Figure 4

Dorsal-ventral sections of a partially ventralized dorsal midbrain. A partially ventralized dorsal midbrain typically displays a dorsal region with an alar structure and a ventralized region with a basal-like organization. A-C. These sections show the histological organization of the ventral midbrain, the ventral-medial position of the normal oculomotor nucleus (encircled by a dashed line) and the emergence of the eutopic III cranial nerve. D-F. These sections show the histological organization of the ventralized region of the dorsal midbrain, the abnormal (lateral) position of a duplicated motor-like nucleus (encircled by a dashed line), and the lateral emergence of an ectopic supernumerary cranial nerve. The original sulcus limitans (SL) can still be recognized between the ventral midbrain and the ventralized region of the dorsal midbrain. A new boundary, an additional sulcus limitans (SL’), separates the ventralized region of the dorsal midbrain from the tectal region. Scale bars: 50 μm.