Origin of the pulmonary venous orifice in the mouse and its relation to the morphogenesis of the sinus venosus, extracardiac mesenchyme (spina vestibuli), and atrium

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Abstract
Background Human embryology textbooks indicate that the trunks of the pulmonary vein and artery originate from the left atrium and aortic sac, respectively, based on histological analyses of limited human specimens. However, our studies show that the pulmonary venous trunk in the mouse as in other nonhuman vertebrates originates from a vascular “sac” at the venous pole, the sinus venosus. Methods Mouse embryos of 9–11 days gestation were obtained and staged according to Theiler's criteria and fixed in Carnoy's solution. Samples were embedded in paraffin and serial sections were prepared. Results Histological analysis showed that at day 9.5 the pulmonary venous rudiment was initially observed along the left margin in the extracardiac mesenchyme that separated the venous pole of the heart from the lung buds. The endothelium of the pulmonary vein was continuous, with a vascular sac we identified as sinus venosus based on its location immediately posterior to the left sinoatrial fold. The sinus venosus became incorporated into the left atrium (days 10–10.5) to form part of the posterior atrial wall. Similarly, the pulmonary vein and associated extracardiac mesenchyme were “drawn” into the atrium. This extracardiac mesenchyme of the venous pole, also called “spina vestibuli” and containing the pulmonary vein at its left margin, formed a wedge-shaped invagination within the atrium that contributed nonmuscular tissue to the primary atrial septum. Conclusions We propose that the orifice of the pulmonary vein establishes a link with the left side of the atrium as a consequence of a venous sac, the sinus venosus, and its associated mesenchyme (in which the root of the pulmonary vein is embedded) being incorporated into the atrium.