Figure 2.

Models of forebrain neurulation from fish to mammals. Fish: telencephalic cells migrate above the eye field without mixing. This could be achieved by either (i) uncoordinated or (ii) coordinated telencephalon migration, independently of eye field cell behavior. (iii) Alternatively, the movement may be driven by folding, through rotation of eye field cells. This mode requires strong adhesion between telencephalon and eye at the boundary as well as inside the telencephalic population. Mammal: Studies are very sparse. Absence of marker analyses precludes strong statement on the exact position of the boundaries between hypothalamus, eye field and telencephalon. The initial folding of the neural plate may occur (i) at the boundary between eye field and telencephalon or (ii) in the middle of the eye field. Scenario (i) resembles the fish situation, in which the telencephalon converges toward the midline while the eye field resists these neurulation movements. This step is then followed by deformation of the eye field by apical constriction of its center. Alternatively, in ii) the folding of the neural plate at the middle of the eye field comes first, followed by differential growth of a small telencephalic initial primordium.