Early Development of the Cardiovascular System - PDF

Embryology Early Development of the Cardiovascular System OB JE C T I V E Describe the site, formation, union, and division of the of the heart tube. Describe the formation and fate of the sinus venosus. Describe the formation of the interatrial a...

Embryology Early Development of the Cardiovascular System OB JE C T I V E Describe the site, formation, union, and division of the of the heart tube. Describe the formation and fate of the sinus venosus. Describe the formation of the interatrial and the interventricular septae. Describe the formation of the two atria and the two ventricles. Describe the partitioning of the truncus arteriosus and formation of the aorta and pulmonary trunk. List the most common cardiac anomalies. The CVS (heart) is the first functional major organ to develop, It begins to beat at 22 to 23 days. (from fertilization). At the start of its development, the cardiovascular system exists as two regions near the cranial end of the embryo. These regions are called cardiogenic fields and they are derived from the mesoderm layer of the embryo. The heart primordium is first evident at 18 days (as an angioplastic cords which soon canalize to form the 2 heart tubes). The heart is the first functional organ to develop. It develops from splanchnic mesoderm in the wall of the yolk sac (cardiogenic area), cranial to the developing mouth & nervous system and ventral to the developing pericardial sac. Angiogenic cell culture develop at area of cardiogenic plate to form right and left endocardial heart tubes. -Both tubes fuse together forming single heart Folding tube. of embryo Lateral folding: fuses the two lateral sides of the embryo. This brings to two cardiogenic fields into the midline so they can fuse and form the primitive heart tube. cephalocaudal folding: Cephalo-caudal folding brings the cardiogenic fields from the cranial end towards the center of the embryo to sit in thoracic region where the heart will be. primitive heart tube. 1. Sinus Venosus. 2. Truncus Arteriosus. 3. Bulbus Cordis. 4. Common Ventricle. 5. Common Atrium. The endocardial heart tube has 2 ends: 1. Venous end(Caudal) : Sinus Venosus. 2. Arterial end(Cranial) : Truncus arteriosus. Cardiac Looping The newly formed heart tube is surrounded by the pericardial sac. As the heart tube grows and elongates, it gets too long for the sac. This means that to fit, it must loop. The primitive ventricle moves ventrally and to the right, while the primitive atrium moves dorsally and to the left. This puts the inflow portion of the heart (veins and atria) behind the outflow portion (ventricles and arteries) – the same shape and orientation as mature hearts.  The sinus venosus: The sinus venosus: formed of (body of sinus venosus) and two horns (right and left). Each horn receives blood from three important veins. a-The Vitelline vein drains the yolk sac. b-The umbilical vein drains the placenta. c-The common cardinal vein drains from the fetal body The two horns are at first equal in size, but later on, the right horn enlarges while the left one regresses. This is caused by a left- to- right shunting of blood. The sinus venosus opens into the primitive atrium by the sinu- atrial orifice. This orifice is guarded by 2 right and left venous valves. These 2 valves fuse together at the cranial end of the orifice to form the septum spurium. Fate of Sinus Venosus 1-The body and right horn: Absorbed into the primitive atrium and for the smooth part of the right atrium. 2-Septum spurium and left venous valves blends with the inter-atrial septum and leaves no trace in the adult. 3-The right valve gives the following derivatives: Its cranial part: forms a part of the crista terminalis. Its caudal part: forms the valve of the inferior vena cava and valve of the coronary sinus. 4-Left horn of sinus venosus: becomes the coronary sinus. 5-Common cardinal vein: varies according to the side: a-On the left side: forms the oblique vein of the left atrium (vein of Marshal). b-On the right side: forms the lower 1/2 of the superior vena cava. N.B: Both the umbilical and vitelline veins on the left side lose connection with the left horn of the sinus venosus.  