Embryology of the Heart

Transcription

1 *Page 1A: Embryology of the Heart Human embryonic disc is divided into three layers: ectoderm, intraembryonic mesoderm, and endoderm. The embryonic disc lies between the amniotic cavity and the primary yolk sac. Intraembryonic mesoderm is divided into: paraxial mesoderm, intermediate cell mass, and lateral plate. The lateral plate will split into somatic layer mesoderm, and splanchnic layer mesoderm. It contains some cavities that join together to form one large cavity called intraembryonic coelom. This space will give rise to the future pericardial, pleural, and peritoneal cavities. Closure of these cavities is completed by lateral folding in order to form a cylindrical embryo. *Pages 4, 5A, 5B: Before folding of the embryo: The ectoderm differentiates to form the nervous system (brain, spine, and peripheral nerves). Parts of the embryo from cranial to caudal: 1. Septum transversum, which is a mass of mesoderm where the liver will form. The liver is an endodermal bud of the foregut that grows toward the septum transversum, these endodermal cells will form the parenchyma of the liver. 2. Primitive heart tube, and pericardial cavity, which is derived from the intraembryonic coelom. The primitive heart tube lies ventral to the pericardial cavity. 3. Buccopharyngeal membrane, also called prechordal plate, where ectoderm and endoderm come in contact with no mesoderm between them. This will form the future mouth. After folding of the embryo: October 27, 2013 Page 1

2 Due to the formation of an extended nervous network by the ectoderm and the formation of the neural tube; head, tail, and lateral body foldings form. During folding, part of the yolk sac is incorporated into the embryo as the gut. Parts of the embryo from cranial to caudal: 1. Buccopharyngeal membrane. 2. Primitive heart tube and pericardial cavity. The primitive heart tube now lies dorsal to the pericardial cavity, and ventral to the future esophageal part of the foregut. 3. Septum transversum. Primitive heart tube is formed by fusion of the right and left endocardial heart tubes during lateral folding. Pericardial cavities also fuse, which gives a single heart tube that is surrounded by a single pericardial cavity, and ventral to the foregut. The primitive heart tube is originally endothelium. Its myocardium and epicardium are derived from the splanchnic mesoderm that surrounds the pericardial cavity, forming a myoepicardial mantle. The primitive heart tube is suspended from the dorsal wall of the pericardial cavity by dorsal mesentery that soon disappears and gets replaced by the transverse pericardial sinus. *Pages 6, 7: Primitive heart tube has an arterial end, and a venous end. Five segments of primitive heart tube are defined, from cranial to caudal: Truncus arteriosus bubus cordis primitive ventricle primitive atrium sinus venosus. Blood flows from caudal end to cranial end. Bulbus cordis and ventricle are inside the pericardial cavity, but atrium and sinus venosus are attached to the septum transversum outside the pericardial cavity. October 27, 2013 Page 2

3 The heart tube undergoes looping (bending to the right) and rotation in a mechanism explained in the handout! If it bent to the left, the heart would form on the right side of the chest. The atrium and the sinus come to lie behind and above the ventricle. Notice that the atrium and the ventricle are not septated yet. The final result of the looping is to bring the arterial and venous ends closer together. Primitive ventricle forms only a part of the left ventricle; the remainder of the left ventricle and the entire right ventricle are derivatives of bulbus cordis. *Pages 8, 9, 10, 11: There are 3 major venous systems that flow into the sinus venosus end of the heart tube: 1. Vitelline veins: drain deoxygenated blood from the yolk stalk. 2. Umbilical veins: carry oxygenated blood from the placenta. The source of blood in the fetus is the placenta, not the lungs. 3. Common Cardinal veins: carry deoxygenated blood from the body wall of the embryo; formed by the union of anterior and posterior cardinal veins. The sinus venosus develops right and left horns. Each horn receives blood from these veins. Vitelline and umbilical veins pass through septum transversum in order to reach the horn. Each of them is divided into cranial part (between septum transversum and sinus venosus) and caudal part (below septum transversum). The right horn becomes larger than the left horn due to two left to right shunts of the blood: 1. Transformation of vitelline and umbilical veins: o Caudal parts of vitelline veins form portal vein. o Parts of vitelline veins passing through septum transversum (the liver) become incorporated into hepatic sinusoids. October 27, 2013 Page 3

4 o Cranial part of the right vitelline vein forms the terminal part of IVC. o Cranial part of the left vitelline vein disappears. o Right umbilical vein disappears. o Caudal part of the left umbilical vein carries blood from the placenta to IVC through ductus venosus (that connects the left portal vein and the IVC), which helps the blood to bypass the liver circulation because no metabolism is needed for the placental blood. Hint: The left umbilical vein is left; the left is left. o Cranial part of the left umbilical vein disappears. 2. The right anterior cardinal vein and the left anterior cardinal vein become interconnected by an oblique anastomosis that forms the left brachiocephalic vein. Eventually, the left horn loses its importance and forms, together with a small part of the left common cardinal vein, the coronary sinus which works as a tributary for the right horn in a mechanism that is fully explained in the handout. The right anterior and right common cardinal veins form the SVC. The final fate of sinus venosus is also fully explained in the handout; it becomes incorporated as the smooth part or the wall of the right atrium and its margins form right and left venous valves. The right valve forms the valves of IVC and coronary sinus, and crista terminalis. You can notice that the valve of IVC is continuous with crista terminalis in the adult heart because they have the same origin. *Page 12: An addition to what is written in the handout is that the two thickenings are formed of endocardium and some connective tissue. Septum intermedium divides the canal into right half, that contains the tricuspid valve, and left half, that contains the mitral valve. So, any anomaly of the endocardial cushions might affect these valves. October 27, 2013 Page 4

