Aortic arch development involves the sequential development and then the involution of six arch pairs. The first pair of arches in the 3-mm CR-length embryo is large when the second pair is just forming (B-H0i Fig. 9-13/1). Caudally, the dorsal aortas fuse to Oform a single vessel, and then vessel fusion progresses cranially. In a 4-mm embryo, the first and second arches have largely disappeared Fig. 9-13B). The third aortic arch is well developed, and the fourth and sixth arches are being formed as ventral and dorsal sprouts of the aortic sac and dorsal aorta, respectively. The ventral portion of the sixth arch already has as its major branch the primitive pulmonary artery even though the arch itself has not yet been completed. Of note, in mammals the fifth aortic arch is rudimentary. By the 10-mm embryo stage, the first two aortic arches have regressed; the third, fourth, and sixth are present; and the truncoaortic sac has been divided by the formation of the aorticopulmonary septum so that the six arches are now continuous with the pulmonary trunk Fig. 9-13Q. Of note, the seventh cervical intersegmental arteries arise from the dorsal aorta near the midline and form the subclavian arteries. In a 14-mm embryo, the dorsal aortas between the third and fourth arches have disappeared and the third arches begin to elongate Fig. 9-13.P). At this point, the dorsal portion of the right sixth arch has disappeared, though the left sixth arch persists as the ductus arteriosus. The aortic sac has been broadened to contribute to the brachiocephalic trunk on the right and part of the definitive aortic arch up to the origin of the left third arch (common carotid artery). Finally, by the 17-mm embryo stage, the right dorsal aorta has become atrophic between its junction with the left dorsal aorta and the origin of the right seventh intersegmental artery has become attenuated and later disappears Fig. 9-13.fi). The remaining components of the right dorsal aorta and right fourth aortic arch form the proximal subclavian artery. After birth, the distal part of the left sixth aortic arch, the ductus arteriosus, normally also involutes to form the ligamentum arteriosum. Thus, most aortic arch anomalies are secondary to abnormal retention or disappearance of various embryonic segments.


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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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