Hemodynamic and Histopathological Changes in the Early Phase of the Development of an Intracranial Aneurysm

Abstract

Hemodynamic stress and chronic inflammation are closely associated with the pathogenesis of intracranial aneurysms (IAs). However, the hemodynamic and biological mechanisms triggering IA formation remain to be elucidated. To clarify them, computational fluid dynamics (CFD) and histopathological analyses in the early phase of IA development using an experimentally induced IA model in rats were conducted. Histological changes in the early phase of IA development were observed under a scanning electron microscope (SEM) and a transmission electron microscope (TEM). Using data from 7-T magnetic resonance angiography (7T-MRA), CFD analyses were performed to determine wall shear stress (WSS) and wall pressure (WP) at the prospective site of IA. A bump-like protrusion named an "intimal pad" was located in the anterior cerebral artery (ACA) immediately distal to the apex of the bifurcation. TEM showed the degeneration of the internal elastic lamina (IEL) and longitudinally elongated smooth muscle cells (SMCs) that switched from the contractile to the proliferative phenotype and penetrated between two divided layers of the degenerated IEL in the prospective site of the IA. However, no inflammatory cells were observed. CFD analyses showed no particular pattern of WSS and WP at the prospective IA site. IEL degeneration and the phenotypic change and longitudinal elongation of SMCs were identified as the early events in IA development. CFD analyses and TEM data suggest that these biological events may be derived from increased circumferential wall stress due to increased blood pressure and increased longitudinal wall strain due to the existence of the intimal pad.

Keywords: hemodynamics; internal elastic lamina; intracranial aneurysm; smooth muscle cell.

Fig. 1

Electron microscopy imaging of the OA-ACA bifurcation before IA induction. (A) The circle of Willis in a rat observed under a stereomicroscope. (B) SEM imaging of the intimal surface at the OA-ACA bifurcation. The white arrow indicates the prospective site of the IA. Bar = 50 μm (C and D) Magnified images of the intimal pad. Bar = 10 μm (E) Measurement of the height of the intimal pad by SEM. (F) TEM imaging of the overview of the OA-ACA bifurcation. The gray arrow indicates the prospective site of the IA. (G and H) Magnified views at the leading edge of the internal elastic lamella (green-painted) at the OA (G) and the ACA (H). Bar = 5 μm. ACA: anterior cerebral artery, IA: intracranial aneurysm, ICA: internal carotid artery, IEL: internal elastic lamina, MCA: middle cerebral artery, OA: olfactory artery, SEM: scanning electron microscopy, TEM: transmission electron microscopy.

Fig. 2

Transmission electron microscopy imaging of the OA-ACA bifurcation one day after IA induction. (A) An overview of the OA-ACA bifurcation. Bar = 50 μm (B) Magnified image of the apex. On the ACA side. C and S denote contractile and synthetic smooth muscle cells, respectively. Bar = 10 μm (C) A more magnified image at the leading edge of the IEL on the ACA side. White arrows indicate the fragmented and thinning IEL. Bar = 5 μm (D and E) A magnified image of the intimal pad (D) and the contralateral side (E). Bar = 5 μm. ACA: anterior cerebral artery, EC: endothelial cell, FB: fibroblast, IA: intracranial aneurysm, IEL: internal elastic lamina, OA: olfactory artery.

Fig. 3

Comparison of the IEL scores on the OA side and the ACA side. The state of the IEL before IA induction (control; n = 4), 1 day after IA induction (D1; n = 5), and 3 days after IA induction (D3; n = 6) was scored using the following three grades under TEM: 0 = intact IEL; 1 = mildly degenerated IEL in which only the leading edge is fragmented; and 2 = severely degenerated IEL, divided into the two layers. The IEL score is expressed in a box plot and compared between the OA side and the ACA side by the Wilcoxon signed-rank test. * P < 0.05. ACA: anterior cerebral artery, IA: intracranial aneurysm, IEL: internal elastic lamina, OA: olfactory artery, TEM: transmission electron microscopy.

Fig. 4

Hemodynamics of the OA-ACA bifurcation by computational fluid dynamics analysis. (A) Streamlines with velocity magnitude, (B) WSS, and (C) WP. WSS and WP are also zoomed from the top view, where the white dotted-line area corresponds to the site of the prospective site of the IA, and the white solid-line area corresponds to the OA and ACA origin at the apex. Note that the WP is defined as a differential pressure between the inlet and outlet arteries due to the principles of CFD. For the actual pressure, blood pressure should be added. ACA: anterior cerebral artery, CFD: computational fluid dynamics, ICA: internal carotid artery, MCA: middle cerebral artery, OA = olfactory artery, WP: wall pressure, WSS: wall shear stress.

Fig. 5

Schematic drawings explaining the mechanisms of the early events of experimentally induced IA development at the OA-ACA bifurcation in rats. (A) A schema of the anatomical structure in the OA-ACA bifurcation. The intimal pad is located in the ACA origin. The IEL at the apex on the ACA side is inherently disrupted. Squares A and B denote the proximal and distal edges of the prospective site of the IA, respectively. (B) A schema of the apex of the OA-ACA bifurcation after IA induction. The IEL in this site is fragmented and divided into two layers from the leading edge of the ACA side. SMCs at this site switch the phenotype to the synthetic type and are elongated longitudinally. These synthetic SMCs penetrate between the two layers of the degenerative IEL. (C) A simplified model showing the mechanobiological responses in the prospective site of the IA. An elevated blood pressure after IA induction results in increased circumferential wall stress in the whole artery. The intimal pad serves as a mechanical constraint for dilation and circumferential wall strain at the site adjacent to the intimal pad, resulting in increased longitudinal wall strain at this site. The increased longitudinal wall strain is assumed to cause the phenotypic change and longitudinal elongation of SMCs. (D) The estimated relationship between the circumferential and longitudinal wall strains in the vascular wall at the prospective site of the IA after abrupt blood pressure elevation. The existence of the intimal pad restricts the circumferential wall strain at the site adjacent to the intimal pad. Consequently, the longitudinal wall strain increases at this site. ACA: anterior cerebral artery, IA: intracranial aneurysm, IEL: internal elastic lamina, OA: olfactory artery, SMCs: smooth muscle cells.