Lung fusion with the liver in chick and quail embryos.

Even in mammals with the diaphragm, the lung and liver are likely to attach mutually without separation by any structure in embryos. The aim of this study was to examine whether or not the lung attaches to the liver in embryonic development of birds without diaphragm. First, we ensured the topographical relation between the lung and liver in 12 human embryos at 5 weeks. After the serosal mesothelium was established, the human lung sometimes (3 embryos) attached tightly to the liver without interruption by the developing diaphragm in the pleuroperitoneal fold. Second, we observed the lung-liver interface in chick and quail embryos. At 3-5 days' incubation (stages 20-27), the lung and liver were fused at bilateral narrow areas just above the muscular stomach. Therein, mesenchymal cells, possibly derived from the transverse septum, were intermingled between the lung and liver. The interface tended to be larger in the quail than the chick. At and until 7 days' incubation, the fusion of the lung and liver disappeared and, instead, a membrane connected them bilaterally. The right membrane extended caudally to attach to the mesonephros and caudal vena cava. At 12 days' incubation, bilateral thick folds, containing the abdominal air sac and pleuroperitoneal muscle (striated muscle), separated the dorsally located lung from the liver. Therefore, the lung-liver fusion occurred transiently in birds. Rather than the presence of the muscular diaphragm, whether the lung and liver were fused seemed to depend on a timing and sequence of development of the mesothelial coverings of these viscera.