<div>Background: Being the vital organ of circulatory system, the development of the heart before birth must be studied to safeguard the animal from the incidence of various developmental anomalies. The ultrastructural details of auricular and atrio-ventricular valvular architecture of heart especially in pre-natal sheep has not yet been reported. &nbsp;&nbsp; 
 
Methods: The collected foeti of sheep were divided into three age groups viz. early prenatal (up to 50 days), mid prenatal (51-100 days) and late prenatal (101 to 150 days). The samples of auricle, bicuspid and tricuspid valves were processed for scanning electron microscopic study (Scanning Electron Microscope, Make: Hitachi and Model: S-3400N) and subsequently, the samples were viewed and the photographs were taken in the facility available at Central Instrumentation Facility (CIF), OUAT, Bhubaneswar. The measurements of various parameters of auricles and valves were also taken at the ultrastructural level by the inbuilt software programming system. The recorded data were subjected to routine statistical analysis. 
 
Result: It was revealed that the endocardium was the inner layer of the auricles of heart. The endothelial surface of the auricles was occupied by the simple squamous epithelium. The endothelial cells were elongated in shape in all the age groups. The subendothelial layer was consisted of connective tissue fibers and conducting fibers. The pectinate muscles covered the inner side of the right and left atria and were interconnected having a network like appearance except in mid prenatal stage, i.e. at 96 days of gestation, where they were arranged linearly. There was presence of pores among the pectinate muscles in both the atria in all the age groups under study. The endothelial surface of the bicuspid or mitral and tricuspid valves was lined by simple squamous endothelium that projected into the lumen of the valve. There was presence of pores among the endothelial cells of the valves. Further, the subendothelial layer was present just below the endothelium and comprised of connective tissue fibers and conducting fibers in both the valves. The width of the fibers in bicuspid valve was not uniform and was further categorized into thick and thin fibers especially in the mid prenatal stage, i.e. at 96 days of gestation. There was presence of clusters of pores among the endothelial cells of the tricuspid valve in the heart especially in the late prenatal stage, i.e. at 120 days of gestation period.</div>

Scanning Electron Microscopic Studies on the Auricular and Atrio-ventricular Valvular Architecture of Pre-natal Non-descript Sheep