Evaluation of the skeletal ossification of sheep fetuses at different gestational ages (20–95 days) using radiography and whole-mount skeletal staining

The emergence of ossification centers is a useful tool for estimating prenatal fetal age, evaluating the process of fetal bone maturity, and diagnosing of some fetal abnormalities. In this study, the sheep fetal skeleton at different gestational ages was evaluated using radiography and whole-mount skeletal staining. The sheep's pregnant uteri were prepared from the slaughterhouse at different gestational ages, with fetal age estimated using ultrasonography. Five fetuses were aligned in each age range of 30–40 days, 40–50 days, 50–60 days, 60–70 days and 70–95 days. The fetus was removed from the uterus and laterally placed on the radiographic cassette. Whole-mount skeletal staining was then performed, with data recorded including the overall shape of the fetal skeleton and the time and area of presence of bone calcification centers at different ages of pregnancy. The results show that the emergence and expansion of ossification centers began from the bones near the midline of the body on the back surface at about 45–55 days of pregnancy. The ossification of the fetal skeleton was completed at the age of 80–95 days of pregnancy in the toes, tooth, and near the abdominal midline. Whole-mount skeletal staining revealed ossification centers from day 23. Therefore, radiography can be used to depict the time of emergence of bone centers and to examine the evolution of the fetus during sheep pregnancy. Opacity of embryonic and fetal fluids did not change during this period, preventing observation of the fetus in fetal membranes and uterus before ossification of the fetal skeleton.