Distinct delayed development of the basisphenoid-ethmoid interface in human fetuses: a histological study using human...

In non-human primates, the presphenoid-ethmoid synchondrosis (PSept) functions as a major growth center of the midline skull base, yet whether humans possess a developmental analogue remains unresolved. To clarify whether a human counterpart exists, we re-examined the basisphenoid-ethmoid (SPH-ETH) junction using sagittal histological sections from 20 human fetuses (11-39 weeks) preserved in the Akita University collection. At 15-16 weeks, chondrocytes in the anterior portion of the SPH-immediately anterior to the pituitary fossa-became slender or irregularly shaped with increased extracellular matrix, forming a distinct low-density zone (<8 cells per 0.1 mm²) compared with adjacent SPH cartilage (>15 cells per 0.1 mm²). In late-term fetuses (≥29 weeks), this zone expanded vertically to reach both the superior and inferior perichondria and appeared less basophilic, often containing fibrous or mesh-like structures. Notably, two late-term fetuses (35 and 39 weeks) displayed an additional anterior ossification center immediately in front of this low-density zone, creating a bipolar ossification pattern analogous to that of the PSept in non-human primates. In these specimens, parallel collagen bundles oriented supero-inferiorly were also observed across the interface. Taken together, these findings identify a reproducible, sparsely populated, fiber-containing cartilage region at the human SPH-ETH junction, frequently associated with an anterior ossification center. We propose that this region represents the human analogue of the PSept and that the "transient presphenoid" becomes incorporated into the definitive ethmoid during late fetal development.