04. CVS (1).pptx

Cardiovascular system development By Guta B. Assistant PROFESSOR of Human Anatomy  Blood Islands During 3rd week of gestation angioblastic blood islands of mesoderm (angiogenic clusters) appear in the yolk sac and chorion The innermost cells of these...

Cardiovascular system development By Guta B. Assistant PROFESSOR of Human Anatomy  Blood Islands During 3rd week of gestation angioblastic blood islands of mesoderm (angiogenic clusters) appear in the yolk sac and chorion The innermost cells of these blood islands are ==hematopoietic cells that give rise to the blood cell lines. The outermost cells give rise to the endothelial cell layer of blood vessels. A series of blood islands eventually coalesce to form blood vessels. Early Heart Tube development Appears in the middle of the 3rd week == i.e. mandatory to meet nutrient needs of rapidly growing embryo Cardiac progenitor cells(cardiac premorial cells) lie in the epiblast, immediately lateral to the primitive streak Cells migrate sequentially destined to form :-  Cranial segments of the heart --- the outflow tract, =migrate first  More caudal portions--- right ventricle, left ventricle, and sinus venosus, sequentially migrate next  The cells proceed toward the cranium and position themselves rostral to the buccopharyngeal membrane and neural fold  Here they reside in the lateral plate mesoderm (splanchnic layer)  Induced by the underlying pharyngeal endoderm to form cardiac myoblasts  Blood islands from the splanchnic mesoderm appear and form a plexus of vessels lying deep to the:-  === horseshoe-shaped a future = pericardial cavity  These small vessels develop into paired:-  =====endocardial heart tubes  The Splanchnic mesoderm proliferates and develops into the myocardial mantle which gives rise to the myocardium.  The epicardium develops from cells that migrate over the myocardial mantle from areas adjacent to the developing heart.  This region is known as the ===cardiogenic field  The bilateral endocardial heart tubes continue to develop and connect with a pair of vessels, the dorsal aortae, located on either side of the midline.  As the embryo develops, the lateral folding and cephalic growth of the embryo shift the endocardial heart tubes medially, ventrally and caudally. They fuse in the midline:- ==== as a single endocardial heart tube The endocardial heart tube is surrounded by the myocardial mantle and between these two layers is the cardiac jelly. The resulting heart tube is suspended in the pericardial cavity by the dorsal mesocardium. When the single heart tube is formed, the embryo is in the fourth week of gestation  After folding, the heart becomes a continuous expanded tube consisting of an inner endothelial lining and an outer myocardial layer.  It receives venous drainage at its caudal pole and begins to pump blood out of the first aortic arch into the dorsal aorta at its cranial pole Then, the developing heart tube bulges more and more into the pericardial cavity. a fold of mesodermal tissue, dorsal mesocardium initially attached the tube, to the dorsal side of the pericardial cavity  No ventral mesocardium is ever formed Fusion of the heart tubes  The Cardiac Loop Formation  Starting on day 21, a serious of expansions and constrictions appear within the heart tube  The primitive heart tube elongates and develops alternate dilatations and constrictions :  The five main dilatations 1. Truncus arteriosus 2. Bulbus cordis 3. Primitive ventricle 4. Primitive atrium 5. Sinus venosus Formation of the Cardiac Loop 1. Truncus arteriosus:- is continous cranially with aortic sac ,from which aortic arches develop 2. Bulbus cordis & ventricle:- grow faster than other regions, so the heart bends upon itself, forming U-shaped bulboventricular loop by the end of 4th week 3. The atrium & sinus venosus :-also come to lie dorsal to truncus arteriosus, bulbus cordis & ventricle. When heart tube elongates forming S-shaped cardiac loop.  The bulbus cordis shifts to the right side of the ventricle. The atrium expands transversely, bulging on either side of the bulbus cordis forming auricles These five dilatations (separated by constrictions)  In a caudo-cranial order, they are: sinus venosus, atrium, ventricle, bulbus cordis, truncus arteriosus  These two ends of the heart tube are connected to the intra-embryonic vessels: ==At the venous end, each horn of sinus venosus receives 3 veins i.e := 1. Vitelline vein from the yolk sac. 2. Umbilical vein from the placenta. 