Multimodal Anatomy of the Human Forniceal Commissure

Anatomical studies investigating the forniceal commissure have mainly been performed in non-human primates. However, ambiguity surrounds the existence, morphology, and functionality of this structure in the human brain. We investigated the morphology of the human forniceal commissure by including advanced in vivo tractography, ex vivo fiber dissection, and histology. A whole-brain tractography from 7T data of 180 participants was produced, each fornix dissected, and fiber percentage maps calculated. In nine human brains, we performed stepwise multidirectional Klinger dissection under the operating microscope. For histological analysis, multiplanar serial floating sections of six specimens were obtained. Virtual fiber dissection did not identify interhemispheric connections between the crura fornicis. Klingler dissection demonstrated a thin soft-tissue membrane spanning between the left and right crus fornicis, but no commissural fibers. These findings were corroborated by the multiplanar histological analysis with myelin staining that visualized delicate cruciform fibers extending between the crura fornicis, which were enclosed by connective tissue. The human forniceal commissure appears much more delicate than previously described and presented in anatomical textbooks. This finding is consistent with the phylogenetic trend of a striking reduction of the forniceal commissure in non-human primates compared to non-primate mammals and might have functional implications in clinical neurology.