The Muscles At The End Of The Seventh Week

Figure 11.23 Development of the conotruncal ridges (cushions) and closure of the interventricular foramen. Proliferations of the right and left conus cushions, combined with proliferation of the inferior endocardial cushion, close the interventricular foramen and form the membranous portion of the interventricular septum. A. 6 weeks (12 mm). B. Beginning of the seventh week (14.5 mm). C. End of the seventh week (20 mm).

the muscular ventricular septum and the fused endocardial cushions permits communication between the two ventricles.

The interventricular foramen, above the muscular portion of the inter-ventricular septum, shrinks on completion of the conus septum (Fig. 11.23). During further development, outgrowth of tissue from the inferior endocardial cushion along the top of the muscular interventricular septum closes the foramen (Fig. 11.23). This tissue fuses with the abutting parts of the conus septum. Complete closure of the interventricular foramen forms the membranous part of the interventricular septum.

Conusseptum
Figure 11.24 Frontal section through the heart of a 7-week embryo. Note the conus septum and position of the pulmonary valves.

Semilunar Valves

When partitioning of the truncus is almost complete, primordia of the semilunar valves become visible as small tubercles found on the main truncus swellings. One of each pair is assigned to the pulmonary and aortic channels, respectively (Fig. 11.26). 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 (Fig. 11.27). Recent evidence shows that neural crest cells contribute to formation of these valves.

CLINICAL CORRELATES Heart Defects

Ventricular septal defect (VSD) involving the membranous portion of the septum (Fig. 11.28) 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

Spplurr ESBindwm

Right vfiojt HBtVQ ._

OVBl löriman flJ^Tf ajrium

CorxiE wplurn

Spplurr ESBindwm

Right vfiojt HBtVQ ._

CorxiE wplurn filgftl vflnWde

Muscular intarïtiradu^âr septum

Septum Conotruncale

Sfiplum (KlmuTi Lohr alnum

Ltfl L'enlrdû

Figure 11.25 Frontal section through the heart of an embryo at the end of the seventh week. The conus septum is complete, and blood from the left ventricle enters the aorta. Note the septum in the atrial region.

Sfiplum (KlmuTi Lohr alnum

ÜLH15W rfwnngl cr Iflft VWtriÄ

filgftl vflnWde

Muscular intarïtiradu^âr septum

Ltfl L'enlrdû

Figure 11.25 Frontal section through the heart of an embryo at the end of the seventh week. The conus septum is complete, and blood from the left ventricle enters the aorta. Note the septum in the atrial region.

Figure 11.26 Transverse sections through the truncus arteriosus at the level of the semilunar valves at 5 (A), 6 (B), and 7 (C) weeks of development.

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.

Tetralogy of Fallot, the most frequently occurring abnormality of the conotruncal region (Fig. 11.29), is due to an unequal division of the conus resulting from anterior displacement of the conotruncal septum. Displacement of the septum produces four cardiovascular alterations: (a) a narrow right

Figure 11.27 Longitudinal sections through the semilunar valves at the sixth (A), seventh (B), and ninth (C) weeks of development. The upper surface is hollowed (arrows) to form the valves.

Interventricular Foramen Heart
Figure 11.28 A. Normal heart. B. Isolated defect in the membranous portion of the interventricular septum. Blood from the left ventricle flows to the right through the interventricular foramen (arrows).

ventricular outflow region, a pulmonary infundibular stenosis; (b) a large defect of the interventricular septum; (c) an overriding aorta that arises directly above the septal defect; and (d) hypertrophy of the right ventricular wall because of higher pressure on the right side. Tetralogy of Fallot, which is not fatal, occurs in 9.6/10,000 births.

Persistent truncus arteriosus results when the conotruncal ridges fail to fuse and to descend toward the ventricles (Fig. 11.30). 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.

Transposition of the great vessels occurs when the conotruncal septum fails to follow its normal spiral course and runs straight down (Fig. 11.31A). As a consequence, the aorta originates from the right ventricle, and the

Superior vena cava

Right coronary artery

Large aortic stem

Large aortic stem

Right coronary artery

Patent Ductus Arteriosus Repair Pictures

Patent ductus arteriosus

Narrow pulmonary trunk

Was this article helpful?

0 0
Getting Back Into Shape After The Pregnancy

Getting Back Into Shape After The Pregnancy

Once your pregnancy is over and done with, your baby is happily in your arms, and youre headed back home from the hospital, youll begin to realize that things have only just begun. Over the next few days, weeks, and months, youre going to increasingly notice that your entire life has changed in more ways than you could ever imagine.

Get My Free Ebook


Responses

  • pandora clayhanger
    Where Is The interventricular Septum In The Heart?
    7 years ago
  • ceredic
    Where are the interventricular septum located?
    7 years ago

Post a comment