This is a lengthwise slice through an airway (a bronchus or bronchiole), showing three layers: the inner lining of cells (which have tiny hairs called cilia) coated in mucus that glows brightly under the microscope, glands beneath that produce the mucus, and a layer of muscle around the outside. The gap you can see between these layers is probably just an artefact from how the tissue was prepared (from fixing or cutting it, causing it to shrink back), rather than a sign of any real disease or abnormality.

🟦 DNA 🟨 Non-specific stain 🟥 Non-specific stain