Junction between membranous and endochondral bones in the developing occipital squamosa

The occipital bone squamosa (OCS) is unique because of its double origin from both endochondral and membranous bones. The present study attempted to demonstrate the process of connection between these two bone types. We examined sagittal and frontal histological sections from 29 human fetuses with a crown-rump length ranging from 38 to 328 mm (approximately 7-39 weeks of gestational age [GA]). An initial cartilage plate appeared in the posterior side of the fourth ventricle at GA 7-8 weeks and extended inferiorly to connect with the cartilaginous basioccipital and condyle. At GA 9-10 weeks, on the superior side of the cartilage plate, membranous bone fragments appeared and adopted an arrangement resembling a chain of irregularly-shaped beads. They did not form a complete plate-like bone until late-term. At GA 11-12 weeks, endochondral ossification centers appeared at the upper and lower ends of the cartilage plate. At GA 12-15 weeks, a bar-like periosteal bone developed near and superior to the upper ossification center. Notably, sinusoidal structures, which were surrounded by growing periosteal bones, contained island-like clusters of calcified cartilage fragments. Therefore, the upper ossification center appeared likely to "migrate" downward and become distant from membranous bones. The extending periosteal bone reached and joined the membranous bone fragments. Consequently, the periosteal bones connected between the endochondral and membranous bones in the OCS. This connection was quite different from the other components of the calvaria, where membranous bones overlap the skull base cartilages at the margin.