At the Guangxi Medical University’s Department of Anatomy, 20 anatomical specimens taken during the fresh autopsy were maintained in 10% formalin solution for at least two weeks. Each specimen was over the age of 18 years. This study was approved by the ethics committee of Guangxi Medical University (ID No. KY-2021-007). The following exclusion criteria were used: 1) craniocerebral trauma, 2) neurological illness, and 3) sinus disease. The members of the families signed individual consent permitting the use of resected samples for research.

There were 12 male and 8 female specimens with the mean age at death of 62 ± 10.33 years (range: 45–80 years). To describe the intraluminal structure in the SSS, the latex was not injected into vein vessels and sinus. The scalps were removed, and by using a surgical power device (Xishan, China), the cranial vault above the axial plane across the nasion and inion was removed. A 4.5-gauge needle was inserted into the SSS, flushing with tap water to remove blood clots. With the cadavers in supine, fixed in Mayfield head holder, an advanced rigid endoscope (Karl Storz, Germany) with a diameter of 4.0 mm and optics of 0 and 30° was inserted into the sinus lumen from the forehead to the coronal. The endoscopes were connected to a digital camera and a video system, enabling photographic recording of the relevant structures. The morphology of the arachnoid granulation and chordae willisii received special attention. Afterward, the SSS samples were carefully removed en bloc using a surgical microscope (OPMI6, Zeiss), and the SSS samples were cut into 1 cm sections from the torcula herophili. AG and its surrounding structures were placed in the observatory area of interest.

Following sectioning, the arachnoid granules and their surrounding structures were prepared for microscopic assessment. To assess those morphological characteristics, H&E staining was used in addition to the particular staining method for detecting collagen fibers (Masson’s trichrome) and elastic fibers (Victoria blue). A Zeiss Axioskop plus microscope (Carl Zeiss Microscopy) was used to analyze and document the histological sections at ×50, ×100, and ×400 magnification. Axio Vision software was used to capture and save the images.

The research involved 268 patients: 167 men and 191 women. At diagnosis, the mean age was 51.63 ± 12.23 years (range: 34–78 years). In addition to conventional cerebral MR sequences, all of these patients acquired 3D high-resolution volumetric MR images [3D T2-SPACE sequence]. Our institutional review board granted approval for this study. This retrospective examination of medical data and imaging studies did not need written informed permission. Consensual analysis of all MR images was performed by two neuroradiologists. Arachnoid granulation was hyperintense on T2WI, and CWs were isointense. Each case was carefully evaluated to determine the numbers and location. Exclusion criteria included the following: (1) cerebral vascular diseases involved with SSS; (2) intracranial tumor involved with SSS; and (3) image data were incomplete or of poor image quality. As previously described, MRI images were captured (14, 20).

SPSS 22.0 for Windows was used to perform all statistical analyses (SPSS Inc., Chicago, Illinois). Descriptive statistics was used to summarize the categorical data, such as arachnoid granulations and percentages. Means, standard deviations, minimums, and maximums were used to express numerical data. Pearson’s chi-square tests were used to determine any statistical difference about proportions. Continuous variables were compared using independent t-test.

AGs were pseudopodia anatomic structures that protrude into the venous sinuses lumen. AGs were detected by an anatomy and MRI scan. Some studies stated the presence of intrasinus structure in the SSS with the aid of a rigid endoscopy. They found arachnoid granulation protruded from the venous lacuna into the lumen of the SSS. With age, the percentage of patients with AGs in SSS increases significantly and there are no AGs in the dura sinuses regarding numerous individuals of various ages (14). The AGs in the cranial bones were discovered for the first time around the SSS at the age of 10, and their number grows dramatically with time. AGs were more prevalent in the cranial bones than in dura sinuses after the age of 60. Three-dimensional high-resolution magnetic resonance imaging sequences such as T2-weighted sampling perfection with application optimized contrasts using different flip-angle evolution and post-contrast T1-weighted magnetization prepared rapid gradient echo was used to diagnose AGs in our study (21). AGs could be clearly observed by an endoscopy and are divided into three types based on their anatomical position. The three types of AGs were also confirmed on the MRI of normal population.

