Development of the Pharyngeal Arch Arteries7–12 As described in Chapter 3, the intrapericardial components of the arterial trunks are formed within the distal part of the outflow tract They are produced by the ongoing migration of nonmyocardial cells into the arterial pole of the heart from the second heart field and the neural crest during the ninth and tenth days of murine development The series of paired arteries that connect the extrapericardial aortic sac to the initially paired dorsal aortas extend through segments of pharyngeal mesenchyme and encircle the tracheoesophageal pedicle The segments of pharyngeal mesenchyme, along with their contained arteries, develop in sequence and in a cranial-to-caudal direction Each protuberance, or arch, is covered by endoderm on the inside and by ectoderm on the outside The first four arches also contain a mesodermal core along with additional mesenchyme derived from the neural crest As far as we know, the sequence and mechanism of formation of the arches and their contained arteries as seen in the mouse is paralleled in the developing human heart Since the sequence has best been investigated in the mouse, we use these images to demonstrate the changes The arterial channels are first seen during the latter half of murine embryonic day 8 and continue to appear during embryonic day 9.5 (Fig 48.2) By the end of murine embryonic day 9.5, which is equivalent to Carnegie stage 11 in the developing human, three pairs of arteries occupy the first through third pairs of pharyngeal arches By the beginning of embryonic day 10.5, by which time the fourth pair of arteries has begun to appear, the arteries in the first and second arches have already begun to regress These vessels form a capillary network within the developing jaw but make no significant contribution to the final structure of the brachiocephalic branches of the aortic arch Formation of the sixth pair of arches takes place during the latter part of embryonic day 10.5 (Fig 48.3) Like the arteries of the first four arches, the sixth arch arteries also occupy a discrete segment of pharyngeal mesenchyme Unlike the first four arches, however, the sixth pharyngeal arches do not contain bony, muscular, or neural elements FIG 48.2 Reconstructed episcopic datasets from developing mice sacrificed during embryonic day 9.5 The upper panels are from a mouse with 19 somites, whereas the mouse providing the material for the lower panels had 27 somites The arterial channels running from the aortic sac to the dorsal aortas have been reconstructed in the panels at left, whereas the panels at right show the relationship of the arterial channels to the pharyngeal mesenchyme FIG 48.3 Reconstructed episcopic datasets from developing mice sacrificed during embryonic day 10.5 showing the ongoing development of the arch arteries concomitant with regression of the arches formed in the first and second arches Note that the dorsal aortas continue cranially as the carotid ducts The images shown in the upper panels are from an embryo with 32 somites, whereas the embryo from which the lower panels were prepared had 40 somites By the beginning of embryonic day 11.5, the bilateral symmetry of the arteries is readily evident, although by this time only three sets of arteries are recognizable This is due to the regression of the arteries of the first and second arches (Fig 48.4) It is during embryonic day 11.5 that collateral channels can be found in up to half of all the embryos (see Fig 48.4, right), although the collateral channels are not seen prior to the formation of the sixth pair of arch