PPM development in control and Tbx18-deficient embryos.

(A–P), Histological analysis by haematoxylin and eosin stainings of PPM development was performed on transverse sections of a posterior and a mid-transverse section plane of control and Tbx18-deficient hearts from E11.5 to E14.5. Magnifications (C, D, G, H) highlight the sinuatrial mesenchymal ridges that protrude into the PPCs in control embryos at E11.5 and E12.5, and their absence in Tbx18-deficient embryos, respectively. Genotypes and stages are as indicated. Asterisks highlight the subcoelomic lateral body wall mesenchyme; black arrowheads mark the attachment point of the PPMs to the lateral body wall; black arrows point to the remaining PPCs; and green arrowheads mark the mesenchymal ridge that is necessary for the closure of the PPCs. icv, inferior cardinal vein; la, left atrium; lcv, left cardinal vein; li, liver; lu, lung; lsh, left sinus horn; lv, left ventricle; pc, pericardial cavity; ppc, pericardioperitoneal canal; pl, pleural cavity; ppm, pleuropericardial membrane; ra, right atrium; rcv, right cardinal vein; rsh, right sinus horn; rv, right ventricle.