Figure 3

Slits are expressed in the floor-plate area of the embryonic chicken spinal cord. In agreement with published reports [29, 68], Slit1 was expressed weakly in the floor plate at HH19, when commissural axons start to extend in the dorsal spinal cord but before they reach the floor plate (black arrow in A). Interestingly, Slit1 was also expressed in commissural neurons throughout axonal pathfinding (HH19-26; arrowheads in A D G). Slit1 was expressed more strongly in cells flanking the floor plate (open arrows in A D G) rather than the floor plate itself (arrow; D G). By HH26, Slit1 was no longer expressed in the floor plate (arrow in G) but persisted in cells flanking the floor plate. Slit2 was strongly expressed in the floor plate (arrow in B E H), in agreement with published findings [29, 68]. Furthermore, Slit2 was expressed by motoneurons (asterisks in B E). Slit3 was strongly expressed in motoneurons (asterisks in C F I), but only at low levels, if at all, in the floor plate (arrows in C F I). Thus Slit expression shows some differences between chicken and rodent embryos [25, 33]. In mouse [33] and in rat [25], Slit1 is expressed in the floor plate rather than in cells flanking the floor plate. Mouse but not rat motoneurons express Slit3 between E10.5 to E13.5. Motoneurons are the predominant site of Slit3 expression in the chick at HH23. Overall, Slit expression is conserved between chicken embryos and rodents, although the patterns of individual Slits differ significantly. Bar: 33 μm in A C, 50 μm in D F, 100 μm in G I.