3. Common cardinal vein from the body wall. =====At the arterial end, the truncus arteriosus divides into 2 horns, each is connected to dorsal arteries by the aortic arches Primitive atrium:- gives rise to the Rt and Lt auricles Primitive ventricle:- gives rise to most of the definitive Lt ventricle  Bulbus cordis:- is separated from the ventricle by the bulboventricular sulcus which differentiate into several regions A. The inferior region forms most of the Rt ventricle B. Conotruncus contributes mainly to the definitive outflow tracts  Conus cordis : proximal part, forms a small part of the Rt ventricle and the proximal end of cardiac out flow tracts Truncus arteriosus:- distal part, forms the ascending aorta & pulmonary trunk Aortic sac:- most cranial segment, leads directly into Rt & Lt first aortic arch The Fate of the sinus venosus In the middle of the fourth week, the sinus venosus receives venous blood from the right and left sinus horns  Each horn receives blood from three important veins 1. The vitelline or omphalomesenteric vein 2. The umbilical vein 3. The common cardinal vein At first communication between the sinus and the atrium is wide Soon, however, the entrance of the sinus shifts to the right  This shift is caused primarily by left-to-right shunts of blood, which occur in the venous system during the 4th and 5th weeks of development With obliteration of the right umbilical vein and the left vitelline vein during the fifth week, the left sinus horn rapidly loses its importance  The opening between the sinus venosus and the atrium (sino-atrial orifice) has the shape of vertical slit.  Its edges are called the right and left venous valves. When the left common cardinal vein is obliterated at 10 week, all that remains of the left sinus horn is the oblique vein of the left atrium and the coronary sinus  As a result of those changes, the right sinus horn and veins enlarge greatly Overall fate of the sinus venosus and its tributeries 1. The sino-atrial orifice moves to the right and opens in the part of the primitive atrium that will become the right atrium 2. The right horn progressively enlarges and becomes incorporated into the right atrium giving the smooth posterior part of the right atrium called sinus venarum.  Aorta Superior vena cava Pulmonary artery Oblique Oblique vein of vein left of left atrium atrium Pulmonar y veins Coronary sinus Inferior vena cava Coronary sinus 22 3. The left horn diminishes forming the coronary sinus. 4. From the 6 veins, 3 veins disappear and 3 persist forming veins that open into the right atrium: a. The right vitelline v. upper part of IVC b. The right common cardinal v. SVC c. The left common cardinal v. oblique vein of left atrium. 5. The venous valves: a. The right venous valve  Upper portion crista terminalis  Lower portion valve of coronary sinus and IVC b. The left venous valve is incorporated with septum secundum 23  the venous valves Interseptovalvul ar Superior vena cava space Septu Septum m Pulmonar Sinus Septum y primum Septum spurium primu venaru secundu m vein m m Crista Right terminali Venous s valve Sinuatrial orifice Left venous Valve of inferior vena cava Inferior endocardial Valve of coronary sinus cushion 24  Formation of the Cardiac Septa The major septa of the heart are formed between the 27th and 37th days of development by two methods:- 1. Tissue growth ► Two of more actively growing masses of tissue which approach each other in the same plane, fuse to divide a single chamber into two ► Such a septum may also be formed by active growth of a single tissue mass that continues to expand until it reaches the opposite side of the lumen  Formation of such tissue masses depends on synthesis and deposition of extracellular matrices and cell proliferation known as 25 endocardial cushions  Endocardial cushions develop in the atrioventricular and conotruncal region assist in formation of  The atrioventricular canals and valves  The atrial and ventricular (membranous portion) septa  The aortic and pulmonary channels 2. Overgrowth  Involves growth of a chamber at all points except for a narrow strip which fails to grow  Leaves a small canal connecting the two chambers A septum formed by overgrowth never completely divides the original lumen but it is usually closed secondarily by tissue contributed by neighboring proliferating tissues.  Such a septum partially divides the atria and ventricles  Formation of the Cardiac Septa A and B. Septum formation by two actively growing ridges that approach each other until C. Septum they fuse formed by a single actively growing cell mass D, E, and F. Septum formation by merging of two 27 expanding portions of the wall of the heart  Atria Separation Separation of the atria and atrio-ventricular canal begins in the 4th week In the AV canal two endocardial cushions, ventral and dorsal project from the walls and fuse together forming the septum intermedium (divides the AVcanal into right and left halves)  The upper part of each half (towards the atria) is added to the corresponding atrium while the lower parts form the mitral and tricuspid orifices.  