Development of CVS 4 Systemic Embryology - PowerPoint Presentation

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Development of CVS 4

Systemic Embryology

SEPTUM FORMATION IN THE ATRIOVENTRICULAR CANAL

At the end of the fourth week,

Superior and inferior atrioventricular endocardial cushions appear at the upper and lower borders of the atrioventricular canal. Atrioventricular canal gives access only to the primitive left ventricle initially; later the atrioventricular orifice has access to the both ventricles.Two lateral atrioventricular cushions appear on the right and left borders of the canal. Superior and inferior cushions fuse resulting in a complete division of the canal into right and left atrioventricular orifices by the end of the fifth week

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Atrioventricular

Valves

Atrioventricular orifice is surrounded by proliferations of mesenchymal tissue. Valves form from proliferations and are attached to the ventricular wall by muscular cords.Muscular tissue in the cords degenerates and is replaced by dense connective tissue. Valves consist of connective tissue covered by endocardium. - Connected to trabeculae in the wall of the ventricle, the papillary muscles, by chordae tendineae.Two valve leaflets (bicuspid or mitral valve) form in the left atrioventricular canal and three (tricuspid valve) form on the right side.

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Heart Defects and vascular abnormalities

Largest category of human birth defects

Due to genetic factors, environmental agents and multifactorial causes. Cardiovascular teratogens include rubella virus, thalidomide, isotretinoin (vitamin A), alcohol. Maternal diseases, such as insulin-dependent diabetes and hypertension. Gene mutations that result in heart defects eg mutations in the heart-specifying gene NKX2.5, produce atrial septal defects.5

Persistent

atrioventricular

canal with a defect in the septum Cushions fail to fuse. Has an atrial and a ventricular component, separated by abnormal valve leaflets in the single atrioventricular orifice.At times, endocardial cushions in the atrioventricular canal partially fuse resulting in a defect in the atrial septum, but the interventricular septum is closed - ostium primum defectTricuspid atresiaInvolves obliteration of the right atrioventricular

orifice

Characterized by the absence or fusion of the tricuspid valves.

Associated with (a) patency of the oval foramen, (b) ventricular

septal

defect, (c) underdevelopment of the right ventricle, and (d) hypertrophy of the left ventricle.

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SEPTUM FORMATION IN THE TRUNCUS ARTERIOSUS AND CONUS CORDIS

During the fifth week,

Pairs of opposing ridges appear in the truncus. Truncus swellings, or cushions, lie on the right superior wall and on the left inferior wall. Right superior truncus swelling grows distally and to the left, and the left inferior truncus swelling grows distally and to the right.Growing toward the aortic sac, the swellings twist around each other, foreshadowing the spiral course of the future septum. After complete fusion, the ridges form the aorticopulmonary septum, dividing the truncus into an aortic and a pulmonary channel.7

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Similar swellings (cushions) develop along the right dorsal and left ventral walls of the

conus cordis.These swellings grow toward each other and unite with the truncus septum. When the two conus swellings have fused, the septum divides the conus into an anterolateral portion (the ouflow tract of the right ventricle) and a posteromedial portion (the outflow tract of the left ventricle).

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Neural crest cells contribute to

endocardial cushion formation in both the conus cordis and truncus arteriosus. Abnormal migration, proliferation, or differentiation of these cells results in congenital malformations such as tetralogy of Fallot, pulmonary stenoses, transposition of the great vessels and persistent truncus arteriosus.Since neural crest cells also contribute to craniofacial development, it is not uncommon to see facial and cardiac abnormalities in the same individual.

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Partitioning of Primordial Ventricle

First indicated by a median ridge—the muscular interventricular (IV) septum—in the floor of the ventricle near its apex. Myocytes from both primordial ventricles contribute to the formation of the muscular part of the IV septum. There is IV foramen between the free edge of the IV septum and the fused endocardial cushions which permits communication between the right and left ventricles.IV foramen closes by the end of the seventh week as the bulbar ridges fuse with the endocardial cushion.

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Closure of the IV foramen and formation of the membranous part of the IV septum result from the fusion

of tissues from the right and left bulbar ridges and the endocardial cushion. Membranous part of the IV septum is derived from the right side of the endocardial cushion.This tissue merges with the aorticopulmonary septum and the thick muscular part of the IV septum.After closure of the IV foramen and formation of the membranous part of the IV septum, the pulmonary trunk communicates with the right ventricle and the aorta communicates with the left ventricle.Cavitation of the ventricular walls—trabeculae carneae.

