GDNF protein is produced by the fetal and early postnatal mouse carotid body. Photomicrographs of tissue sections through the carotid bifurcation from an E17.5 (A) and newborn (C) mouse show GDNF immunoreactivity within the carotid body at both ages (arrows). Tissues were stained using a GDNF-specific polyclonal antibody (see Materials and Methods). B shows the absence of GDNF immunoreactivity in an adjacent section from the E17.5 mouse in which the primary antibody was omitted during the staining procedure. Scale bar, 50 μm.