Local endothelial proliferations form the cusps of the AV valves (3 cusps in the tricuspid valve & 2 cusps in the mitral valve). The primitive atrium is divided into right and left chambers by the development of an inter-atrial septum  In the intrauterine life, this septum keeps a gap connecting both atria. 28  Steps of atrial septum formation 1. The Septum Primum: A thin crescentic membrane grows from the roof of the atrium and descends towards the septum intermedium, but is separated from it by the foramen primum (ostium primum) which allows blood between the right and left sides.  The foramen primum becomes progressively smaller and is finally closed Just before complete closure of the foramen primum, the upper part of the septum primum ruptures forming another opening, the foramen secundum, widens cranially. 2. The Septum Secundum: Another crescentic membrane grows from the roof of the atrium, immediately to the right side of the septum primum. Now, the opening below the septum secundum is called the foramen ovale 30  Blood in the right atrium passes to the left atrium through the foramen ovale and foramen secundum Blood passage in opposite direction is prevented by the valvular action of the remaining lower part of the septum primum, closing the foramen ovale. After birth, the pressure in both atria becomes equal and the 2 septa fuse.  The septum primum in the site of the foramen ovale forms the fossa ovalis  The margins of the septum secundum around the fossa form the limbus fossa ovalis (annulus ovalis). ===The Left atrium receives a common pulmonary vein which is formed by union of four pulmonary veins from the lungs.  Proximal part of this pulmonary venous system is absorbed into the wall of the developing left atrium. 33 ====that is why the four pulmonary veins open directly into the left atrium.  In the posterior wall of the primitive left atrium, a pulmonary vein develops which progressively branches. Each atrium is derived from 3 sources: Right atrium Left atrium 1. Right half of the primitive 1. Left half of the primitive atrium atrium 2. Absorbed right horn of sinus 2. Absorbed pulmonary veins venosus 3. Upper part of the right A-V 3. Upper part of the left A-V canal canal 34  Atrioventricular Valves After the AV endocardial cushions fuse, each AV orifice is surrounded by local proliferations of mesenchymal tissue Valves are formed when the bloodstream hollows out and thins these proliferating tissues on the ventricular surface and remain attached to the ventricular wall by muscular cords Finally, muscular tissue in the cords degenerates and is replaced by dense connective tissue  The valves then consist of connective tissue covered by endocardium They are connected to thick trabeculae in the wall of the ventricle by the papillary muscles, in turn the muscles connect to the cusps by means of chordae tendineae. In this manner  Two valve leaflets, the bicuspid/mitral valve, form in the left AV canal 35  Three valve leaflets, constituting the tricuspid valve, form in the right AV canal  SEPTUM FORMATION IN THE TRUNCUS ARTERIOSUS AND CONUS CORDIS lower part of the bulbus cordis the common ventricle rough inflowing parts of both ventricles The remaining smooth part of the bulbus cordis forms the smooth outflowing parts of both ventricles The truncus arteriosus is divided into ascending aorta and pulmonary trunk Septa divide these chambers 1. Muscular interventricular septum 2. Membranous interventricular septum 3. Septa in the conus cordis Bulbar septa 37 4. Septum in the truncus arteriosus  Truncal septum  Muscular interventricular septum: grows as a cresentric partition from the floor of the chamber between both ventricles Membranous interventricular septum: fills the gap between the upper crescentic border of the muscular septum and the septum intermedium, which was called the interventricular foramen  It is derived from the septum intermedium and the proximal bulbar septum Proximal bulbar septum: comes from right and left bulbar ridges which fuse together separating the infundibulum of right ventricle (ventrally) and the vestibule of left ventricle (dorsally)  It extends and shares in the formation of the membranous IV septum Distal bulbar septum: comes from right and left bulbar ridges which fuse together separating the pulmonary orifice (ventrally) and the aortic orifice (dorsally)  Each ventricle is