Fig. 3. Normal Rac activity is essential for macrophage migration. Embryos were stained with anti-Peroxidasin antibody to label macrophages. (A,C,E,G) Stage 13 embryos; (B,D,F,G) stage 15 embryos. (A) In wild-type stage 13 embryos, macrophages have migrated from the anterior and posterior regions toward the middle along the ventral cord. Part of the ventral abdominal region of the embryo is still devoid of macrophages (arrows). (B) At late embryogenesis, macrophages are evenly distributed throughout a wild-type embryo. (C,D) Expression of Rac1^N17 under the control of the gcm-Gal4 causes an arrest of macrophage migration. Only a few macrophages move anteriorly and posteriorly for short distances. (E,F) Expression of Rac1^V12 arrests macrophage migration and most macrophages remain in the anterior region forming a cluster around the foregut. (G,H) Expression of Rac1^L89 causes a delay in macrophage migration. These embryos show a larger macrophagefree area ventrally in the stage 13 embryo (G; area between arrows) than wild-type embryos. A ventral region devoid of macrophages persists also at later stages in Rac1^L89-expressing embryos (H; arrows). (I-K) Whole-mount embryos were stained with anti-Cqr antibody (red). Confocal images of Cqr expression were superimposed with differential interference contrast images to reveal cell profiles. (I) Cqr is a macrophage-specific scavenger receptor that labels the plasma membrane and early phagosomes (Franc et al., 1996; Franc et al., 1999). Wild-type macrophages contain approximately four phagosomes per cell (Franc et al., 1999). Macrophages expressing Rac1^N17 (J) or Rac1^V12 (K) show normal expression of Cqr, as seen within the dense cluster of macrophages surrounding the foregut. These cells contain few or no phagosomes.