Orbital roof cartilage and bone in human fetuses with special reference to changing territories among the ala minor of the sphenoid, frontal bone, and ethmoid

The orbital roof in adults consists mainly of the frontal bone (FB), with the ala minor of the sphenoid at the posterior margin. In fetuses, these bones have been observed to overlap at the junction. The present study examined sagittal, frontal, and horizontal histological sections from 38 human fetuses at a gestational age (GA) of approximately 7-39 weeks. At GA 7-8 weeks, the ala minor extended anterolaterally from the orbitosphenoid, covering the posteromedial quadrant of the roof until GA 9 weeks and reaching to almost the center of the roof at late-term. Simultaneously, the FB appeared in front of the cerebral frontal lobe, reached the anterolateral corner of the roof, and at late-term, it extended posteromedially to cover at least the anterior half of the orbit. In addition, a superolateral plate of the ethmoid, originating from the future cribriform plate, covered the medial marginal part of the roof and had a maximum area at GA 11-16 weeks. At the junction, the FB overlapped and extended below the ala minor or ethmoid. Therefore, at birth, the FB and ala minor seemed to overlap widely at the central one-third of the orbital roof. Because the ala minor was ossified at late-term, postnatal degeneration and absorption were unlikely. The fetal anterior skull base was not flat because of the delayed elevation of the nose and the deeply caved cranial fossa. The overlapping bone and cartilage might slide and migrate, providing materials for reconstruction and later growth of the skull base.