Fig. 7. Mouse cells are unable to migrate into chick host tissue. (A,C) Homotopic E8.75 mouse to HH stage10 chick grafts including r3/4 (A) and basal r4 (C). (B,D) Dorsal views of flat-mounted hindbrains from HH stage 24 chimaeric embryos after anti-mouse Hoxb1 immunocytochemistry. The position of the grafts is encircled with a broken red line. In B, only the grafted r4 region is Hoxb1-positive. In D, the basal plate of r4 was grafted; however, chimaeric embryos show a spread of mouse cells from medial to lateral. Note the total absence of Hoxb1-positive cells in the chick surrounding tissue in B,D. The inset in D shows an example of a mouse hindbrain explant immunostained with anti-Hoxb1 antibodies. The arrow indicates Hoxb1-positive cells in r5 along the floor plate corresponding to migrating mouse FBM neurones. br4, basal r4; fp, floor plate; mr3/4, mouse rhombomeres 3 and 4; mr4, mouse rhombomere 4.