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The pituitary (hypophyseal) fossa or sella turcica is a midline, dural lined structure in the sphenoid bone, which houses the pituitary gland.
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The anterior, inferior and posterior walls are bony, while the lateral walls and roof are formed by dural slings between the anterior and posterior clinoid processes (see below). There is a fenestration in the roof of the pituitary fossa for the infundibulum. There are four parts to the pituitary fossa although some authors describe the sella synonymously with the fossa:
Middle clinoid process (MCP) is a small projection of the sphenoid bone located at the anterolateral margin of sella turcica. MCP is variable in appearance and size in different human subjects 9 .
There is, perhaps surprisingly, some controversy about the exact anatomy of the dural layers of the pituitary fossa and adjacent cavernous sinuses. A generally accepted description, which has the added benefit of being epistemologically easier to visualize and understand, is to think of the pituitary fossa as being formed by a sac of only the meningeal layer of the dura mater, with the endosteal layer only being present where there are bony structures (anterior, inferior and posterior) 4-6 . This meningeal layer is continuous with the lateral wall of the cavernous sinus, passing up and over the pituitary fossa to form the diaphragma sellae. Centrally within the diaphragm, a depression is formed which represents the pituitary fossa. Therefore, the lateral walls of the pituitary fossa, between the pituitary gland and the cavernous sinus, are formed by a single, thinned, layer of meningeal dura 4-8 . This also helps visualize the presence of intercavernous venous communications that can be seen below the pituitary gland (between the meningeal and endosteal layers) 4 . It should be noted that connective tissue surrounds the cranial nerves contained in the lateral wall of the cavernous sinus. This connective tissue is variably described as arising from the endosteal layer 7 or from the meningeal layer 8 . Regardless of this distinction, it is true that the lateral wall of the cavernous sinus is double-layered whereas the medial wall, between the cavernous sinus and the pituitary, is formed by a single layer of dura 7,8 .
It is worth noting that, in addition to the aforementioned ‘dural sac’, the anterior lobe of the pituitary also has its own capsule 5 .
Only superiorly is there as a small recess of CSF, the hypophyseal cistern, which is located above the anterior lobe of the pituitary gland and in front of the infundibulum. When enlarged, this cistern can fill and even expand the sella (particularly in the setting of intracranial hypertension) and result in a largely empty pituitary fossa.
History and etymology
Sella turcica literally means “Turkish saddle” in Latin, owing to its shape resembling a saddle.