Thursday, January 23, 2014

Heart Development and Embryonic Circulation

Week 5-Highlights: Heart development, brain & spinal cord formation, arm/leg buds appear, gut tube development

The heart is the first organ in the embryo to develop and function, it pumps blood by week five. The circulatory system including the heart comes from the middle embryonic layer- the mesoderm. For the heart to get its start, the mesoderm first branches off into two distinct cell layers- the somatic (parietal) mesoderm and the splanchnic (visceral) mesoderm. (Parietal and visceral are another term pair we see in anatomy where parietal is the outer layer and visceral is the inner layer). The latter gives rise to cardiac precursor cells, these precursor cells will eventually give rise to the endocardium (lining of the heart chambers and valves) and the myocardium (as the suffix ‘myo’ suggests these are the musculature of the ventricles and atria- the chambers of the heart).  The overall process of the organ development of the heart is pretty fascinating, in my opinion the heart is one of the most complex organs in the body so the idea that it is the first to be functionally in the embryo indicates how much complex work is being done so early in embryonic development. Using the balloon animal artist metaphor the heart begins as a basic tube, patterned along the anterior/posterior (head/anus) axis to set where the eventual regions of the heart will be oriented. First up the tube needs to gain some flexible if it’s going to start to look like the adult heart with its four chambers and four major veins and arteries coming in and out of it. In order for this to happen the mesoderm around the tube degenerates so that only the arterial and venous ends are tethered. This sets the field for the heart to fold ventrally (towards the belly) and bulges into five early regions- Anterior to posterior the regions are- Truncus Arteriosus, Bulbus Cordis, Primitive Ventricle, Primitive Atrium, and the Sinus Venosus.

Looping occurs which changes the anterior/posterior and left/right polarity of the heart- the developing ventricle region shifts to the left and the developing atrium region moves to the right. After this reorientation the developing heart chambers will separate into two each- so that there are two atria and two ventricles- this occurs as cell growth and migration makes septa to separate the given chamber into two (as you can probably guess this involves a lot more terminology which I’ll skip). First the atria forms two chambers and then the ventricle. After this is complete the outflow tract also needs to become separated by septa. The outflow tracts are the two separate routes blood takes- the pulmonary circulation route via pulmonary artery and the systemic (body) circulation route via aorta.  This involves the top two regions mentioned above- the Truncus arterious and the Bulbus Cordis- they separate into distinct areas and twist around each other forming the aorticopulmonary septum which divides the aorta from the pulmonary artery.  Based on the research I did it seems a pretty safe thing to say that these two embryonic structures, the Truncus arterious and Bulbus Cordis become part of the adult structures the aorta and the pulmonary artery (also called the pulmonary trunk). After all of this the four heart valves are formed, separating the distinct chambers and ensuring blood flow is unidirectional.

Embryonic blood flow is distinct from what will become in the fully developed organism. This is due to the fact that the placenta is playing the role of the lungs, kidney, and liver in the developing embryo and thus blood is shunted to avoid these areas (or lack thereof) until they are developed. Let’s briefly take a look at the major differences in embryonic circulation compared to adult circulation (and by adult I mean that loosely really once all the organs are developed it is ‘adult’). Essentially what is different is a detour shunting blood away from areas of the embryo that are undergoing construction. Also we learned earlier the large multifactorial role the placenta plays during embryonic development. The placenta swoops in and allows for full function of gas exchange (which will be done by the lungs once they are good to go) and filtration (which the liver and kidney will take over when they are ready). This means that blood doesn’t really need to go to these two construction zones and it order for it not to the embryo builds three shunts or bypasses that help redirect blood away from the developing lung and the developing liver and kidneys.  In adults the major one sentence summary is the pulmonary circulation system carries deoxygenated blood away from the heart to the lung to become oxygenated and sends it back to the heart freshly oxygenated, while the systemic circulation path brings the oxygenated blood from the heart to all over the body. There are two main ‘out’ valves- the aorta and the pulmonary artery and two main ‘in’ valves- vena cava and the pulmonary veins and the flow of blood goes from the vena cava → right atrium → right ventricle → pulmonary artery → lungs (then back to the pulmonary veins) and the pulmonary veins → left atrium → left ventricular → aorta → body (then back to the vena cava). In the developing organism the blood is shunted from the right atrium to the left atrium, bypassing the whole system that brings blood to the lungs. As well another shunt is present to collect any blood that gets passed the first shunt and made its way into the right ventricle (from the right atrium).  This blood makes it from the right ventricle to the pulmonary artery, from here it is sent to the aorta. Some blood does make it passed these shunts to the developing lung and it will stay there to aid lung development. The other potentially obvious components of fetal circulation, since we know now the gas exchange occurs in the placenta, is that there are both umbilical veins that bring oxygenated blood to the embryo and umbilical arteries that carry the deoxygenated blood back to the placenta.  I’ve left out a few more veins and arteries as well as name of the shunts for a somewhat simpler picture of what’s going on overall.  

Fast Facts- The fetus' heartbeat can be detected around the sixth week and it is about twice as fast as the mother's.

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