derived from 2 sources: Right ventricle Left ventricle Inflowing Most of the bulbus and Most of the primitive part small part of the bulbo- ventricle and small part ventricular chamber of the bulbo-ventricular chamber Outflowing Ventral 1/ 2 of the proximal Dorsal 1/ 2 of the part part of bulbus cordis proximal part of bulbus cordis 42  Prenatal and postnatal circulations The separation of the atria is complicated by the fact that blood must shunt from the Rt to the Lt atrium throughout uterine life which closes at birth In the fetus (single - circuit) Heart --- systemic or placental circulation ----- Heart In the adult (double - circuit) Heart --- systemic / pulmonary circulation ---- heart Two shunts which allow blood to by pass the fetal pulmonary circulation  Inter atrial shunt (b/n Rt & Lt atria)  Ductus arteriosus (b/n fetal pulmonary trunk & arch of aorta) At birth, the shunt is closed by a sudden rise in left atrial pressure 43  The fetal cardiovascular system is designed to serve prenatal needs and permit modifications at birth that establish the neonatal circulatory pattern Good respiration in the newborn infant is dependent on normal circulatory changes occurring at birth, result in oxygenation of the blood occurring in the lungs when fetal blood flow through the placenta ceases. Prenatally, the lungs do not provide gas exchange and the pulmonary vessels are vasoconstricted. The three vascular structures most important in the transitional circulation are the DuctusVenosus, foramen ovale, and Ductus Arteriosus. 44  Highly oxygenated, nutrient-rich blood returns under high pressure from the placenta in the umbilical vein. On approaching the liver, approximately half of the blood passes directly into the DV, a fetal vessel connecting the umbilical vein to the IVC consequently, this blood bypasses the liver. The other half of the blood in the umbilical vein flows into the sinusoids of the liver and 45 enters the IVC through the hepatic veins.  After a short course in the IVC, the blood enters the right atrium of the heart. Because the IVC also contains poorly oxygenated blood from the lower limbs, abdomen, and pelvis, the blood entering the right atrium is not as well oxygenated as that in the umbilical vein, but it still has a high oxygen content. Most blood from the IVC is directed by the crista dividens (inferior border of the septum secundum), through the oval foramen into the left atrium Here it mixes with the relatively small amount of poorly oxygenated blood returning from the lungs through the pulmonary veins. N.B The fetal lungs use oxygen from the blood instead of replenishing it. From the left atrium, the blood then passes to the left ventricle and 46 leaves through the ascending aorta.  Then to the heart, neck, head, upper limbs receive well-oxygenated blood from the ascending aorta. The liver also receives well-oxygenated blood from the umbilical vein. The small amount of well-oxygenated blood from the IVC in the right atrium that does not enter the foramen ovale mixes with poorly oxygenated blood from the SVC and coronary sinus and passes into the right ventricle. 47 This blood, with a medium oxygen content,  10% of this blood flow goes to the lungs; But most blood passes through the Dectus artriosus (DA) into the descending aorta to the fetal body returns to the placenta through the umbilical arteries. The DA protects the lungs from circulatory overloading and allows the right ventricle to strengthen in preparation for functioning at full capacity at birth. 48 49  Congenital anomalies Acardia  Absence of heart  Mostly occurs in conjoined monozygotic twins Ectopia/c Cordis  A heart protruding through a median gap Into the neck or Protruding outside chest Dextracardia  Formation of cardiac loop to the left rather than to the Rt  Heart is located in right hemithorax  Most cases associated with situs inversus  Heart, great vessels, other thoracic & abdominal organs may present a 50 mirror image of the norm. 51 Septal Problems Atrial septal defects (ASDs)  Failure of fusion between the two septa  This is called Probe-patent foramen oval.  May be caused by  Underdeveloped septum secundum.  