**2. Applied anatomy and general information**

The pituitary gland is located in the hypophyseal fossa, which is a depression in body of the sphenoid bone located in the middle cranial fossa. Anteriorly, this space is limited by the tuberculum sellae, posteriorly by the dorsum sellae, laterally by the medial wall of the cavernous sinus on each side which extends from the anterior clinoid process and superior orbital fissure anteriorly to the posterior clinoid process posteriorly (**Figure 1A**). The chiasmatic sulcus is a shallow depression running between tuberculum sellae and the limbus sphenoidale where the optic chiasm spans between two optic nerves. The anterior tip of the chiasmatic sulcus, or limbus sphenoidale, is the limit between the anterior and middle cranial fossae. The pituitary gland is an intradural extra-arachnoidal structure with an ovoid shape composed of two lobes: a larger anterior lobe and a smaller posterior one. The pituitary stalk (aka the infundibulum) provides the pathway for ascending neural connections arising from superior surface of the posterior lobe to the hypothalamus.

#### **Figure 1.**

*Intracranial view showing sellar and parasellar areas anatomy. A: Superior view of cranial base. Hypophyseal fossa, or sellae turcica, bounded anteriorly by tuberculum sellae, posteriorly by the dorsum sellae, laterally by the medial wall of the cavernous sinus on each side which extends form anterior clinoid process and superior orbital fissure (SOF) anteriorly to posterior clinoid process posteriorly. Anterior tip of chiasmatic sulcus called limbus sphenoidale (marked by asterisk) which is the junction between anterior and middle cranial fossa. Anterior optic strut separates optic canal superomedially from SOF inferolatearlly and maxillary strut separates SOF from foramen rotundum. Middle clinoid process (MCP), which present in 50% of population, is a projection from lateral margin of sellae turcica. It corresponds transsphenoidally to medial opticocarotid recess. B: Superior view showing the roof hypophyseal fossa and cavernous sinus. Diaphragm sellae roof the superior surface of pituitary gland with the exception of a small opening that allows the stalk to pass from the gland to the hypothalamus. It is continuous with the dura covering the planum sphenoidale anteriorly and the dorsum sellae and clivus posteriorly. The roof of cavernous sinus formed by the oculomotor triangle (blue highlighted triangle) and clinoidal triangle. Oculomotor nerve (CNIII) enter the cavernous sinus at the middle of oculomotor triangle. The roof of left cavernous sinus is opened to show the contents of the cavernous sinus. Only the ICA and abducens nerve (CN VI) are running inside the sinus. CNIII, trochlear (CNIV), ophthalmic, and maxillary nerve are running in the lateral wall of cavernous sinus. CN VI enters the cavernous sinus by passing under Gruber's ligament (aka petrosphenoidal ligament) which spans from the petrous apex to the posterior clinoid process and form the roof of Dorello's canal. In this specimen, Gruber's ligament is duplicated. ACP, anterior clinoid process; CAV. ICA, cavernous segment of ICA; MCP, middle clinoid process; PCP, posterior clinoid process; ON, optic nerve; PETR. ICA, petrous segment of ICA.*

Due to the lack of a robust blood-brain barrier, the pituitary gland exhibits intense enhancement on contrasted magnetic resonance imaging (MRI). The larger anterior pituitary gland is composed of 3 parts:


Five types of endocrine cells are contained inside the anterior lobe that secrete 6 different hormones (**Table 1**). The secretion of hormones is under either stimulatory control from hypothalamus or inhibitory control through feed-back mechanisms. Prolactin is the only pituitary hormone that is under inhibitory control from hypothalamus by prolactin releasing inhibitory factor, mainly dopamine.

The roof of the sellae is formed by a structure known as the diaphragm sellae, which covers the entire superior surface of pituitary gland with the exception of a small opening that allows the stalk to pass from the gland to the hypothalamus. It is formed by a dual layer of dura that is continuous with the dura covering the planum sphenoidale anteriorly and the dorsum sellae and clivus posteriorly (**Figure 1B**).


