Cells of ethmoid bone

The cells of ethmoid bone (a.k.a. ethmoidal air cells) are a complex network of small air-filled cavities, which are intricately positioned within the ethmoidal labyrinths, situated on either side of the nasal cavity’s midline. Originating as outgrowths from the nasal cavity, they progressively erode and integrate into the ethmoid bone, becoming essential components of the paranasal air sinuses. Inside the cells, ciliated respiratory epithelium lines the internal surfaces, playing a crucial role in humidifying and filtering air during respiration.

Anatomically, the ethmoidal air cells are bordered laterally by the orbital plate, a thin bony partition that delineates them from the orbital cavity. Medially, the ethmoidal labyrinth's medial wall separates them from the nasal cavity. These air cells are organized into anterior, middle, and posterior clusters, each with its own distinct location and drainage pathway. Notably, the anterior and middle air cells drain into the middle meatus of the nasal cavity. Specifically, the anterior air cells drain through the frontonasal duct, which connects to the ethmoidal infundibulum. Conversely, the middle ethmoidal air cells open onto the surface of the ethmoidal bulla within the middle meatus. The posterior air cells, meanwhile, drain into the superior meatus.

One interesting aspect of the ethmoidal air cells is their expansion beyond the typical boundaries of the ethmoid bone. This can be observed when examining an isolated ethmoid bone specimen, where the external walls of these cells appear open. However, within a complete skull, these cells are effectively covered and enclosed by the surrounding bones that articulate with the ethmoid bone. These include the frontal bone, the frontal process of the maxilla, the lacrimal bone, the sphenoidal conchae, and the orbital process of the palatine bone.

From a vascular perspective, the ethmoidal air cells are nourished by branches of the anterior and posterior ethmoidal arteries. Innervation is provided by branches from the ophthalmic nerve–specifically, the anterior and posterior ethmoidal branches of the nasociliary nerve–and from the maxillary nerve via its orbital branches that originate from the pterygopalatine ganglion.

During embryonic development, the ethmoidal air cells begin to form around the third month after conception, and they are not radiographically visible before this stage.