Excessive resorption of septum primum  The most serious abnormality in this group is complete absence of the atrial septum cortriloculare biventriculare Premature /Prenatal closure of the oval foramen  Leads to massive hypertrophy of the right atrium & ventricle, and underdevelopment of the Left side of the heart  Death usually occurs shortly after birth Persistent atrioventricular canal 52  Endocardial cushion fail to fuse --- causes ostium primum defect Probe-patent foramen oval Normal heart patent foramen oval 53  Ventricular Septal Defects (VSDs): ==Membranous VSD - due to failure of formation of the membranous part of the interventricular septum. ==Muscular VSD -- one or multiple holes in the muscular septum due to excessive cavitation during trabeculation of the medial walls of both ventricles Tricuspid atresia  Involves obliteration of the right atrioventricular orifice  Characterized by the absence or fusion of the tricuspid valves 54 Abnormal division of the conotruncal region: Persistent truncus arteriosus  When conotruncal ridges fail to fuse & descend toward the ventricles  The pulmonary artery arises some distance above the origin of the undivided truncus  The undivided truncus thus overrides both ventricles and receives blood from both sides Transposition of the great vessels  When the conotruncal septum fails to follow its normal spiral course & descends straight downward  The aorta originates from the right ventricle, and the pulmonary artery originates from the left ventricle 55  Usually accompanied by an open ductus arteriosus Vascular Development Arterial system  Venous system Lymphatic system 56 Arterial system 57  Arterial System  Pharyngeal arches that develop in humans embryo (4th-5th wk) it is an evolutionary derivatives of the brachial arches (gill bars) of fishes =Each pharyngeal arch contains an artery i.e aortic arch =Each aortic arch arises ventrally from the aortic sac & terminate in the Lt and Rt dorsal aorta =The five pairs of human pharyngeal arches are homologous to gill arches 1, 2, 3, 4 & 6 and are so numbered (I, II, III, IV and VI) =During further development, these arteries become modified and some of them regress completely  Division of the truncus arteriosus by the aortico-pulmonary septum divides the outflow channel of the heart into the aorta and the pulmonary trunk  During 8th week ,the primitive aortic arch pattern is transformed into final fetal arteries 58 they are  The Rt & Lt dorsal aortae fuse during 4th week and form midline dorsal aorta i.e z future descending aorta  The human aortic arches arise in caranio-caudal sequence  The 1st pair of aortic arches is formed on day 22 to 24 The remaining 4 pairs (2nd, 3rd, 4th & 6th -) develop later  The 2nd pair is formed on day 26  The 3rd & 4th pairs … on day 28  The 6th pair … on day 29  Reminants of aortic arches I & II give rise to minor vascular elements in the head  The 1st arch on each side forms the terminal segment of maxillary a( to the maxilary bone) 60  The 2nd arch on each side forms the embryonic stapedial artery( small bone in middle ear) & Hyoid rtery  The 2nd aortic arches disappear leaving small parts forming the stapedial 61 artereis (run through the ring of the stapes, a small bone in middle ear).  The 3rd arch artery common carotid artery proximal part of internal carotid artery (on both sides), it joins with the dorsal aorta which form the distal part of internal carotid artery.  The external.carotid.artery develops as a new branch from 3rd arch  the 4th arch arch of aorta… on left side, proximal part of right subclavian artery  The distal part of Rt.subclavian artery develops from the right dorsal aorta & right 7th intersegmental artery.  The left subclavian artery from the left 7th intersegmental artery  The portion of dorsal aorta connecting the 3rd & 4th arches disappears on both sides 62  The 5th arch artery disappears  The dorsal aorta on the right side caudal to 4th arch disappears down to the single dorsal aorta  but persists on left side to form descending aorta  The 6th arch artery:  Proximal part on both sides … forms the pulmonary artery  Distal part of left artery,,,, forms ductus arteriosus which connects left pulmonary artery with arch of aorta  Distal part of right artery disappears 64 Aortic arches give rise to vessels of head, neck, and upper thorax  The arch of aorta is derived from 3 sources:  Left horn of aortic sac.  4th left aortic arch.  Part of the left dorsal aorta till the 7th intersegmental a.  The brachiocephalic trunk is derived from  The right horn of the aortic sac  The subclavian arteries:  The right:- is derived from the right 4th aortic arch + part of the right dorsal aorta + the right 7th intersegmental artery.  The left: -is derived from the left 7th intersegmental artery.  The descending aorta is derived from  The part of the left dorsal aorta caudal to the left 7th intersegmental 66 branch + the single distal aorta. 67  The dorsal aorta develops ventral, lateral, & dorsolateral branches A) Ventral branches, vitelline aa ,supply blood to the gut tube & its derivatives 1. The thoracic foregut is served by about 5 vitelline aa 2. The abdominal gut is served by 3 vitelline aa  Celiac artery→ foregut.  