Morphology of the embryonic and hatchling american alligator ductus arteriosi and implications for embryonic cardiovascular shunting

Abstract

The ductus arteriosi (DA) are embryonic blood vessels found in amniotic vertebrates that shunt blood away from the pulmonary artery and lungs and toward the aorta. Here, we examine changes in morphology of the right and left DA (LDA), and right and left aorta (LAo) from embryonic and hatchling alligators. The developing alligator has two-patent DA that join the right and LAo. Both DA exhibit a muscular phenotype composed of an internal smooth muscle layer (2-4 cells thick). At hatching, the lumen diameter of both DA decreases as the vessels begin to close within the first 12 h of posthatch life. Between day 1 and day 12 posthatching, the vessel becomes fully occluded with endothelial and smooth muscle cells filling the lumen. A number of DA from hatchlings contained blood clots along their length. The lumen of the full term alligator DA is reduced in comparison with the full term chicken DA. The developing alligator embryo has an additional right-to-left shunt pathway in the LAo arising from the right ventricle. The embryonic LAo diameter is twice the diameter of either the right DA or LDA, providing a lower resistance pathway for blood leaving the right ventricle. On the basis of these findings, we propose that the paired DA of the embryonic alligator have a reduced role in the embryonic right-to-left shunt of blood from the right ventricle when compared with the avian DA.

Cardiovascular shunting in vertebrates: a practical integration of competing hypotheses

“…In one of the few physiological studies of shunting in embryonic reptiles, experimental manipulations including surgical occlusion of the left aorta demonstrated that embryonic alligators use the left aorta to shunt right ventricular blood in the systemic circulation (R–L shunt), although this study indicated that such shunting is not necessary for maintaining embryonic oxygen consumption (Eme, Crossley II, & Hicks, ). The role of a patent ductus for shunting in embryonic crocodilians has been suggested to be limited, given the presence of the left aorta (Jacobs et al ., ). In both embryos and adults of non‐crocodilian reptiles (Fig.…”

“…For example, while atrial shunts in mammals via the foramen ovale have been extensively studied, the role of atrial shunts, even whether they exist, is unknown for most reptiles. In the prenatal American alligator, there are two patent ductus arteriosi that join the right and left aortae (Jacobs, Goy, & Dzialowski, ). However, the developing alligator embryo can also affect a R–L shunt via the left aorta, arising from the right ventricle (Fig.…”

Left Right Patterning, Evolution and Cardiac Development

“…In the case of the VIth artery, the distal portion of the right VIth regresses in mammals while the left is maintained until birth and becomes the ductus arteriosus linking the pulmonary and systemic circulations. This asymmetry is not seen in either birds or reptiles, both of which maintain both VIth arteries and develop two ductuses arteriosus [99,102]. Reptiles maintain a symmetrical arrangement in which one ductus feeds into each aorta, but in birds the left ductus must be routed into the single right aorta (Figure 5f).…”