The appearance and distribution of desmin, a muscle type intermediate filament, was examined in the truncus arteriosus of the chick embryonic heart during AP septation by an indirect immunofluorescence method. Prior to septation, on the 4th day of incubation, staining with antidesmin antibody was observed in the AP septum anlagen in the distal truncus. No staining with the antibody was detected in the developing tunica media of the great arteries in the distal truncus at this stage. During septation, on the 5th day of incubation, intense staining with the antidesmin antibody was observed in the cell condensation of the AP septum. In the 6-day-old embryo, the staining in the AP septum was reduced, but fluorescence by staining with the antibody was observed in the developing tunica media. No fluorescence was detected in other mesenchymal cells in the truncal swellings.
Because of the time of appearance and the localization of antidesmin reactivity in the developing AP septum, it is suggested that muscle-type cells exist in the AP septum, and that these cells may perform an important function in septation.