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

Background: Human embryology textbooks indicate that the trunks of the pulmonary vein and artery originate from the left atrium and aortic s ec, respectively, based on histological analyses of limited human spec imens. However, our studies show that the pulmonary venous trunk in th e 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 c riteria and fixed in Carnoy's solution, Samples were embedded in paraf fin and serial sections were prepared. Results: Histological analysis showed that at day 9.5 the pulmonary venous rudiment was initially obs erved along the left margin in the extracardiac mesenchyme that separa ted the venous pole of the heart from the lung buds, The endothelium o f the pulmonary vein was continuous, with a vascular sac we identified as sinus venosus based on its location immediately posterior to the l eft sinoatrial fold, The sinus venosus became incorporated into the le ft atrium (days 10-10.5) to form part of the posterior atrial wall, Si milarly, the pulmonary vein and associated extracardiac mesenchyme wer e ''drawn'' into the atrium, This extracardiac mesenchyme of the venou s pole, also called ''spina vestibuli'' and containing the pulmonary v ein at its left margin, formed a wedge-shaped invagination within the atrium that contributed nonmuscular tissue to the primary atrial septu m, Conclusions: We propose that the orifice of the pulmonary vein esta blishes 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 incoporated into th e atrium. (C) 1996 Wiley-Liss, Inc.