Trachea mucous membrane. Coloured scanning electron micrograph (SEM) of a fractured mucous membrane of a trachea (wind pipe). The upper epithelial surface is covered in a mixture of secretory and ciliated cells (pink), beneath which is the tall columnar epithelium (purple). At bottom is connective tissue (lamina propria, brown), which is perforated by blood vessels, which contain red blood cells (erythrocytes, red). Magnification: x500 when printed 10 centimetres wide.