Fig. 4. Math1expression is restricted to postmitotic cells in the cochlea. Timed-mated pregnant Math1/EGFP transgenic animals were injected with BrdU at either E12.5 (A) or E13.5 (B) and then sacrificed at E14.5. Sections through the cochlear epithelium were double labeled with antibodies against BrdU (green) and EGFP (red). Cells double labeled with both antibodies are yellow. Brackets indicate the sensory primordium. (A) Section through an E14.5 cochlea exposed to BrdU on E12.5. Math1/EGFP cells (arrowheads) are double labeled with BrdU (yellow) indicating that on E12.5, cells that are destined to become Math1/EGFP positive are still in the cell cycle and incorporating BrdU. (B) Section through an E14.5 cochlea exposed to BrdU at E13.5. No double-labeled cells are present, indicating that at E13.5, cells that are destined to become Math1-positive (red, indicated by arrowheads) at E14.5 have exited the cell cycle and do not incorporate BrdU. Scale bar: 50 µm.