11.12.08 organ systems: cardiovascular development 2

this is the second lecture in the cardiovascular development series and focuses on development of prenatal circulation and the transition to post natal circulation. it starts with where we left off with the last lecture, looking at the development of the larger structures in the heart, like the aortic arch being formed from the truncus arteriosus, forming the aorta on the left. the left recurrent laryngeal nerve is underneath the aorta and the right recurrent laryngeal nerve is underneath the right subclavian artery.

then we go back to some possible defects in structural development in the BC/TA trunk. when the spiral aortic-pulmonary septum forms from the endocardial cushion, there are several possibilities for abnormal development. these are: lack of septum altogether, called persistent truncus arteriosus, lack of a spiral in the septum, causing transposition of the aorta and pulmonary trunks, and an unequal divide in the trunk, causing one of the trunks to be bigger than the other. the tetralogy of fallot is a condition that combines the unequal divide with a VSD, resulting in right ventricular hypertrophy (induced by the pulmonary stenosis) as well as "aortic override".

finally, we look at the development of fetal circulation on a larger scale, as it interacts with its placenta. the trophoblast that is embedded in the endometrium differentiates into two layers: the syncytiotrophoblast, which contains spiralling maternal blood vessels and interstitial spaces, and the cytotrophoblast, which has "choroinic villi" that extend into the interstitial spaces and contain the fetal blood vessels. we then shift over to look at the difference between pre and post natal circulation, which is mainly in prenatal's use of shunts which divert blood from the liver and lungs. the ductus venous diverts blood from the liver, the ductus arteriosus and foramen ovale divert blood from the lungs, instead going directly back into circulation, which apparently "reduces oxygenated blood" in the lower limbs. two more random facts: blood supply from umbilical cord and vena cava remain in separate streams when entering the right atrium. the right ventricle and pulmonary trunk receives more deoxygenated blood and the left ventricle receives more oxygenated blood.

11.11.08 organ systems: cardiovascular development 1

this unit covers some basic ideas in early cardiovascular development. the first section focuses on the development of blood and blood vessels: blood production starts mainly in the yolk sac and moves to the liver/spleen, then to the bone marrow during development and involves differentiation of hemangioblast cells. these cells are derived from mesenchymal cells and triggered by the growth factor FGF to differentiate into blood cell precursors, hematopoetic stem cells, or blood vessel (capillary) walls, which then converge into larger and larger vessels. VEGF is a growth factor that stimulates nearby mesenchyme cells to differentiate into smooth muscle or pericyte cells.

the next sections describe the early structural development of the heart. the formation of the heart chambers begins when the endocardial cushion forms in the middle of the heart via a fusion of projections of tissue from the anterior and posterior walls. in the atriums, the primary septum is formed with a secondary foramen, and the secondary septum is formed with a foramen ovale. in the prenatal heart, blood flows from the right atrium, into the foramen ovale, through the primary septum, directly into the left atrium, bypassing the lungs. (this is the second "shunting" we've learned about- the first being the ductus arteriosus) in the post natal heart, the pressure from the pulmonary artery in the left atrium causes the primary septum to close and fuse with the secondary, creating the fossa ovalis. the ventricles are partitioned when the muscular and the membranous aspects of the interventricular septum join, closing up the interventricular foramen. finally, the endocardial cushion projects a spiral aortic pulmonary septum that projects into the bulbus cordis / truncus arteriosus vessel and separates it into the aortic and pulmonary trunks. the points of fusion between the bulbus cordis and truncus arteriosus becomes the semilunar valves.

we then look at some defects possible in the development of these structures. in the atriums, atrial septal defect can occur, called probe patent formation, when there is an imperfect adhesion between the primary and secondary septums. in the ventricles, VSD can occur when the membranous aspect of the interventricular septum does not close completely.