Embryology - Biology 104, Spring 2006 - Albert Harris and Corey Johnson
OUTLINE OF EIGHTEENTH LECTURE: Feb 27, 2006, by Corey JohnsonHeart DevelopmentEstablishment of the vascular systemBlood vessels form primarily from the splanchnic mesoderm. Blood vessels form by three different mechanisms. Vasculogenesis we've seen in the yolk sac, where angioblasts (cells that give rise to blood vessels) coalesce to form vesicles that may combine to form tubes. Angiogenesis is the process where cell divisions increases the size and branching of existing vessels. The third mechanism (I don't know if it has a name) occurs where angioblasts migrate toward, and incorporate into existing vessels.The circulatory system has three major "loops." There's the yolk sac circulation which brings nutrients and blood cells to the body through the vitelline arteries and returns via the vitelline veins. There's the allantoic circulation which brings O2 (and nutrients in mammals) to the embryo (allantoic/umbilical vein) and removes CO2 and wastes (allantoic/umbilical arteries). And then there's the intraembryonic circulation that distributes blood to the body (dorsal aorta) and returns blood to the heart (cardinal veins). The heart pumps blood through a series of up to 6 pairs of aortic arches, which coalesce, returning blood to a large single vessel, the dorsal aorta. The aorta runs down the back of the embryo beneath the neural tube and notochord. Most of the blood travels from the dorsal aorta to the head, to the yolk sac (vitelline artery) or to the allantois (umbilical/umbilical) artery. The heart itself begins formation from the splanchnic mesoderm. The cells that will organize to form the heart begin on two sides of the body, in front of the head. Cells form blood islands as in the vitelline vessels of the yolk sac. The islands eventually coalesce forming the rudiments of two tubes. As the body folds (see lecture on extraembryonic membranes) are established in amniotes, the cardiacogenic (heart forming) region is tucked under the head. This tucking also forms the anterior end of the foregut. The two tubes fuse together forming a single tube. If prevented from fusing the embryo will form 2 hearts! In amniotes, the single tube formed is continuous with the vitelline veins which have developed from the yolk sac. The atrial region is posterior to the ventricular region. As the heart begins to beat, blood is oozed anteriorly from the vitelline veins, to the atrial region, to the ventricular region, to the aortic arches, and then to the dorsal aorta. Here's an : animation of early heart formation The regions of the heart from posterior to anterior are called:
Sinus venosus: region where the vitelline (and other) veins drain; blood goes to the... Identify the parts: In fish, this is the general configuration of the adult heart. Blood is pumped from the heart to the aortic arches. These arches expand to form the capillaries of the gills. Gas exchange occurs here: CO2 is dumped off and O2 is picked up. Blood then travels to the body where the opposite occurs. Then back to the heart to be pumped to the gills for gas exchange. Amphibian larvae have this arrangement too. They develop a 3 chambered heart later when they develop lungs. 2 atria, 1 ventricle. In amniotes, the heart soon grows too long for the space in which it is located and undergoes looping. Looping occurs in vitro (in culture) so there are forces internal to the heart that dictate its shape. It's not its growth that causes the change in shape. Such growth is rarely responsible for changes in embryonic shape. I can't seem to find a good picture of this. You'll have to do with my drawings. The atrium is moved anteriorly by this looping and the ventricle moved posteriorly. Eventually 2 septa divides the atria and the ventricles forming 4 chambers. The truncus arteriosus also divides (involving neural crest, of course!) forming the aorta and pulmonary trunk/artery. Picture of a chick heart that has begun looping The ventricle bends ventrally and to the right. The atrium and sinus venosus move dorsally. This is the first visible sign of right-left asymmetry. The heart undergoes a widening and separates in to right and left sides. The outflow (truncus arteriosus) divides into two sides, which lead into the aorta and the pulmonary trunk (artery). The heart and the kidneys represent two very different ways of developing. They both function early on, but must be "upgraded" for better equipment as development proceeds. The kidneys replace outdated machinery: the pronephros is replaced by the mesonephros. It works for a while but is eventually replaced by a better kidney, the metanephros. The heart, in contrast, begins as a simple tube. It's sufficient for the job, but later must develop a circulation to the lungs. So instead of replacing it, the embryo remodels the existing heart to have 4 chambers (in birds, mammals, and some reptiles). Each aortic arch is surrounded by mesenchymal tissue derived from the neural crest and mesoderm. In the region of the neck, NC cells migrate from the neural folds ventrally and proliferate in such great numbers that they form large bars of tissue. Each of these bars of mesenchyme are called pharyngeal arches. Through the center there is 1 aortic arch. The pharyngeal arches are lined with ectoderm on the outside and have indentations between adjacent arches, called pharyngeal clefts. The inside of the pharyngeal arches are lined with endoderm. They also have indentations between adjacent arches, called pharyngeal pouches. The region of the pharyngeal pouches corresponds to the foregut. More on these when we get to discuss endoderm. Some of the Aortic arches in birds, reptiles, and mammals degenerate, and others become other arteries not found in fish. The main arch of the aorta in adult humans, for example, comes from the left 4th arch.
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