The mastoid air cells vary greatly in number, shape, and size; they may be extensive or minimal or even absent.

[1]: 746 The cells are typically interconnected and their walls lined by mucosa that is continuous with that of the mastoid antrum and tympanic cavity.

[1]: 746 They may excavate the mastoid process to its tip, and be separated from the posterior cranial fossa and sigmoid sinus by a mere slip of bone or not at all.

They may extend into the squamous part of temporal bone, petrous part of the temporal bone zygomatic process of temporal bone, and - rarely - the jugular process of occipital bone; they may thus come to adjoin many important structures (including the bony labyrinth, tympanic cavity, external acoustic meatus, pharyngotympanic tube, superior jugular bulb, posterior cranial fossa, middle cranial fossa, carotid canal, abducens nerve, sigmoid sinus) to which they may disseminate infection in case of infective mastoiditis.

The air cells then gradually extend into the bone of the mastoid during the first years of life.