The Right Horn forms the smooth posterior part of the right atrium. The Left Horn and Body atrophy and form the Coronary Sinus. The Left Common cardinal vein forms the Oblique Vein of the Left Atrium. The development of the atria passes in the following stages: Division of the atrioventricular canal into 2 halves by ventral and dorsal endocardial cushion. Division of the primitive common atrium into right and left halves by development of the interatrial septum. Absorption of the atrioventricular canal into the 2 atria. Absorption of the right horn of sinus venosus into the right atrium. Absorption of the pulmonary veins into the left atrium. RIGHT ATRIUM: The right atrium develops from most of the primitive atrium, and part of the sinus venosus. The sinus venosus receives blood from the right and left sinus horns, bringing blood into the primitive heart tube. The right sinus horn is partially absorbed by the primitive atrium as it grows. This forms the superior and inferior vena cava. The left sinus horn becomes the coronary sinus which drains venous blood from the coronary vessels into the right atrium. LEFT ATRIUM : The left atrium develops from a small part of the primitive atrium, and the proximal portions of the pulmonary veins. The pulmonary veins begin as a single vein entering the left atrium. This vein is formed from four branches which converge to form one vein draining into the developing left atrium. As the left atrium grows, it absorbs the single pulmonary vein, absorbing all the way to the four branches. This means that when the left atrium has finished growing, it is receiving blood from four pulmonary veins, as seen in mature hearts.  Formation of the Cardiac Septa The atrioventricular canal - It is the elongated constriction between primitive atrium and ventricle: -2 endocardial cushions (ventral and dorsal) appear, and fuse together forming septum intermedium. -Septum intermedium divides the canal into right and left parts. Primitive atrium is divided into right and left chambers by an inter-atrial septum. Throughout intra uterine life, there is always a gap connecting both atria: A) Septum primum: It is crescentic membrane grows from roof towards the septum intermedium, and separated from the septum by foramen primum Just before its complete closure, septum primum ruptures in its center forming another foramen (foramen secundum). The upper part of septum primum that is attached to the roof of the common atrium shows gradual resorption forming an opening called ostium secondum. Another septum descends on the right side of the septum primum called Septum Secundum. It forms an incomplete partition between the two atria. Consequently a valvular oval foramen forms, (Foramen Ovale)  Ventricles -At 1st, the bulbus cordis lies to the right of the primitive ventricle with a deep sulcus in between. It then moves slightly ventral to the ventricle. -Due to expansion of the bulbus cordis and the primitive ventricle, the fold projecting between the 2 cavities regresses and the sulcus between them disappears, thus a common bulbo- ventricular chamber is formed. Septation of the bulbo-ventricular chamber: 1-Development of the interventricular septum divides the common bulbo-ventricular chamber into right and left ventricles. 2-The right ventricle is derived mainly from the bulbus cordis and partly from the primitive ventricle, while the left ventricle develops mainly from the primitive ventricle and partly from the bulbus cordis. Ventricular Separation The formation of the ventricular septum take place in two steps. -First, a muscular portion of heart tissue grows upwards from the floor of the primitive ventricle towards the endocardial cushions. It doesn’t quite reach the cushions, forming the primary interventricular foramen. -A membranous portion then grows down from the endocardial cushions to meet the membranous portion and close the foramen. Bulbus Cordis The bulbus cordis forms the smooth upper part of the two ventricles. Right Ventricle: Conus Arteriosus or (Infundibulum): which leads to the pulmonary trunk. Left ventricle: Aortic Vestibule: leading to ascending aorta Partition of Truncus Arteriosus The bulbus cordis and truncus arteriosus form one tube allowing outflow from the heart. This tube needs to be spilt in order to form the aorta and pulmonary trunk. In the 5th week, proliferation of mesenchymal cells (Endocardial Cushions) appear in the wall of the truncus arteriosus ,they form a Spiral Septum called the aorticopulmonary septum – consequently forming the aorta and pulmonary trunk: A. It divides the Lower part of the TA into Right & Left parts B. It divides the Middle part of TA into Anterior & Posterior parts. C. It divides the Upper part of the TA into Left & Right parts.  Major Cardiac Anomalies 1-Atrial Septal Defects (ASD) There are three types: 1- Absence of septum primum and septum secundum, leads to common atrium. 