Physiopathology of the cochlear microcirculation

Abstract

Normal blood supply to the cochlea is critically important for establishing the endocochlear potential and sustaining production of endolymph. Abnormal cochlear microcirculation has long been considered an etiologic factor in noise-induced hearing loss, age-related hearing loss (presbycusis), sudden hearing loss or vestibular function, and Meniere's disease. Knowledge of the mechanisms underlying the pathophysiology of cochlear microcirculation is of fundamental clinical importance. A better understanding of cochlear blood flow (CoBF) will enable more effective management of hearing disorders resulting from aberrant blood flow. This review focuses on recent discoveries and findings related to the physiopathology of the cochlear microvasculature.

Figure 1. Schematic view of CoBF supply

A, The SMA, a major artery, supplies blood to the cochlea image from (Axelsson, 1968)]. B, A characterization of the vascular pattern on the outer wall of the cochlea is shown. Radiating arterioles arching over the roof of the scala vestibuli run in bony channels, branching as they emerge from the upper margin of the spiral ligament. Two distinct capillary networks in the spiral ligament and stria vascularis are apparent in the lateral wall. The networks parallel each other without cross connections [image adapted from (Mudry et al., 2009)]. V/SL: vessels of the spiral ligament; V/SV: vessels of the stria vascularis.

Figure 2. Cochlear pericytes on cochlear microvessels in adult guinea pig

Pericytes are idenified with double-staining for desmin (red), a pericyte marker protein, and nitric oxide (DAF-2DA, green). A: an arteriole; B: a capillary of the spiral ligament (SL); C: a capillary of the stria vascularis (SV). Pericytes have a body (short arrows) and many primary processes (long arrows) which tightly embrace the endothelial tube. Pericytes on the outer wall of vessels have a characteristic “bump on a log” shape.

Figure 3. Shapes of pericytes on different cochlear microvessels

The pericytes were double-labeled with a pericyte marker protein: desmin (red), combined with fluorescent indicator for intracellular nitric oxide DAF-2DA (green). Panels A–C show the morphology of a pericyte on a true capillary. The pericyte has a polygonal-shaped cell body (Panel A, 10 sections; interval: 1 µm), relatively few long longitudinal processes, and short, fine circumferential projections (Panel B, 10 sections; interval: 1 µm). Panel C is a merged image of Panels A and B. Panels D–F show the morphology of a pericyte on a precapillary. The pericyte has a “bump-shaped” soma (Panel D, 11 sections; interval: 1 µm) and relatively large processes that encircle the capillary (Panel E, 10 sections; interval: 1 µm). Panel F is a merged image of Panels D and E. Panels G–I show the morphology of a pericyte on a postcapillary. These pericytes have a flattened cell body (Panel G, 11 sections; interval: 1 µm) and short processes encircling the vessel (Panel H, 11 sections; interval: 1 µm). Panel I is a merged image of Panels G and H. Panels J–L show the morphology of a pericyte on a branch point of the postcapillary. The pericyte has a spindle-shaped cell body (Panel J, 10 sections; interval: 1 µm) and long processes distributed over the two branches (Panel K, 10 sections; interval: 1 µm). Panel L is a merged image of Panels J and K

Figure 4. Morphological details of fibro-vascular coupling is shown in confocal and TEM images

(A) Type V fibrocytes positive for S100 (green) abut capillary walls labeled by isolectin IB4 (blue). (B) Type V fibrocytes are positive for Na⁺/K⁺ ATPase β1 (red). (C) Type V fibrocytes also contain high levels of NO, as detected with the intracellular NO indicator, DAF-2DA (gray). (D) Magnification of panel B shows foot processes in contact with a capillary. (E) A multiple-foot process of a fibrocyte abuts capillary wall. (F) A high magnification image shows a fibrocyte end-foot structure at the soma of a pericyte. The soma of pericytes were labeled by an antibody for NG2, (red), and processes were labeled with an antibody for the structural protein, desmin (blue). Capillary walls are labeled by phalloidin (green). (G) and (H) Fibrocytes contact capillaries with enlarged endings. (I) The endings display electron-dense membrane regions rich in mitochondria. Abbreviations: FC, fibrocyte; EC, endothelial cells; PC, pericyte; Mt, mitochondria. Calibration bars in H and I are 500 nm.

Figure 5. A working model of fibro-vascular coupled signaling in the inner ear

Cochlear blood flow is anatomically distant from sensory hair cells, but the cells are morphologically coupled to supporting and fibrocytes by gap junctions. Mechanical activity (red line) or metabolic activity (red dotted line) increases COX-1 enzymatic activity in type V fibrocytes, but the exact pathway is unknown. Activation of COX-1 may result in conversion of arachidonic acid into metabolic intermediates such as PGE2. The PGE2 diffuses into the perivascular space and elicits vasodilatation through the mediation of fibrocyte-coupled pericyte activity.

Figure 6. Cellular structure of the blood-labyrinth barrier

Endothelial cells in normal BLB are identified with an antibody for mouse endothelial IgG (A, blue), pericytes with an antibody for desmin (B, green), and macrophages with an antibody for F4/80 (C, red). The merged image (D) shows the complexity of the blood-labyrinth-barrier.

Figure 7. Noise induces breakdown of the blood-labyrinth-barrier and causes irregularities in pericyte coverage

A & C, Serum protein IgG is confined to blood plasma (IgG/arrow) in vessels of the stria vascularis in normal mice (A) and guinea pigs (C). B and D, Serum protein IgG leaks from vessels (arrow/IgG) in noise-exposed mice (B) and guinea pigs (D). Arrowheads indicate sites of vascular leakage. GP: guinea pig. Pericytes containing desmin filaments are evenly distributed on the vessel walls of the stria vascularis in both guinea pigs (E) and mice (F). Pericytes are labeled with an antibody for desmin (green), and vessels with an antibody for isolectin IB4 (red). G and H: Confocal fluorescent images from noise-exposed guinea pigs and mice show abnormal pericyte morphology and increased pericyte coverage. Arrows point to irregular pericyte foot processes turning away from the vessel wall (G) and detached from it (H). I and J: Drawings illustrate the pattern of pericyte distribution on vessel walls in normal and noised-exposed animals. V/SV, vessel of the stria vascularis; NE, noise exposure; GP, guinea pig; MS, mouse.

Figure 8. Classification of isolated stria vascularis capillary proteins identified ATP1A1 as the most abundant protein in the blood-labyrinth barrier

The pie graph shows a spectral count-weighted tabulation of the GO annotation by biological process. Proteins involved in transport (42%) and metabolism (19%) are highly expressed in the blood-labyrinth barrier

Figure 9. The 3D volume rendering of mouse cochlea is segmented and displayed in four different orientations to provide a detailed view of the cochlear microvasculature

A & B show a 3D volumetric perfusion image of the entire cochlea (Media3 & Media4). C is a segmented 3D volumetric microvascular perfusion at the Modiolus (Media5), and D a 3D volumetric reconstruction of the microvascular perfusion together with cochlear structures (Media6).