Transmission electron micrograph (TEM) of a section through respiratory epithelium. In the upper frame the cilia are seen in cross-section showing the structure of their central core (axoneme), which consists of twenty microtubules arranged as a central pair, surrounded by nine peripheral doublets. The respiratory epithelium is the lining of the respiratory tract, which moistens and protects the airways. Mucous and cilia act to trap potential pathogens and foreign particles, preventing infection.