5 *Pages 13, 14: Dr. Faraj mentioned nothing more than what s already written in the handout, but I will rewrite it in another way so that it might become clearer, but you still have to refer to the handout for some unmentioned details. During fetal circulation, right atrial pressure is higher than left due to the large amount of blood coming from the placenta into the right atrium, and to the high pulmonary resistance, because lungs are collapsed in the fetus. The foramen ovale remains open during fetal life, in order to shunt blood from right to left atrium. Structures involved in the septation: The septum primum (ostium primum) grows inferiorly from the roof of the atrium toward the endocardial cushions, but it doesn t fuse with it. The foramen primum is located between the inferior edge of septum primum and the endocardial cushions. The foramen secundum (ostium secundum) forms within septum primum just before the foramen primum closes to maintain the right to left shunting of blood. The septum primum fuses with the endocardial cushions, obliterating the foramen primum. The thick septum secundum forms to the right of the septum primum from the roof of the atrium. It descends and partially covers the foramen secundum, but doesn t fuse with the endocardial cushions. The foramen ovale is the opening between septum primum and septum secundum. It s formed only of septum primum. The part of septum primum that forms the lower edge of foramen secundum is mobile. So, when blood tends to flow October 27, 2013 Page 5

6 from right to left before birth, it moves away and there s no obstruction of blood flow. The lower margin of the septum secundum, called crista dividens, divides the oxygenated fetal blood upon reaching the foramen ovale into two volumes, the greater volume of blood enters the left atrium, and the remainder, joined by blood from the SVC, passes into the right ventricle. After birth, closure of foramen ovale immediately occurs due to decreased atrial pressure caused by closure of umbilical vein, and decreased pulmonary resistance. Blood now tends to move from left to right, but this blood faces septum secundum that closes foramen secundum, which prevents it from moving to the right atrium; septum primum adheres with septum secondum, which closes the foramen ovale, forming fossa ovale. October 27, 2013 Page 6

7 Cardiovascular System Embryology Blood interatrial flow before birth Formation of interatrial septum Blood can t flow after birth October 27, 2013 Page 7

8 *Pages 15, 16: Unlike the atrial septum, the IV septum will develop and close completely before birth, without any shunting of blood between ventricles. The adult IV septum consists of two parts: muscular part, and membranous part. The muscular IV septum develops in the floor of the ventricle, ascends, but doesn t fuse with the septum intermedium, leaving the IV foramen. The membranous IV septum closes the IV foramen. It forms by the fusion of: 1. A downward extension from the right margin of the septum intermedium. It forms the greater portion of the membranous septum. 2. The proximal bulbar septum, which is formed by the union of two bulbar ridges. It also divides the bulbus cordis longitudinally into the infundibulum of the right ventricle, and the vestibule of the left ventricle. The membranous part of the interventricular septum is very susceptible to anomalies because of its multiple origins. The anterior part of it separates the right and left ventricles (interventricular), but the posterior part separates the left ventricle from the right atrium (atrioventricular). Why? 1. Because the interatrial septum and the interventricular septum don t attach the septum intermedium at the same point; the interventricular septum binds the right margin of the septum intermedium. So, the septum intermedium undergoes two pulling forces of different directions; one from the interatrial septum above, and the other from the interventricular septum below; the October 27, 2013 Page 8

9 membranous part formed by endocardial cushions lengthens and forms the atrioventricular part. 2. The septal cusp of the tricuspid valve divides the membranous part into anterior and posterior parts. Distal bulbar septum forms the pulmonary and aortic valves above the infundibulum and the vestibule. October 27, 2013 Page 9

10 *Summary: Embryonic structure Truncus arteriosus Proximal part of bulbus cordis Middle part of bulbus cordis Distal part of bulbus cordis Primitive ventricle Primitive atrium Right horn of sinus venosus Left horn of sinus venosus Absorbed pulmonary veins Adult structure Ascending aorta, pulmonary trunk Trabeculated part of right ventricle Infundibulum of the right ventricle, and aortic vestibule of the left ventricle Pulmonary and aortic valves, through the distal bulbar septum Trabeculated part of the left ventricle Anterior rough wall of the right atrium including its auricle, and also the left auricle Smooth part of the right atrium (sinus venarum) Coronary sinus Smooth part of the left atrium Best wishes from me to you. Your colleague: Aseel Nsairat. I look up at the night sky, and I know that, yes, we are part of this Universe, we are in this Universe, but perhaps more important than both of those facts is that the Universe is in us. When I reflect on that fact, I look up many people feel small, because they re small and the Universe is big, but I feel big, because my atoms came from those stars. Neil degrasse Tyson October 27, 2013 Page 10