2-Absence of Septum Secundum 3-Large (Patent) foramen ovale 2-Ventricular septal defect(VSD) Roger’s disease: 1- Absence of the membranous part of interventricular septum. 2- This means that the primary interventricular foramen remains open. (There is a space between 2 ventricles , so it leads to mix of Venus and arterial blood ) 3-Tetralogy of fallot Includes four heart malformations present together: 1-ventricular septal defect(VSD). (Absence of the membranous part ) 2- Pulmonary stenosis(narrowing of pulmonary valves , so the aorta will be larger than pulmonary). 3- Right ventricular hypertrophy. 4-4-overriding of aorta (blood enters the aorta from both ventricles). 4-Transposition of great arteries (TGA) - TGA is due to abnormal rotation or malformation of the aorticopulmonary septum (spiral), so the right ventricle joins the aorta, while the left ventricle joins the pulmonary artery. One of the most common causes of cyanotic heart disease in the newborn (blue baby). Often associated with ASD (atrial septal defect) or VSD (ventricular septal defect) Formation of the Great Vessels Vessels arise from the truncus arteriosus as aortic arches. There are five pairs of arches, numbered I, II, III, IV, VI (arch V doesn’t form in humans, so there are 5 arches numbered between 1-6 without a number 5). These arches, along with the trucncus arteriosus, contribute to the formation of large arteries: Truncus arteriosus: divided by the aorticopulmonary septum to form the pulmonary trunk and the aorta. Sixth arch: goes on to form the right and left pulmonary arteries arising from the pulmonary trunk. The left artery maintains its connection to the rest of the vessels via the ductus arteriosus, however the right artery loses this connection. When the baby is born and the ductus arteriosus closes, this separates the pulmonary circulation from the systemic circulation. o The sixth arch is also known as the pulmonary arch. Fourth arch: on the left side becomes the arch of the aorta, and on the right becomes the right subclavian. Third arch: becomes the common carotids and the first part of the internal carotids. Second and first arches: disappear.  Third arch Fourth arch Sixth arch Fetal Circulation The fetal circulation works very differently to adult circulation. The following steps outline the flow of blood from the placenta to the systemic circulation of the fetus: 1. Oxygenated blood is carried from the placenta into the fetal circulation via the umbilical vein. 2. The oxygenated blood enters the inferior vena cava and mixes with deoxygenated blood. It bypasses the developing liver via the ductus venosus. o The liver is very metabolically active, so bypassing it means that oxygenated blood can maintain its oxygen saturation for when it reaches the heart to be pumped around the body (most importantly, the brain) 3. The blood enters the right atrium and passes into the left atrium via the foramen ovale, therefore bypassing the pulmonary circulation. The blood is able to shunt from the right to left side of the heart because pressure in the right side of the heart is higher than the left in the fetus. o The baby is not breathing and instead the fetus' blood is being oxygenated by the mother's blood. Therefore blood doesn’t have to go to the alveoli for oxygenation, so the pulmonary circulation can be bypassed. 4. The blood is pumped from the left ventricle into the aorta. 5. Blood that doesn’t pass through the foramen ovale, and instead is pumped into the pulmonary trunk from the right ventricle, enters the systemic circulation at the arch of the aorta via the ductus arteriosus. This system exists so that the right ventricle still has some blood to pump against. Changes to the Circulation After Birth When the baby takes its first breath, the pO2 increases. This change causes the ductus arteriosus to close. o Ductus arteriosus becomes the ligamentum arteriosum. More blood now flows through the pulmonary circulation as blood in the pulmonary trunk cannot leave via the ductus arteriosus anymore. This causes increased venous return to the left atrium, leading to an increase in left atrial pressure. The pressure in the left atrium exceeds that of the right, which causes the foramen ovale to close. o Foramen ovale becomes the fossa ovalis. When the umbilical cord is cut, there is no longer blood flowing through the umbilical vein, causing the ductus venosus to collapse. o Ductus venosus becomes the ligamentum teres.