development, also known as antenatal development, is the development of the embryo following fertilization, and continued as fetal development. By the end of the tenth week of gestational age the embryo has acquired its basic form and is referred to as a fetus. The next period is that of fetal development where many organs become fully developed. This fetal period is described both topically (by organ) and chronologically (by time) with major occurrences being listed by gestational age.
In other animals the very early stages of embryogenesis are the same as those in humans. In later stages, development across all taxa of animals and the length of gestation vary.
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