Anatomical Histological And Imaging Research Of The Normal Suprasellar Structures

PartⅠ:Anatomical and histological research of the membraneous structures related to the pituitary stalkBackgroundIt's very early for human beings to discover the membraneous structures of nervous system, but much later to really recognize them. Vinas had ever divided the whole cognitive process into anatomic, neuroradiological and microneurosurigical stages. The main improvements of the microneurosurgical stage were that with the use of the operating microscope, it was much clear for us to observe the distribution and boundary of the arachnoid mater and cisterns in normal and pathological conditions.The sellar region lies in the center of the skull base. In concept, the suprasellar cistern consists of the chiasmatic cistern, the cistern of lateral sulcus, the cistern of lamina terminals and the ambient cistern, and contains the Willis arterial circle, optic chiasm, basilar part of hypothalamus and pituitary stalk. Respecting the high risk and difficulty of the sellar operation, the disposition of suprasellar cistern and arachnoid mater had ever been clarified by Yasargil and subsequent authors. But few of them dealt with the membraneous structures related to the pituitary stalk.Furthermore, we couldn't find answers in the literatures to the following questions: 1) Does the pituitary stalk directly descend through a focal defect on the basal arachnoid membrane covering diaphragma sellae to reach the pituitary gland? 2) Is the inferior part of pituitary stalk enveloped in a downward arachnoid sleeve formed by basal arachnoid membrane covering diaphragma sellae as seen in other cranial nerves? And if so, where does the arachnoid sleeve terminate? 3) Is the pituitary stalk a structure in subarachnoid space, or with part in subarachnoid space and part in extraarachnoid space?ObjectiveThe aim of this study was to evaluate the anatomical and histological characteristics of the arachnoid membranes related to the pituitary stalk.MethodsIn sixteen adult cadaveric formalin-fixed heads used for anatomical dissection, red latex was injected into arterial vessels in ten. The cranial vault and brain tissue 2 centimeters above the axial plane across the nasion and the inion were removed. Then the brain tissue over and around the supratentorial basal cisterns was subpially removed in piecemeal fashion, with special attention not to stretch the underlying leptomeninges. By stepwise opening the pia mater covering the carotid cistern, chiasmatic cistern and interpeduncular cistern, the arachnoid membrane with close relationship to the pituitary stalk was investigated. A Leica M651 surgical microscope was used for microsurgical dissections, with a Samsung SCC-101BP digital video camera attached for photographic documentation of relevant structures.In another five adult cadaveric formalin-fixed heads used for histological study, the sellar-suprasellar regions were removed from each head en bloc. The bone in each sample was decalcified. Serial histological sections were obtained in sagittal and coronal direction at 5-μm intervals. All these sections were stained with Masson's trichrome stains and immunohistochemically labeled for vimentin. Observation was made under the Olympus- DP71 light microscope.ResultsThe pituitary stalk penetrated the basal arachnoid membrane to reach the pituitary gland. At the penetrating site, the basal arachnoid membrane extended upwards along the pituitary stalk, which formed an arachnoid sleeve enveloping the pituitary stalk (ASPS) in all specimens.In ten specimens, there was an interval between the basal arachnoid membrane and the superior surface of the pituitary gland at the opening of diaphragma sellae, which made the lower part of the pituitary stalk being free of arachnoid envelop in variable length (typeⅠ). The distance between the lower end of the ASPS and the superior surface of the pituitary gland was nearly equal on the anterior and posterior surface of the pituitary stalk in six specimens (type la), and longer on the posterior surface than that on the anterior surface of the pituitary stalk in four (type Ib). In the other six specimens the basal arachnoid membrane directly contacted with the superior surface of the pituitary gland at the opening of diaphragma sellae, and the ASPS enveloped the pituitary stalk of nearly full length (typeⅡ). In all specimens, stripping the ASPS upwards could be accomplished at its lower part, but could not proceed beyond the middle part of the pituitary stalk without tearing the ASPS due to the tight adherence.In twelve specimens, the well-developed diencephalic leaf of Liliequist's membrane extended upwards, attached to the anterior edge of the mamillary bodies. It separated the chiasmatic cistern and