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

). 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.