Anatomical view of the sellar region from an endoscopic endonasal (a), interhemispheric (b), frontolateral (c), and pterional (d) perspective. (1) Pituitary stalk, (2) optic chiasm, (3) optic nerve, (4) internal carotid artery, (5) anterior cerebral artery, (6) anterior communicating artery, (7) superior hypophyseal artery, (8) middle cerebral artery, and (9) lamina terminalis
As Atul Goel emphasized the importance of membranes in that: “the anatomical membranes are more primitive embryologically and are physically stronger than many other tissues in the body …… it may appear from an external appearance that the tumour has broken into the anatomical membrane, but on a ‘closer’ look it can be clear that the membrane may be thinned out or rolled over but never actually torn and transgressed …… it is crucial to understand the ‘anatomy’ of the tumour growth so that a preoperative impression of the nature of the tumour can be made and accordingly the surgical strategy can be planned” .
2.2 The Diaphragma Sellae
The pituitary gland is located within a dural sac, which generally does not easily lend itself to tumor invasion. This dural sac, however, has three potential weak points, at where intrasellar tumor may invade and progress. One is the opening of the diaphragma sellae, and the other two are the lateral walls of the pituitary dural sac. Although the lateral wall is single-layered and much thinner than the other dural walls of the pituitary fossa (Fig. 2.2), it is extremely rare that a newly diagnosed infra-diaphragmatic craniopharyngioma would assume the characteristics of pituitary adenoma in terms of cavernous sinus invasion.
Due to the remarkable variation, the actual dimensions of the opening of the diaphragma sellae determine the least resistant path in an infra-diaphragmatic craniopharyngioma (Figs. 2.3). When the opening of the diaphragma sellae is large and/or the tumor origin is close to the opening, the tumor may extrude through the opening by tracking up alongside the pituitary stalk. In contrast, when the opening of the diaphragma sellae is small and/or the tumor arises deep within the pituitary gland, the tumor’s rounded dome is covered by the stretched but intact diaphragma sellae, which withstands more pressure than bone because it cannot be eroded.
2.3 The Arachnoid Sleeve Enveloping the Pituitary Stalk (ASPS)
The pituitary stalk is enveloped by the arachnoid mater sleeve (ASPS), which is a direct upward extension of the basal arachnoid membrane covering the diaphragma sellae (Figs. 2.4 and 2.5) [3, 4]. At the upper part of the pituitary stalk, the ASPS ends at the top of the pars tuberalis. The arachnoid sleeve is reinforced by the arachnoid trabeculae originating from the adjacent basal and cisternal arachnoid membranes. For a suprasellar craniopharyngioma, the ASPS is, in our opinion, the most important structure in directing the growth of the tumor. The Liliequist membrane and membranes surrounding the internal carotid artery (such as the medial carotid membrane) are also very important and provide an effective barrier for craniopharyngioma to encroach the vascular-rich interpeduncular, carotid, and Sylvian cisterns, when they are dense sheet-like membranes with less and smaller openings.