DEVELOPMENT OF THE SINOATRIAL VALVES IN THE DOMESTIC CHICKEN (GALLUS DOMESTICUS) AND IN THE ALBINO RAT (RATTUS NORVEGICUS ALBINUS): A SCANNING AND TRANSMISSION ELECTRON-MICROSCOPIC STUDY (HEART, SINUS VALVES, EMBRYOLOGY)

Development of the sinoatrial valves was studied in chick and rat embryonic, fetal, and neonatal hearts. Tissue was prepared by routine techniques for scanning and transmission electron microscopy.;The sinus valves were still well developed in the hatched chick heart and at six days post-natally in the rat heart. At 10 days post-natally, the valves in rat pups have become noticeably reduced in their height, possibly due to their incorporation into the wall of the expanding right atrium.;Transmission electron microscopy of the sinus valves demonstrated the presence of developing myocytes early in valvular development. Dense-cored granules were also observed in the myocytes through out the period of the study.;Both sinus valves were observed in embryonic chick heats at four days of incubation and embryonic rat hearts at 12 days of gestation. The sinoatrial orifice in the chick was oval or ellipsoidal in all stages studied. In the rat embryo this shape of the orifice was acquired by transformation from a small, circular foramen. The right sinus valve demonstrated the greatest projection into the right atrial chamber in both species. Cranially, in the chick, the septum spurium was formed primarily by the cephalic continuation of the left valve rather than as an equal contribution by both valves as in the rat. Caudally, the terminal portion of the right valve coursed under the cover of the left valve in the chick heart, to merge with the fibrous annulus of the right atrioventricular canal. The left valve fuses with the interatrial septum. The caudal termination of the sinus vales in the embryonic rat heart can be described as a circumscription about the inferior vena caval and coronary sinus orifices. The endocardial covering of the sinus valves in embryonic chick and rat hearts demonstrated fenestrations that allow communication of the atrial chamber with the subendocardial space.