Mastoid cells

Mastoid cells, or the mastoid air cells, are numerous tiny cavities situated in the lower part of the mastoid process. These cells vary widely in size, number, and shape. For example, the cells at the upper and front part of the mastoid process are large, irregular, and filled with air, whereas those toward the bottom become smaller. The cells at the very tip of the mastoid process are often quite small and contain bone marrow. In rare cases, mastoid cells may be completely absent, resulting in a solid mastoid process.

These cells are connected to the middle ear cavity or tympanic cavity through the mastoid antrum—a channel that opens into the back wall of the middle ear, specifically in its upper recess, the epitympanic recess. Both the mastoid antrum and the tympanic cavity are covered by the same thin bone, called the tegmen tympani. The lining of the mastoid cells is continuous with that of the mastoid antrum and middle ear cavity. Therefore, an infection in the middle ear, like otitis media, can quickly spread to the mastoid antrum and cells, potentially causing mastoiditis.