Some of these

bundles become the papillary muscles and

tendinous

cords (

chordae

tendineae

).

Tendinous

cords run from the papillary muscles to the AV valves

SEPTUM FORMATION IN THE VENTRICLES

By the end of the

fourth week, Primitive ventricles begin to expand. Continuous growth of the myocardium on the outside and diverticulation and trabecula formation on the inside.Medial walls of the expanding ventricles become apposed and merge forming the muscular interventricular septum. Sometimes the two walls do not merge completely leaving a cleft between the two ventricles. Space between the free rim of the muscular ventricular septum and the fused endocardial cushions permits communication between the two ventricles.

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The

interventricular foramen shrinks on completion of the conus septum.Tissue outgrowth from the inferior endocardial cushion along the top of the muscular interventricular septum closes the foramen. Tissue fuses with the abutting parts of the conus septum.Complete closure of the interventricular foramen forms the membranous part of the interventricular septum.

Semilunar

Valves

Primordia of the semilunar valves on the truncus swellings becomes visible when partitioning of the truncus is almost complete.One of each pair is assigned to the pulmonary and aortic channels, respectively.A third tubercle appears in both channels opposite the fused truncus swellings. Gradually the tubercles hollow out at their upper surface, forming the semilunar valves. Recent evidence shows that neural crest cells contribute to formation of these valves.14

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Heart Defects

Ventricular septal defect (VSD) involving the membranous portion of the septum is the most common congenital cardiac malformation, occurring as an isolated condition in 12/10,000 births. Although it may be found as an isolated lesion, VSD is often associated with abnormalities in partitioning of the conotruncal region. Depending on the size of the opening, blood carried by the pulmonary artery may be 1.2 to 1.7 times as abundant as that carried by the aorta. Occasionally the defect is not restricted to the membranous part but also involves the muscular part of the septum.

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Tetralogy

of FallotNot fatal, occurs in 9.6/10,000 births.Most frequently occurring abnormality of the conotruncal regionDue to an unequal division of the conus resulting from anterior displacement of the conotruncal septum. Displacement of the septum produces four cardiovascular alterations: A narrow right ventricular outflow region, a pulmonary infundibular stenosisA large defect of the interventricular septumAn overriding aorta that arises directly above the septal

defect

Hypertrophy

of the right

ventricular wall

because of higher pressure on the right side.

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Persistent

truncus arteriosus results when the conotruncal ridges fail to fuse and to descend toward the ventricles.In such a case, which occurs in 0.8/10,000 births, the pulmonary artery arises some distance above the origin of the undivided truncus. Since the ridges also participate in formation of the interventricular septum, the persistent truncus is always accompanied by a defective interventricular septum. The undivided truncus thus overrides both ventricles and receives blood from both sides.

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Transposition of the great vessels occurs when the

conotruncal septum fails to follow its normal spiral course and runs straight down.As a consequence, the aorta originates from the right ventricle, and the pulmonary artery originates from the left ventricle. This condition, which occurs in 4.8/10,000 births, sometimes is associated with a defect in the membranous part of the interventricular septum. It is usually accompanied by an open ductus arteriosus. Since neural crest cells contribute to the formation of the truncal cushions, insults to these cells contribute to cardiac-defects involving the outflow tract.

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Valvular

stenosis of the pulmonary artery or aorta occurs when the semilunar valves are fused for a variable distance. The incidence of the abnormality is similar for both regions, being approximately 3 to 4 per 10,000 births. In the case of a valvular stenosis of the pulmonary artery, the trunk of the pulmonary artery is narrow or even atretic. The patent oval foramen then forms the only outlet for blood from the right side of the heart. The ductus

arteriosus

, always patent, is the only access route to the pulmonary circulation.

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In

aortic valvular stenosis, fusion of the thickened valves may be so complete that only a pinhole opening remains. The size of the aorta itself is usually normal.When fusion of the semilunar aortic valves is complete–aortic valvular atresia –the aorta, left ventricle, and left atrium are markedly underdeveloped. The abnormality is usually accompanied by an open ductus arteriosus, which delivers blood into the aorta.Ectopia cordis is a rare anomaly in which the heart lies on the surface ofthe chest. It is caused by failure of the embryo to close the ventral body wall.