interpeduncular cistern. In eleven specimens, a few arachnoid trabeculae originated from the superior surface of Liliequist's membrane and attached to the posterior, lateral surface of the upper and middle parts of the pituitary stalk, and to the inferolateral surface of the optic chiasma, which made the middle and upper part of the ASPS thicker. In one specimen, there were no arachnoid trabeculae interconnecting Liliequist's membrane and the pituitary stalk. In four specimens, the diencephalic leaf of Liliequist's membrane was absent and the chiasmatic cistern communicated freely with the interpeduncular cistern. Under these circumstances, there were no pituitary stalk related arachnoid trabeculae from Liliequist's membrane.The sagittally oriented medial carotid membrane, separating the chismatic cistern and the carotid cistern, varied greatly among different specimens and from side to side in individual specimen. In five specimens, the medial carotid membrane was absent unilaterally. The appearance of the medial carotid membrane varied from porous trabeculated wall to relatively intact membrane with small openings. The medial carotid membrane was originated from the basal arachnoid membrane inferomedial to the supraclinoid internal carotid artery and posterior communicating artery, and paramedian superior surface of Liliequist's membrane. Extending superiorly and medially, the medial carotid membrane attached to the pia mater covering the inferior surface of the optic chiasm, adjacent optic nerve, lateral surface of infundibulum and the upper part of ASPS. Perforating arteries originating from the internal carotid artery and posterior communicating artery traveled on or through the medial carotid membrane to supply the pituitary stalk, optic chiasma, optic tract and hypothalamus.The ASPS could be seen in all histological specimens. Above the site where the pituitary stalk penetrated the basal arachnoid membrane to reach the pituitary gland, the upward extension of basal arachnoid membrane ran along the pituitary stalk. Therefore, the ASPS was arachnoid mater membrane, but thinner than the basal arachnoid membrane. At the upper part of the pituitary stalk, the ASPS could be seen attached to the top of pars tuberalis. In histological sections with immuno histochemical labeling for vimentin, it was clear that the arachnoid sleeve ended at the top of pars tuberalis and near the base of infundibulum.Conclusions1. The pituitary stalk penetrated the basal arachnoid membrane to reach the pituitary gland. At the penetrating site, the basal arachnoid membrane extended upwards along the pituitary stalk, which formed an arachnoid sleeve enveloping the pituitary stalk (ASPS) in all specimens.2. The pituitary stalk is enveloped by arachnoid mater sleeve (the ASPS) in variable length. Type I:there was an interval between the basal arachnoid membrane and the superior surface of the pituitary gland at the opening of diaphragma sellae, which made the lower part of the pituitary stalk being free of arachnoid envelop in variable length. The distance between the lower end of the ASPS and the superior surface of the pituitary gland was nearly equal on the anterior and posterior surface of the pituitary stalk (typeⅠa); and longer on the posterior surface than that on the anterior surface of the pituitary stalk (typeⅠb). TypeⅡ:the basal arachnoid membrane directly contacted with the superior surface of the pituitary gland at the opening of diaphragma sellae, and the ASPS enveloped the pituitary stalk of nearly full length.3. The ASPS constantly presents, and is reinforced by the arachnoid trabeculea originating form the basal arachnoid membrane, Liliequist's membrane, and the medial carotid membrane in the majority of specimens.4. The ASPS begins at where the pituitary stalk penetrates the basal arachnoid membrane and ends at the top of the pars tuberalis.5. The pituitary stalk can be divided into three parts according to their relationship with the ASPS, which includes subarachnoid, intraarachnoid, and extraarachnoid part.PartⅡ:The imaging anatomical research of the normal suprasellar structuresBackgroundThe sellar region, a common place to develop tumors of neurosystem, lies in the centre part of skull base and is characteristic of complicated and distinctive anatomy. Due to the narrow space, expansion and restriction effects existing commonly between the sellar tumors and surrounding structures determine their actual growth pattern and appearance consequently. Given the inferior osseous sellae turcica with little compensation space and the superior optic chiasm that is relatively fixed by restriction of bilateral optic nerves and pituitary stalk, the anterior and posterior aspects are the principal directions of expansion with regard to supresellar mass lesions in