Mammalian ciliary membranes form direct contacts in vivo. (A-D) Electron micrograph views of photoreceptor inner segments (IS) and sensory cilia (OS) in a mouse retina. Close apposition of immediately adjacent OS membranes is visible (black arrows in A, B, C). OS membrane of a rod cell forms contact with the connecting cilium (CC) of a neighboring cell (white arrow in A, D). (E-H) Cilia from different cholangiocytes come together. Images are maximum intensity projections from confocal z-stacks of immunostained, paraffin-embedded mouse liver sections from P2 (E and F) and adult (G and H) mice. White arrowheads in E, F, and G indicate cilia contact; in H the arrowheads are at the centrosomes. Hoechst staining shows the position of nuclei (blue); acetylated (E-G) or detyrosinated (H) tubulin staining was used to visualize primary cilia (red); and gamma tubulin staining indicates the base of the cilia in H. Scale bars are 1 μm in A-D and 5 μm in E-H.