Pituitary adenomas extend to the suprasellar region via diaphragmal passage and extend to the cavernous sinus via the medial wall (MW). Better understanding of the dynamics of suprasellar and parasellar extension of sellar region pathologies requires microanatomical comparison of diaphragma sellae (DS) and the MW of the cavernous sinus. This study provides the first detailed quantitative assessment between DS and MW of the cavernous sinus. Microanatomical details and histopathological examinations of the DS and MWs of the cavernous sinus were studied in sphenoid block samples obtained from adult cadavers, and the thicknesses of the DS and the MW of the cavernous sinus were measured. Mean thickness of the DS was 216.73 +/- 51.26 mu m in the center and 367.33 +/- 133.66 mu m in the periphery. Mean thickness of the lower third of the MW was 161.53 +/- 53.86 mu m and that of the upper third was 278.46 +/- 162.79 mu m. Difference between the thicknesses of the upper and lower thirds was significant (P < 0.001). When the central thickness of the DS and lower third of the MW were compared, a significant difference was found (P < 0.01). Our measurements suggest that when the DS is thick and the stalk opening is complete, the thickness of the lower third of the medial wall could be the major determinant of parasellar extension.