sagittal section.In the long term, the valuable anatomic data and experience of this region for neurosurgeons are mostly acquired from macromicrodissection. What is easily ignored, however, in contrast to the anatomical research of the cadaveric formalin-fixed heads, lively and detailed anatomy of the intracranial structures within living bodies could be more exactly demonstrated in situ by MR imaging, which is generally acknowledged a revolutionary advance in diagnosing central nervous system diseases and has been routine examination nowadays. In review of literature, little attempt was made to investigate the normal sellar region by morphometry.ObjectiveIn this study, we endeavor to investigate the normal sellar region in midsagittal plane by MR imaging anatomic measurement and to probe related clinical significance.Methods120 specimens of normal standard midsagittal T2-weighted images were collected retrospectively. There were 68 male and 52 female cases (male/female ratio 1.33), ranging in age from 1 to 80 years (mean 36.78 years), including 44 cases (36.7%) younger than 18-year-old and 76 cases (63.3%) over 18 years.Anchor points and lines were set to facilitate the morphometric study as follows: the line between anterior commissure and posterior commissure (AP), the major axis of optic chiasm (OC), the major axis of pituitary stalk (PS), the point of tuberculum sellae (TS), the chiasm-tuberculum distance (TC), the point of dorsum sellae (DS), the distance between TS and DS (TD), the nearest point of skull base to optic chiasm (M), the shortest distance between them (Min), the shortest distance between DS and inferior surface of the optic chiasm (DC). In each case, seven investigated indexes were measured, image classifications by the position of optic chiasm, the relationship between pituitary stalk and dorsum sellae, and the relationship between M and TS were also performed.All measurements were performed in each case with electronic calipers and conimeters by JW-PACS imaging working system of our hospital and then averaged. Statistical analysis on the data acquired during this study was fulfilled using 2 independent samples two-tailed t tests, paired-samples two-tailed t test,2 independent samples nonparametric test and Pearson correlation analysis. Mean, standard deviation (SD) and coefficient correlation (r) of investigated indexes were displayed in tables. The criterion for statistical significance for all tests was P<0.05.ResultsThe measurements of 120 cases (mean±standard deviation) were as follows: OC-AP angle (37.37±5.99°), PS-AP angle (68.20±10.63°), OC-PS angle (31.39±10.35°), Min (5.05±1.79mm), TC (5.33±1.68mm), DC(5.21±2.04mm), TD (10.11±2.12mm). Prefixed optic chiasm (14 cases,11.7%), normal type (89 cases,74.1%), postfixed type (17 cases,14.2%); M was situated before TS in 46 cases (38.3%), behind TS in only 1 case, two points coincided in 73 cases (60.8%). The relative position of the stalk and the dorsum sellae, type A (58.3%):no contact; type B (19.4%):slight contact without compression and angulation; type C(22.2%):obvious compression with angulation and deformity.In statistical comparison, adults had larger TD, smaller OC-AP angle,PS-AP angle than minors. No difference was found between female and male. The distribution of three classifications between adults and minors was not statistically significant. TC (5.33±1.68) was larger than Min (5.05±1.79),P=0.000. The Min has significant difference among prefixed, normal and postfixed chiasms, P=0.000.Positive correlation between Min, TC, TD and age was statistically significant. According to the cut point of 5mm,120 cases could be classified into four separate types by both Min and DC. TypeⅠ(25/120,20.8%):Min<5mm, DC<5mm; typeⅡ(30/120,25%):Min<5mm, DC≥5mm; typeⅢ(29/120,24.2%):Min≥5mm, DC<5mm; typeⅣ(36/120,30%):Min≥5mm, DC≥5mm.Conclusions1. It is unacceptable to identify the chiasm-tuberculum distance with the shortest size of prechiasmatic space. Min is a more accurate indicator than TC to evaluate the size of the prechiasmal space.2. Min and TC is not in absolute parallel with optic chiasm classification and it is unreasonable to categorize optic chiasm according to Min value.3. The prechiasmatic space and the anterior-posterior diameters of sellae turcica increase with increasing age.4. The PS-AP angle is expected to become a conventional imaging anatomical parameter, and the data of the current study could serve as standard measurements of the normal pituitary stalk inclination.5. A surprising proportion of stalks have close relationship with dorsum sellae in general population, which may suggest that the pituitary stalk is a more vulnerable structure than we ever thought.6. According to Min and DC, the normal suprasellar space could be divided into four different types.