Planar relationships of the semicircular canals in two strains of mice

Abstract

The mouse is increasingly important as a subject of vestibular research. Although many studies have focused on the vestibular responses of mice to angular rotation, the geometry of their semicircular canals has not been described. High-voltage X-ray computed tomography was used to measure the anatomy of the semicircular canals of two strains of mice, C57Bl/6J and CBA/CaJ. The horizontal plane of a stereotaxic coordinate system was defined by the midpoints of the external auditory meati and the point where the incisors emerge from the maxilla. The centroids of the lumens of the bony canals were calculated, and planes that describe the canals were fit using a least-squares regression analysis to the resulting points. Vectors normal to each regressed plane were used to represent the corresponding canal's axis of rotation, and angles of these vectors relative to skull landmarks as well as to each other were calculated. The horizontal canal of the mouse was found to be angled anteriorly upward 17.8 degrees for CBA/CaJ and 32.6 degrees for C57Bl/6J from the reference horizontal plane. Angles between ipsilateral canals deviated up to 12.3 degrees from orthogonal, and angles between contralateral synergistic canals (left anterior-right posterior, right anterior-left posterior, and horizontal-horizontal) deviated from parallel by up to 14.8 degrees. The orientations of the canals within the head as well as the orientations of the canals relative to each other were significantly different between the two strains, suggesting that care must be taken in the design and interpretation of developmental and physiologic studies involving different mouse strains.

Fig. 1

Image of a scanned microCT slice for a mouse of strain CBA/CaJ. Canals on the animal's right side are labeled as follows: AC, anterior canal; CC, common crus; HC, horizontal canal; PC, posterior canal.

Fig. 2

Stereoscopic representations of the left labyrinth of a mouse of strain CBA/CaJ. Outlines of bony canals are shown in gray, whereas centroids of canal lumens are represented with black dots. Top row: front view with animal facing reader; middle row: lateral view with animal facing to the left; bottom row: dorsal view with animal facing to top of page (identical to views in Fig. 4). To achieve three-dimensional effect, initially, view each row at extremely close range and allow each eye to focus on one image only. Slowly lengthen viewing distance until three images are present. The center image is a three-dimensional representation of the labyrinth.

Fig. 3

Plan and elevation views of representative canal (posterior canal from labyrinth depicted in Fig. 2). The canal is depicted so that the best-fit plane to the data is parallel to the page in the plan view (upper left). Orthogonal elevation views are at right and below. Black dots: centroids of canal lumens; gray arc: curve fitted to centroids; small gray circle: center of fitted curve; dotted gray line: vector between fitted center and point giving greatest angle upward out of the canal plane; dashed gray line: vector between fitted center and point giving greatest angle downward out of the canal plane. Gap in data points reflects region where scanned slices were tangent to the curve of the canal and the centroids of the lumen could not be reliably determined, as discussed in the text.

Fig. 4

Graphical representation of normal unit vectors. Top row: skull images indicating orientation of view for each column. Middle row: population of individual canal unit vectors plotted for CBA/CaJ mice (filled symbols) and unit vectors plotted for C57Bl/6J mice (open symbols). Bottom row: average of vectors shown in middle row for CBA/CaJ mice (filled symbols) and for C57Bl/6J mice (open symbols). LHC, left horizontal canal; RHC, right horizontal canal; LAC, left anterior canal; RAC, right anterior canal; LPC, left posterior canal; RPC, right posterior canal.