Eph Receptors and Ephrins Are Key Regulators of Morphogenesis 131
Fig. lA-D. Eph/ephrin signalling and segmentation. A Interruption of EphA4
function following injection of RNA encoding a dominant-negative form of
the receptor into the fertilised egg of the zebrafish leads to abnormal rhombomere boundary formation. In the top panel expression of the zebrafish receptor EphA4 is seen in the normal hindbrain in rhombomeres 3 and 5. Below
is shown a hindbrain from an embryo into which synthetic RNA encoding an
interfering form of the receptor has been injected. The rhombomere boundaries are abnormally formed and there remains a connection of EphA4-expressing cells between rhombomere 3 and 5 (arrowhead; after Xu et al. 1995). B
Schematic representation of expression of ephrin-B2 (dark shading rhombomerel (Rl), R4 and R7), EphA4 (light shading R3; hatching R5) and EphB4
(medium shading R2, R6; hatching R5). Ephrin-B2 activates both receptors
and can account for segmental boundary formation at the sites of the arrows. C
Somite boundary formation is abnormal in zebrafish embryos which have been
injected with synthetic RNA encoding interfering forms of EphA4 or ephrinE. Bodipy dye-stained zebrafish embryos showing the formation of the
somites in normal embryos (top panel) and in an embryo injected with a synthetic RNA encoding a soluble form of ephrin-B2 (bottom panel). In the experimental embryo somite boundaries are abnormally formed (arrows; after
Durbin et al. 1998). D Expression of ephrin-B2 (dark shading) in posterior
halves (P) of the somite and in two bands in the unsegmented presomitic
mesoderm, and EphA4 (light shading) in anterior halves (A) of somites and in
a single band in the unsegmented presomitic mesoderm. Interactions at alternating interfaces between anterior and posterior half segments leads to boundary formation (arrows; based on Durbin et al. 1998)
Fig. lA-D. Eph/ephrin signalling and segmentation. A Interruption of EphA4
function following injection of RNA encoding a dominant-negative form of
the receptor into the fertilised egg of the zebrafish leads to abnormal rhombomere boundary formation. In the top panel expression of the zebrafish receptor EphA4 is seen in the normal hindbrain in rhombomeres 3 and 5. Below
is shown a hindbrain from an embryo into which synthetic RNA encoding an
interfering form of the receptor has been injected. The rhombomere boundaries are abnormally formed and there remains a connection of EphA4-expressing cells between rhombomere 3 and 5 (arrowhead; after Xu et al. 1995). B
Schematic representation of expression of ephrin-B2 (dark shading rhombomerel (Rl), R4 and R7), EphA4 (light shading R3; hatching R5) and EphB4
(medium shading R2, R6; hatching R5). Ephrin-B2 activates both receptors
and can account for segmental boundary formation at the sites of the arrows. C
Somite boundary formation is abnormal in zebrafish embryos which have been
injected with synthetic RNA encoding interfering forms of EphA4 or ephrinE. Bodipy dye-stained zebrafish embryos showing the formation of the
somites in normal embryos (top panel) and in an embryo injected with a synthetic RNA encoding a soluble form of ephrin-B2 (bottom panel). In the experimental embryo somite boundaries are abnormally formed (arrows; after
Durbin et al. 1998). D Expression of ephrin-B2 (dark shading) in posterior
halves (P) of the somite and in two bands in the unsegmented presomitic
mesoderm, and EphA4 (light shading) in anterior halves (A) of somites and in
a single band in the unsegmented presomitic mesoderm. Interactions at alternating interfaces between anterior and posterior half segments leads to boundary formation (arrows; based on Durbin et al. 1998)