Many studies focused on the histological characteristics of AGs. AGs were made of four distinct components: a central core, a cap cell cluster, an arachnoid cell layer, and a fibrous capsule (22). The arachnoid cell layer that encircled the central core was mostly covered by a thin fibrous capsule with an endothelial investment. The arachnoid cell layer was thickened in places, forming cap cell clusters (23). The central core is contained by arachnoid cells network mixed with connective tissue fibers. Vimentin was found to be localized to intermediate filaments as determined by ultrastructural immunohistochemistry. Depending on their location, the arachnoid cells showed a marked variety in both the cell forms and the number of intermediate filaments or desmosomes. The ultrastructure of arachnoid cell membranes was also investigated by a conventional transmission electron microscope in human AGs. Arachnoid cells exhibited extensive membrane in granulations, namely, desmosomes, gap junctions, tight junctions, and intermediate junctions (9). The arachnoid cells in AGs are not only densely adherent to form a firm structure for CSF transit, but the arachnoid cells also lining the CSF channel exhibit intensive cell–cell contact (24, 25). Similar to previous studies, AGs refer to a narrow neck, broad body, and wide bottom in the dura. The bottom portion protruding into the dura mater formed a single or finger-like dural sheath. We found that type I AGs were larger than type II AGs and the arrangement of collagen fibers in the bottom of type I was more disordered than that of type III. Furthermore, the arachnoid cells were evenly distributed in the body, bottom, and neck.

The morphological characteristics of CW in the SSS resulted in the classification of CW into three distinct forms: lamellae resembling valves, longitudinal lamellae, and trabeculae. The most prevalent form was valve-like lamellae, whereas the longitudinal lamellae were the least common form (26). CMs were visualized and described with the aid of a rigid endoscopy. They also identified three types of CW in all examined specimens (27, 28). Similar to previous research, they also confirmed that CW was the most common in the parietooccipital region of the SSS and its most common type was the valve-like. The relationship between CW and dura sinus walls was demonstrated, and CW divided the lumen of the dura sinus into two separate parts. The thickness of CWs was variable in different parts of dural sinuses (29). In our study, we paid more attention to CW around AGs. We found that valve-like lamellae were presented in type II AG, trabeculae in type I AG, and longitudinal lamellae in type III AG. The collagen fibers on the sinus wall were loosely arranged in type I AG.

Parasagittal meningioma grew inside the dural sinus and may displace or conform to the CW with lumen occlusion without expanding through it (30). The chordae may thus provide a barrier to its spreading into adjacent dura sinus. If the tumor enlarges and extends through CW well behind, the chordae may complicate the process by acting as a barrier to getting behind it and entirely removing the tumor. The dura sinus wall incision, which is often limited to the area where the tumor infiltrates the wall, does not have to provide appropriate exposure, making it necessary for extension behind CW (29). Based on anatomical and histological characteristics of AGs and CWs, we summarize the different growth patterns of PMS ( Figure 6 ). The tumors originating from type I AGs grew inside or outside the SSS lateral wall, and tumors outside the sinus wall could completely achieve tumor resection. To ensure venous blood flow in the SSS, tumor protruding into sinus was partially removed. Residual tumor was treated with radiotherapy three months after operation and observed by imaging. The tumor originating from type II AGs grew into the lateral sinus and subdural space. For CWs blocked tumor growth into the sinus lumen, resectioning the tumor on the outside of CW could safely and completely achieve maximum tumor resection. The tumors originating from type III AGs grew into subdural space. The dura mater, which was invaded by the tumor, could also be resected completely.

We recognize that our study has a number of limitations. First, cadaveric heads vascular replica did not perfectly reflect the flexibility of intracranial vessels. Second, it makes no recommendations for avoiding intraoperative damage to CWs during tumor removal. Third, it does not identify which part of the tumor invaded the dura mater.

This study uses anatomical and histological techniques to reveal the different anatomical types of AGs. Meanwhile, the morphological structure of CWs around AGs was described. Based on the anatomic characteristics of AG, we speculate the different growth patterns of PMS, which guided the surgeon to remove the tumor safely.