Testing the model of optic chiasm formation in human beings.

Recent data from animal studies allow us to understand how the optic commissure formed. However, their validity in humans has not yet been demonstrated. Clues from human teratological cases provide useful information to test the validity of animal models and suggest other morphogenetic mechanisms. Janiceps are non-viable, rare cases of co-joined embryos fused along the frontal plane, with two composite faces, half formed by each of the embryos. Their development after fusion is of interest to study the optic chiasm. The analysis of the optic nerves and of the skull by magnetic resonance imaging or direct inspection of three different cases of Janiceps showed a shared hypophyseal fossa that laterally could contain a compound optic commissure between the eyes of the same composite face (heterologues), but not between the eyes from the same individual (homologue). The current model of the optic commissure formation correctly predicts these findings in humans because the optic commissure are driven toward the midline by local factors and not by target regions. However, our data suggest that the overall geometry of the region is important for a successful decussation of the axons, and additional mechanisms are involved in midline differentiation. Copyright 2002 Elsevier Science Inc.