Sup. Mesenteric artery→ midgut  Inf. mesenteric artery → hindgut B) Lateral branches, Supply the structures derived from the intermediate cell mass mesoderm 1. Phrenic aa --- diaphragm 2. Suprarenal aa – the suprarenal glands 3. Gonadal aa – the gonads 68 4. Renal aa –the kidneys Dorsal aorta … ventral branches 69 dorsal aorta … lateral branches 70 C) Dorsolateral branches, == intersegmental art.to vascularize the body wall, vertebral column, spinal cord, & limbs. ====The intersegmental aa. grow out b/n the somites.  The intercostal, lumbar, & lateral sacral aa.  Produces the vertebral aa. as well as the deep cervical, internal thoracic, and superior & inferior epigastric aa. 71 ===The limbs receive blood through intersegmental artery branches - The axis artery of the upper limb – 7th cervical intersegmental a. - The axis artery of the lower limb – 5th lumbar intersegmental a. D) The umblical art., which develop in the connecting stalk, initially join the dorsal aortae but shift their origin to the internal iliac aa. 72  Venous System The primitive embryonic venous system is divided into vitelline, umblical, and cardinal systems A. The vitelline system drains the yolk sac and the gut tube, B. The umblical system brings oxygenated blood from the placenta C. The cardinal system drains the body wall, neck, and head of the embryo The vitelline system gives rise to the liver sinusoids & ductus venosus, the portal system, and to cranial segment of IVC 73   Branches of Rt & Lt vitelline Veins ramify --- liver sinosoids  The ductus venosus develops by the coalescence of liver sinosoids (conducts blood from umblical Vein to the IVC)  The Rt Vitelline Vein forms cranial segment of IVC as well as portal vein (SMV of the portal system) Left vitelline Vein forms some of the portal vessels, including the splenic & IMV The Rt umblical Vein dissapears completely during development The left umblical vein also disappears except the caudal part which remains(from the umbilicus to the left branch of portal v) 74 The Lt umblical Vein is the only one to carry blood from the placenta to the liver  With the increase of the placental circulation, a direct communication forms between the left umbilical vein and the right hepatocardiac channel, the ductus venosus  This vessel bypasses the sinusoidal plexus of the liver.  After birth the left umbilical vein and ductus venosus are obliterated and form the ligamentum teres hepatis and ligamentum venosum, respectively 79 80  Cardinal system Anterior & posterior cardinal Vs 81  Anterior cardinal veins:  Become connected by an oblique anastomosis which forms the left brachiocephalic v  the right cardinal vein forms the right brachiocephalic v. + the upper part of SVC.  The distal portions form the internal juguar Vs  posterior cardinal veins:  The terminal portion of the left posterior cardinal vein entering into the left brachiocephalic vein is retained as a small vessel, the left superior intercostal vein  It receives blood from the 2nd and 3rd intercostal spaces 83  Common cardinal veins:  Right → lower part of SVC (below the azygos v.).  Left→ oblique vein of left atrium  The coronary vessels develop from blood islands deep to epicardium The anastomosis between the subcardinal veins forms the left renal vein  When this communication established, the left subcardinal vein disappears, and only its distal portion remains as the left gonadal vein  Hence the right subcardinal vein becomes the main drainage channel and develops into the renal segment of the IVC 84 85  The right and left supracardinal veins in the lumbar region form anastomosis  The left degenerate but the right become the supracardinal segment of the IVC  The anastomosis between the caudal posterior cardinal veins forms the posterior cardinal segment of the inferior vena cava  Distal to the anastomosis remain as common iliac veins with their branches ( internal & external iliac vs.) 86  After remodeling, the already anastomosis connection between subcardinal and supracardinal segments now comes to be straight with each other and with posterior cardinal segment Now, the inferior vena cava, consisting of hepatic, renal, supracardinal and posterior cardinal segments, is complete  The IVC is thus a composite structure of (from cranial to caudal)  Rt vitelline vein  Subcardinal vein  Supracardinal vein  Posterior cardinal vein 87 Supracardinal veins / thoracic part  Azygos vein Posterior cardinal veins/ lumbar part 88  Anastomose … form posterior cardinal  Hemiazygos veins segment of IVC. And common iliac vs (distally)  DEVELOPMENT … Changes just after birth Pressure is equalized in both atria because:  Respiration begins. so, blood returns from the functioning lungs to the left atrium.  