Tracking Appearance and Development of the Ossification Centers in Cranium and Skull Base Bones of Iraqi Sheep Fetuses (Ovis aries) Using Double Stain Method

The skull is a group of flat and irregular bones that protect the brain and special sense organs. The shape of the head depends on the shape of the skull, which is closely related to certain structural and phenotypic features commonly used to identify and characterize different breeds of animals, their genetic profiles, and their relationship with the surrounding environment. 29 samples of indigenous Iraqi sheep fetuses were collected through daily visits to the slaughterhouses in Mosul city. The crown-rump length was measured to determine gestational ages in days. In the current study, the gestational ages of the fetuses ranged between 40 and 71 days, and a crown-rump length between 2.2 and 17 cm was used to accurately determine the locations of the ossification centers using the double stain (alizarin red S and alizarin blue stain) using an anatomical microscope. Histological examination of the head of the fetus was to determine the different stages of ossification. At 42 days of gestation, several primary ossification centers appeared in the right frontal bone near the right orbit. At 45 days of gestation, ossification centers appeared in the zygomatic processes of the temporal bone and the parietal bone. At 48 days of gestation, ossification centers appeared in the temporal bone. At 50 days of gestation, ossification centers appeared in the sphenoid bone. At 57 days of gestation, ossification centers appeared in the squamous and basilar parts of the occipital bone. At 61 days of gestation, the ossification of the inter-parietal bone and the occipital condyles were observed. Histological examination showed that the ossification center consisted of separate groups of osteoblasts in the membranous neurocranium, which ossified intramembranous. In contrast, the base of the skull was cartilaginous in origin, which ossified the endochondral. The very significant result of this study is that the appearance of ossification centers and their development happen within the seventh to ninth weeks of pregnancy, which makes this period the most dangerous time for bone growth, particularly cranium and skull base bones. Thus, it's important to prevent giving any medicine or treatment during that period, which might interrupt or delay this vigorous progression.