Rat utricular macula: blood flow and stereological assessment of capillary morphology

Vascular compromise has long been proposed as a cause of inner ear disorders. However, the examination of blood flow and its control mechanisms in the vestibular system has been very limited. Combining stereological techniques with the microsphere injection technique, capillary morphology and regional blood flow were determined for the rat utricular macula. Results are as follows: total utricular blood flow 0.158 +/- 0.078 microL/min; blood flow to the neuroepithelium (excluding nerve) 0.0995 +/- 0.046 microL/min; blood flow per unit volume 7.71 +/- 4.31 microL/min per cubic millimeter, neuroepithelial volume 0.01344 +/- 0.0018 mm3; absolute capillary surface area 0.159 +/- 0.039 mm2; mean capillary diameter 5.84 +/- 0.56 microns; absolute capillary length 8.45 +/- 1.6 mm; and capillary lumen volume fraction 0.0175 +/- 0.004. Comparisons to previous data for the posterior canal ampulla indicate that the capillary diameter in the rat utricular macula is smaller; the capillary length is greater; and the end organs are similar with respect to neuroepithelial volume, capillary surface area, and blood flow. The size of the microsphere used in the present study (9.21 microns), in comparison to the mean capillary diameter (5.84 microns) of the utricular neuroepithelium, would indicate that the blood flow data likely represent a minimum value. These findings indirectly indicate that the utricular macula metabolic rate is greater than that of the posterior canal crista, and that there is variation from end organ to end organ in mean capillary diameter.