Septum Formation In The Ventricles

By the end of the fourth week, the two primitive ventricles begin to expand. This is accomplished by continuous growth of the myocardium on the outside and continuous diverticulation and trabecula formation on the inside (Figs. 11.8, 11.17, and 11.25).

The medial walls of the expanding ventricles become apposed and gradually merge, forming the muscular interventricular septum (Fig. 11.25). Sometimes the two walls do not merge completely, and a more or less deep apical cleft between the two ventricles appears. The space between the free rim of

Conotruncal Formation

Figure 11.22 Scanning electron micrographs of hearts from mouse embryos showing formation of the conotruncal ridges (cushions) that form a septum in the outflow tract to divide this region into aortic and pulmonary channels. A. Frontal section showing cushion contact (arrows) in the outflow tract. B. Cross section through the atrioventricular canal (arrowheads) and outflow tract (arrow). Cushions in both regions have made initial contact. C. Cross section through the aortic (A) and pulmonary (P) vessels showing their entwined course caused by spiraling of the conotruncal ridges (see Fig. 11.23). Note the thickness of the aorta.

Figure 11.22 Scanning electron micrographs of hearts from mouse embryos showing formation of the conotruncal ridges (cushions) that form a septum in the outflow tract to divide this region into aortic and pulmonary channels. A. Frontal section showing cushion contact (arrows) in the outflow tract. B. Cross section through the atrioventricular canal (arrowheads) and outflow tract (arrow). Cushions in both regions have made initial contact. C. Cross section through the aortic (A) and pulmonary (P) vessels showing their entwined course caused by spiraling of the conotruncal ridges (see Fig. 11.23). Note the thickness of the aorta.

Right And Left Conotruncal Ridge

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).

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