Fig. 2. mRNA in situ hybridization shows complementary expression of plexin B1 and its ligand Sema4d in epithelial and mesenchymal compartments, respectively, during genesis of several, but not all, organs in mice. (A) Expression of plexin B1 in the developing tooth cusp at embryonic day 13.5 (E13.5) and at day of birth (P0, long arrows), and of Sema4d in the adjacent mesenchyme (short arrows). (B) Expression of plexin B1 in proliferative olfactory epithelium layer (long arrows) and of Sema4d at the border between the olfactory epithelium and mesenchyme (short arrows). Note the reciprocal gradients of plexin B1 and Sema4d expression. (C) Expression of plexin B1 in bronchi (E13.5, long arrow) and branching bronchioles (E15.5, long arrow). Sema4d is expressed on the inner aspect of plexin B1-expressing bronchioles (short arrows), but is not found in the surrounding mesenchyme. In all panels, boxed areas are magnified in insets on the right. Scale bars: 10 µm in A-C.