*\*Prolactin is the only hormone that is under direct inhibition from hypothalamus. ACTH, adrenocorticotropic hormone; FSH, follicle-stimulating hormone; GH, growth hormone; GHRH, growth hormone releasing hormone; LH, luteinizing hormone; PRL, prolactin; TSH, thyroid-stimulating hormone.*

#### **Table 1.**

*Pituitary glandular cell types and function.*

Two layers of dura cover the sellar anterior wall and the floor, namely: the inner (meningeal) layer, and the outer (periosteal) layer. These dural layers run adherent to each other on the anterior and floor of the hypophyseal fossa. Laterally, these 2 dural layers split, as the outer layer continues laterally and form the anterior wall of the cavernous sinus, and the inner layer adheres to lateral wall of pituitary gland to form the medial wall of cavernous sinus (**Figure 2**). Inferior and superior intercavernous sinuses are venous channels that connect the bilateral cavernous sinuses to each other. These venous channels run in the space between the two dural layers in superior and inferior aspects of the hypophyseal fossa (**Figure 2**). In extended transsphenoidal approaches, it is important to coagulate those venous channels before dural opening to avoid significant venous bleeding.

The floor of the sellae forms the posterior wall of sphenoidal sinus, which offers a shortcut in approaching the sellar region. The sphenoidal sinus can be classified based on the degree of pneumatization: conchal, presellar, and sellar. The sellar type is the most common and it is found in 80% of population, representing of a fully pneumatized sphenoid sinus. The conchal type is present in 3% of the population, and it represents a non-pneumatized form. It is common to see this type in the pediatric age population as aeration begins at 10 months of age and rapidly progresses between ages 3 and 6 years—eventually achieving final pneumatization around the 3rd decade of life. The presellar type is an intermediate classification between the conchal and sellar types in which partial pneumatization is observed.

During the endonasal transsphenoidal approach to the sellar and suprasellar regions important structures can be identified as bony prominences on the posterior wall of sphenoidal sinus depending on the degree of aeration. These include the cavernous carotid artery prominences, optic nerves prominences, pituitary gland prominence, and paraclival carotids segments prominences. The lateral optic

#### **Figure 2.**

*Transsphenoidal endoscopic stepwise dissection of sellar floor. (A) Sellar floor bone over the right side anterior wall of cavernous sinus and pituitary gland has been removed and kept intact on the left side. Important landmarks can be appreciated on the sellar floor. Optic nerve, cavernous and paraclival ICA segments prominences can be seen. Lateral opticocarotid recess (LOR) seen superolateral to carotid prominence and inferior to optic nerve prominence (ON Prom.), and it corresponds to optic strut. Limbus sphenoidale spans between optic chiasm/chiasmatic sulcus and planum sphenoidale. (B) Bone over sellar floor removed completely. Periosteal layer of dura (PoL) has been peeled from the meningeal layer (MenL) on the right half of the gland and kept on the left. The anterior wall of cavernous sinus formed by PoL after separating form MenL on the lateral aspect of the pituitary gland and MenL remains stuck to the gland forming the medial wall of cavernous sinus. Note the ligament (marked by "\*") that anchor the medial wall of cavernous sinus. Also, those 2 layers separate at superior and inferior aspects of pituitary gland to form superior intercavernous sinus (Sup. InterCavS.) and inferior intercavernous sinus (Inf. InterCavS.), respectively, which are venous channels connecting the bilateral cavernous sinuses. Inferior hypophyseal artery (IHA) is a branch from meningohypophyseal trunk in majority of cases and supply the pituitary gland with blood. (C) Dura over sellae and suprasellar area has been removed to show the superior hypophyseal artery (SHA) which is a direct branch from supraclinoidal segment of ICA to supply the stalk and gland in addition to optic chiasm and nerves. The arrow heads pointing to diaphragma sellae. Note the opening in the diaphragm through which the stalk ascends from the gland to hypothalamus.*

carotid recess is a depression seen between internal carotid artery (ICA) and the optic nerve prominences. This structure correlates with the optic strut/anterior clinoid process when viewed transcranially (**Figure 2**).
