Ultimobranchial body (UBB) of wild-type, T/ebp-heterozygous, and T/ebp-null mutant mice. A-C: Immunostaining for T/ebp on transverse sections of embryonic day (E) 12.5 embryos. T/ebp-positive cells are shown in brown. Dorsal is up. Black arrow indicates UBB. Red arrow points to the laryngotracheal groove. Most cells in the UBB of wild-type (A) and T/ebp-heterozygous mutant mice (B) express T/ebp. Note that the UBB forms and migrates properly in both T/ebp-heterozygous and null mice (B,C) just like wild-type control, as seen in B,C. D-F: Terminal deoxynucleotidyl transferase-mediated deoxyuridinetriphosphate nick end-labeling (TUNEL) staining of UBBs using transverse sections of E13.5 embryos. Cells stained in brown represent apoptotic cells (shown by arrowheads). Dorsal is up. Note that many cells are positive for apoptosis in the UBB of T/ebp-null mutants (F), whereas a few apoptotic cells are observed in both wild-type (D) and T/ebp-heterozygous mice (E). Scale bar = 100 μm in A-C, 50 μm in D-F.