Transposition Of The Great Vessels

Hypertrophy^

Pulmonary stenosis J

Interventricular septal defect

Fallot Scanner

Figure 11.29 Tetralogy of Fallot. A. Surface view. B. The four components of the defect: pulmonary stenosis, overriding aorta, interventricular septal defect, and hypertrophy of the right ventricle. C. Ultrasound scan showing a normal heart with atria (asterisks), ventricles (V), and interventricular septum (arrow). D. Scan of a heart showing the characteristic features of the tetralogy, including hypertrophy of the right ventricle (R) and overriding aorta (arrow). A, atrium; L, left ventricle.

Figure 11.29 Tetralogy of Fallot. A. Surface view. B. The four components of the defect: pulmonary stenosis, overriding aorta, interventricular septal defect, and hypertrophy of the right ventricle. C. Ultrasound scan showing a normal heart with atria (asterisks), ventricles (V), and interventricular septum (arrow). D. Scan of a heart showing the characteristic features of the tetralogy, including hypertrophy of the right ventricle (R) and overriding aorta (arrow). A, atrium; L, left ventricle.

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.

Development Interventricular Septum
AiJKB
Transposition The Great Vessels
Figure 11.31 A. Transposition of the great vessels. B. Pulmonary valvular atresia with a normal aortic root. The only access route to the lungs is by way of a patent ductus arteriosus.

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 (Fig. 11.31 B). The patent oval foramen then forms the only outlet for blood from the right side of the

Truncos Arteriosos

—Stenosis of aortic valves

Patent ductus arteriosus

Patent oval foramen

—Stenosis of aortic valves

Atresia of aortic valves

Patent ductus arteriosus

Patent oval foramen

Atresia of aortic valves

Atresia Aorta

Figure 11.32 A. Aortic valvular stenosis. B. Aortic valvular atresia. Arrow in the arch of the aorta indicates direction of blood flow. The coronary arteries are supplied by this retroflux. Note the small left ventricle and the large right ventricle.

heart. The ductus arteriosus, always patent, is the only access route to the pulmonary circulation.

In aortic valvular stenosis (Fig. 11.32), 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 (Fig. 11.32B)-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 of the chest. It is caused by failure of the embryo to close the ventral body wall (see Chapter 10).

Formation of the Conducting System of the Heart

Initially the pacemaker for the heart lies in the caudal part of the left cardiac tube. Later the sinus venosus assumes this function, and as the sinus is incorporated into the right atrium, pacemaker tissue lies near the opening of the superior vena cava. Thus, the sinuatrial node is formed.

The atrioventricular node and bundle (bundle of His) are derived from two sources: (a) cells in the left wall of the sinus venosus, and (b) cells from the atrioventricular canal. Once the sinus venosus is incorporated into the right atrium, these cells lie in their final position at the base of the interatrial septum.

Vascular Development

ARTERIAL SYSTEM Aortic Arches

When pharyngeal arches form during the fourth and fifth weeks of development, each arch receives its own cranial nerve and its own artery (see Chapter 15). These arteries, the aortic arches, arise from the aortic sac, the most distal part of the truncus arteriosus (Figs. 11.8 and 11.33). The aortic arches are embedded in mesenchyme of the pharyngeal arches and terminate in the right and left dorsal aortae. (In the region of the arches the dorsal aortae remain paired, but caudal to this region they fuse to form a single vessel.) The pharyngeal arches and their vessels appear in a cranial to caudal sequence, so that they are not all present simultaneously. The aortic sac contributes a branch to each new arch as it forms, giving rise to a total of five pairs of arteries. (The fifth arch either never forms or forms incompletely and then regresses. Consequently the five arches are numbered I, II, III, IV, and VI [Fig. 11.34].) During further development, this arterial pattern becomes modified, and some vessels regress completely.

Division of the truncus arteriosus by the aorticopulmonary septum divides the outflow channel of the heart into the ventral aorta and the pulmonary artery. The aortic sac then forms right and left horns, which subsequently give rise to the brachiocephalic artery and the proximal segment of the aortic arch, respectively (Fig. 11.35, B and C).

By day 27, most of the first aortic arch has disappeared (Fig. 11.34), although a small portion persists to form the maxillary artery. Similarly, the

Anterior cardinal vein

Common cardinal vein

Dorsal aorta

Aortic arches (II and III)

Internal carotid artery

Anterior cardinal vein

Common cardinal vein

Dorsal aorta

Trunco Aortic Sac

Chorionic villus

Chorion

Aortic sac

Umbilical vein and artery

Vitelline vein

Vitelline artery

Figure 11.33 Main intraembryonic and extraembryonic arteries (red) and veins (blue) in a 4-mm embryo (end of the fourth week). Only the vessels on the left side of the embryo are shown.

Chorionic villus

Chorion

Aortic sac

Umbilical vein and artery

Vitelline vein

Vitelline artery

Figure 11.33 Main intraembryonic and extraembryonic arteries (red) and veins (blue) in a 4-mm embryo (end of the fourth week). Only the vessels on the left side of the embryo are shown.

Obliterated

Obliterated

Primitive Chorionic Villi

Left 7th

^ Septum between aorta and pulmonary artery Pulmonary trunk

4-mm stage

10-mm stage

Figure 11.34 A. Aortic arches at the end of the fourth week. The first arch is obliterated before the sixth is formed. B. Aortic arch system at the beginning of the sixth week. Note the aorticopulmonary septum and the large pulmonary arteries.

intersegmental artery

Primitive pulmonary artery

Left 7th

^ Septum between aorta and pulmonary artery Pulmonary trunk

4-mm stage

10-mm stage

Figure 11.34 A. Aortic arches at the end of the fourth week. The first arch is obliterated before the sixth is formed. B. Aortic arch system at the beginning of the sixth week. Note the aorticopulmonary septum and the large pulmonary arteries.

second aortic arch soon disappears. The remaining portions of this arch are the hyoid and stapedial arteries. The third arch is large; the fourth and sixth arches are in the process of formation. Even though the sixth arch is not completed, the primitive pulmonary artery is already present as a major branch (Fig. 11.34 A).

In a 29-day embryo, the first and second aortic arches have disappeared (Fig. 11.34B). The third, fourth, and sixth arches are large. The truncoaortic sac has divided so that the sixth arches are now continuous with the pulmonary trunk.

With further development, the aortic arch system loses its original symmetrical form, as shown in Figure 11.35A, and establishes the definitive pattern illustrated in Figure 11.35, B and C. This representation may clarify the transformation from the embryonic to the adult arterial system. The following changes occur:

The third aortic arch forms the common carotid artery and the first part of the internal carotid artery. The remainder of the internal carotid is formed by the cranial portion of the dorsal aorta. The external carotid artery is a sprout of the third aortic arch.

The fourth aortic arch persists on both sides, but its ultimate fate is different on the right and left sides. On the left it forms part of the arch of the aorta, between the left common carotid and the left subclavian arteries. On the right it forms the most proximal segment of the right subclavian artery, the distal part of which is formed by a portion of the right dorsal aorta and the seventh intersegmental artery (Fig. 11.35B).

rinrcal anrta rinrcal anrta

Aortic arches

Cartoid And The Vagus Nerves

Internal carotid artery Right vagus nerve

Right dorsal aorta

Aortic arches

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