Comparative Morphology of Rodent Vestibular Periphery. I. Saccular and Utricular Maculae

Abstract

Calyx afferents, a group of morphologically and physiologically distinct afferent fibers innervating the striolar region of vestibular sensory epithelia, are selectively labeled by antibodies to the calcium-binding protein calretinin. In this study, the population of calretinin-stained calyx afferents was used to delineate and quantify the striolar region in six rodent species: mouse, rat, gerbil, guinea pig, chinchilla, and tree squirrel. Morphometric studies and hair cell and calyx afferent counts were done. Numbers of hair cells, area, length, and width of the sensory epithelium increase from mouse to tree squirrel. In the mouse and rat, calretinin is found in 5-9% of all type I hair cells, 20-40% of striolar type II hair cells, and 70-80% of extrastriolar type II hair cells. Numbers of calyx afferents increase from mouse to squirrel, with more complex calyx afferents in larger species. About 10% of calyx afferents are branched. Based on our counts of total numbers of calyx afferents in chinchilla maculae and in comparison to fiber counts in the literature, the proportion of calyx afferents is greater than previously described, constituting nearly 20% of the total. Because morphometric measures increase with body weight, we obtained additional data on vestibular end organ surface areas from the literature and used this to construct a power law function describing this relationship. The function holds for species with body weights less than approximately 4 kg. Greater than 4 kg, the surface area of the sensory epithelia remains constant even with increasing body weight.

Sox2 and Jagged1 Expression in Normal and Drug-Damaged Adult Mouse Inner Ear

“…Our data do not support the idea that there are twice as many Type I HCs in the striola region compared with Type II HCs; a 2:1 ratio was proposed by earlier studies (Engström and Wersäll 1958;Lindeman 1969;Watanuki and Meyer zum Gottesberge 1971). Our ratio is also lower than the 1.5±0.02 (mean±SE) Type1/Type II ratio reported by Desai et al (2005). The inclusion of juxtastriolar cells in our analysis may account for some of the discrepancy, as Desai et al (2005) quantified the striolar region specifically, not the larger striolar/juxtastriolar region.…”

“…Our estimates of total HC density (striola, 1.4±0.3 HC/ 100 μm 2 SE [mean±SD]; extrastriolar, 1.7±0.4) agree well with those reported by Desai et al (2005) for adult CBA/J mice (striola, 1.6±0.05 HC/100 μm 2 SE [mean±SE]; extrastriolar, 1.7±0.09). Roughly equivalent numbers of Sox2-negative and Sox2-positive HCs were observed in the striolar/juxtastriolar region, a ratio of 1.1±0.2 Sox2-negative to Sox2-positive HCs.…”

“…Our ratio is also lower than the 1.5±0.02 (mean±SE) Type1/Type II ratio reported by Desai et al (2005). The inclusion of juxtastriolar cells in our analysis may account for some of the discrepancy, as Desai et al (2005) quantified the striolar region specifically, not the larger striolar/juxtastriolar region. An alternative explanation is that Sox2 labeling does not fully delineate Type I from Type II HCs.…”

“…An additional observation linking the Sox2-positive HCs to Type II HCs is that the Sox2-positive HC nuclei tend to be located slightly higher (closer to lumen) in the epithelium than the nuclei of the Sox2-negative HCs. Nuclei of Type II HCs are reported to be positioned closer to the lumen than Type I HC nuclei, which are generally located just above the SC nuclei (Desai et al 2005).…”

“…This is suggestive of a 1.5 Type I/Type II HC ratio in the extrastriolar region of the utricular macula. The two kinds of HCs have been reported to occur in nearly equal numbers in the extrastriolar regions of mammalian otolithic organs (Engström and Wersäll 1958;Lindeman 1969;Watanuki and Meyer zum Gottesberge 1971), and a Type 1/Type II ratio of 1.2±0.01 (mean±SE) was reported by Desai et al (2005).…”