The sinonasal tract is an area of the head that includes the nasal cavity (the inside of the nose) and the paired paranasal sinuses (small air-filled surrounding the nasal cavity). The paranasal sinuses include the maxillary sinus, frontal sinus, ethmoid sinus, and sphenoid sinus.
The inside of the nasal cavity and paranasal sinuses are lined by specialized epithelial cells that form a barrier called the epithelium. The tissue underneath the epithelium is called stroma and it is made up of blood vessels and small round structures called glands that make a substance called mucin. The epithelium and underlying stroma combine to create a layer of tissue called sinonasal mucosa.