The umbilical cord is ligated, so, no blood returns via the occluded umbilical vein to the right atrium. As a result: The septum primum and secundum fuse together → closure of the foramen ovale. The occluded umbilical v. → The ligamentum teres of the liver. The occluded distal parts of the umbilical arteries→ the medial umbilical ligaments. The occluded ductus venosus→ the ligamentum venosum. 89 The occluded ductus arteriosus → the ligamentum arteriosum. Fetal circulation before birth Pulmonar Ductus arteriosus y Superior vena cava vein Pulmonary vein Crista dividens Oval foramen Pulmonary artery Inferior vena cava Descending aorta Ductus venosus Portal Sphincter in vein ductus venosus Inferior vena cava Umbilica l vein 90 Umbilical arteries  Ligamentum arteriosum Pulmonary artery Superior vena cava Closed oval foramen Pulmonary vein Human circulation after birth Inferior vena cava Descending aorta Portal vein Ligamentum teres hepatis 91 Superior vesical artery Medial umbilical  Lymphatic System The lymphatic system begins its development later than the cardiovascular system at the end of the 6th week Lymphatic vessels develop in a manner similar to that previously described for blood vessels Six primary lymph sacs are formed Two jugular, at the junction of the subclavian and anterior cardinal veins Two iliac, at the junction of the iliac and posterior cardinal veins One retroperitoneal, near the root of the mesentery 92 One cisterna chyli, dorsal to the retroperitoneal sac  Lymph sacs develop in the 6th week of fetal development Lymph vessels will ultimately join these lymph sacs in an organized fashion.  Numerous channels connect the sacs with each other and drain lymph from the limbs, body wall, head, and neck Two main channels, the right and left thoracic ducts, join the jugular sacs with the cisterna chyli, and soon an anastomosis forms between these ducts The thoracic duct then develops from the distal portion of the right thoracic duct and the cranial portion of the left thoracic duct The right lymphatic duct is derived from the cranial portion of the right thoracic duct Both ducts maintain their original connections with the venous system and empty into the junction of the internal jugular and subclavian veins Numerous anastomosis produce many variations in the final form of the thoracic duct Lymphatic development Two large channels (right and left thoracic ducts) connect the jugular lymph sacs with the cistern. Soon a large anastomosis forms between these channels.  At 9-10 weeks the right thoracic duct crosses the midline at about T4-T5  Lymphatic vessels soon join the jugulo-axillary lymph sacs, the lumbar and iliac plexuses , the retroperitoneal lymph sac and the cisterna chyli. Development of the Lymph Nodes Except for the superior part of the chyle cistern, the lymph sacs are transformed == into groups of lymph nodes during the early fetal period. Mesenchymal cells= invade each lymph sac and break up its cavity into a network of lymphatic channels- the primordia of the lymph sinuses. Other mesenchymal cells give rise to the capsule and connective tissue framework of the lymph nodes. 98  Thus we see the aggregation of lymph nodes in certain areas (related to the embryologic precursors – the lymph sacs): – Jugulo-axillary lymph sacs – Retroperitoneal lymph sac and the cisterna chyli – Posterior lymph sacs  Development of the Lymphocytes Development of the Lymphocytesare derived originally from stem cells The lymphocytes in the :- umbilical vesicle (yolk sac), - mesenchyme - the liver and spleen. = These early lymphocytes eventually enter the bone marrow, where they divide to form lymphoblasts. ==The lymphocytes that appear in lymph nodes before birth are derived from the thymus. == Small lymphocytes leave the thymus and circulate to other lymphoid organs. ===Later, some mesenchymal cells in the lymph nodes also differentiate into lymphocytes. 100  Development of the Spleen and Tonsils ==The spleen develops from an aggregation of mesenchymal cells in the dorsal mesogastrium. ==The palatine tonsils develop from nearby mesenchyme. ==The tubal tonsils develop from aggregations of lymph nodules around the pharyngeal openings of the pharyngotympanic tubes. == The pharyngeal tonsils (adenoids) develop from an aggregation of lymph nodules in the wall of the nasopharynx. ==The lingual tonsil develops from an aggregation of lymph nodules in the root of the tongue. == Lymph nodules also develop in the mucosa of the respiratory and digestive systems. 101  ! ! u ! yo n k h a