Development of the occipitopetrosal junctions: A histological study using human fetuses.

The temporal bone petrosa is connected to the occipital bone at two sites: one junction with the basilar part (medial junction) and another with the lateral part (lateral junction). Using histological specimens from 45 human fetuses (approximately 7-39 weeks of gestational age or GA), we aimed to describe possible changes in histology and topographical anatomy during processes toward the ossified union. In the early term, the junctions were characterized by dense mesh-like fibers that likely corresponded to the thickened basal laminae of the composite cartilage cells. Notably, the mesh-like structure disappeared until 12 weeks of GA and, irrespective of whether a remnant fibrous tissue was present or absent, cartilage tissues became continuous between the occipital and petrosal sides. Until 25 weeks of GA, the inferolateral marginal part of the spheno-occipital junction cartilage extended to the medial junction and attached to the ossified petrosa. The medial junction sometimes contained an irregularly shaped cartilage mass or a woven bone with bone trabeculae, but it was usually attached directly to the bony petrosa without any interruption by calcified cells. In contrast, at the late-term lateral junction, a cartilage mass was sandwiched by bipolar ossification centers on the temporal and occipital bone sides. As the endochondral ossification advances, bones at and around the lateral junction seemed to grow and cover the expanding mastoid air cells. However, the fetal medial junction might grow, in accordance with midline growth at the spheno-occipital junction cartilage. Alternatively, the bony union of these two junctions might establish postnatally.