Bone Modeling in the Otic Capsule of the Rat

Ovariectomized Rat Model and Shape Variation in the Bony Labyrinth

Postmenopausal osteoporosis is a serious concern in aging individuals, but has not been explored for its potential to alter the shape of the inner ear by way of increased remodelling in the otic capsule. The otic capsule, or bony labyrinth, is thought to experience uniquely limited remodelling after development due to high levels of osteoprotegerin. On this basis, despite the widespread remodelling that accompanies osteoporosis, we hypothesize that both the shape and volume of the semicircular canals will resist such changes. To test this hypothesis, we conducted three-dimensional geometric morphometric shape analysis on microcomputed tomographic data collected on the semicircular canals of an ovariectomized (OVX) rat model. A Procrustes ANOVA found no statistically significant differences in shape between surgery and sham groups, and morphological disparity testing likewise found no differences in shape variation. Univariate testing found no differences in semicircular volume between OVX and control groups. The range of variation in the OVX group, however, is greater than in the sham group but this difference does not reach statistical significance, perhaps because of a combination of small effect size and low sample size. This finding suggests that labyrinthine shape remains a tool for assessing phylogeny and function in the fossil record, but that it is possible that osteoporosis may be contributing to intraspecific shape variation in the bony labyrinth. This effect warrants further exploration at a microstructural level with continued focus on variables related to remodelling. This article is protected by copyright. All rights reserved.

Early life malnutrition and fluctuating asymmetry in the rat bony labyrinth

Maternal malnutrition during gestation and lactation is known to have adverse effects on offspring. We evaluate the impact of maternal diet on offspring bony labyrinth morphology. The bony labyrinth develops early and is thought to be stable to protect vital sensory organs within. For these reasons, bony labyrinth morphology has been used extensively to assess locomotion, hearing function, and phylogeny in primates and numerous other taxa. While variation related to these parameters has been documented, there is still a component of intraspecific variation that is unexplained. Although the labyrinthine developmental window is small, it may provide the opportunity for developmental instability to produce corresponding shape differences, as measured by fluctuating asymmetry (FA). We hypothesized that (a) offspring with poor maternal diet would exhibit increased FA, but (b) no unilateral shape difference. To test these hypotheses, we used two groups of rats (Rattus norvegicus; Crl:WI[Han] strain), one control group and one group exposed to a isocaloric, protein-restricted maternal diet during gestation and suckling. Individuals were sampled at weaning, sexual maturity, and old age. A Procrustes analysis of variance identified statistically significant FA in all diet-age subgroups. No differences in level of FA were identified among the subgroups, rejecting our first hypothesis. A principal components analysis identified no unilateral shape differences, supporting our second hypothesis. These results indicate that bony labyrinth morphology is remarkably stable and likely protected from a poor maternal diet during development. In light of this result, other factors must be explored to explain intraspecific variation in labyrinthine shape.

Shape-shift: semicircular canal morphology responds to selective breeding for increased locomotor activity

Variation in semicircular canal morphology correlates with locomotor agility among species of mammals. An experimental evolutionary mouse model was used to test the hypotheses that semicircular canal morphology (1) evolves in response to selective breeding for increased locomotor activity, (2) exhibits phenotypic plasticity in response to early-onset chronic exercise, and (3) is unique in individuals possessing the minimuscle phenotype. We examined responses in canal morphology to prolonged wheel access and selection in laboratory mice from four replicate lines bred for high voluntary wheel-running (HR) and four nonselected control (C) lines. Linear measurements and a suite of 3D landmarks were obtained from 3D reconstructions of μCT-scanned mouse crania (μCT is microcomputed tomography). Body mass was smaller in HR than C mice and was a significant predictor of both radius of curvature and 3D canal shape. Controlling for body mass, radius of curvature did not differ statistically between HR and C mice, but semicircular canal shape did. Neither chronic wheel access nor minimuscle affected radius of curvature or canal shape These findings suggest that semicircular canal morphology is responsive to evolutionary changes in locomotor behavior, but the pattern of response is potentially different in small- versus large-bodied species.

Adaptive bone modeling and remodeling in acute otitis media caused by non-typeable or type B Haemophilus influenzae or Moraxella catarrhalis

Experimental studies have shown that acute otitis media caused by Streptococcus pneumoniae alters modeling dynamics in bone tissue structures surrounding the middle ear cavity. Initial resorption of bone is followed by formative activity, seen as massive osteoneogenesis. However, neither resorptive nor formative activity occurs in the otic capsule, supporting the existence of a perilymphatic zone of specialized bone. This study investigates adaptive bone modeling in acute otitis media caused by other bacteria frequently encountered in this disease. Seventy-five rats were inoculated with either non-typeable or type b Haemophilus influenzae, or Moraxella catarrhalis (25 rats in each group). Five rats from each group were sacrificed on days 4, 8, 16, 60 and 180 post-inoculation. Qualitative as well as quantitative histopathology revealed increasing apposition of new bone on both sides of the original bony wall of the middle ear bulla, i.e. at the inner and outer periosteum. Remodeling activity was seen on later days of sacrifice, as typical osteone (Haversian system) formation. Measured bone thickness in four anatomically well-defined localities progressed to a peak 2 months post-inoculation, followed by some degree of normalization. However, bone thickness was still massively increased 6 months after the acute incident. Except in the otic capsule, resorptive and formative activity was found in all bone tissue structures surrounding the middle ear cavity. These findings were irrespective of the type of inoculated bacteria. However, non-typeable or type b Haemophilus influenzae induces significantly more new bone formation than Moraxella catarrhalis. We conclude that acute otitis media caused by either of the bacteria is accompanied by massive and progressive net osteoneogenesis, already evident on day 4 and peaking 2 months post-inoculation, followed by some degree of normalization. Non-typeable and type b Haemophilus influenzae induce more new bone formation than Moraxella catarrhalis, whereas other features of bone histomorphology were equivalent. The present findings further support the existence of a perilymphatic zone of specialized bone.

Bone modeling dynamics in acute otitis media

OBJECTIVE

A number of middle ear diseases are associated with pathologic bone modeling, either formative or resorptive. As such, the pathogenesis of a sclerotic mastoid has been controversial for decades. Experimental studies on acute middle ear infection have shown varying degrees of both osteoresorption and osteoneogenesis. This study presents data on the dynamics of bone modeling in a rat model of acute pneumococcal otitis media, studied longitudinally from day 1 through 6 months after inoculation.

RESULTS

Qualitative, as well as quantitative histopathology revealed initial osteoresorption, followed by increasing apposition of new bone in the middle ear cavity, initiated at the outer periosteum. Measured bone thickness in four anatomically distinct locations peaked 3 months after inoculation, followed by some degree of normalization. However, bone thickness was still massively increased 6 months after the acute incident. Except in perilymphatic spaces of the otic capsule, resorptive and formative activity were found in all bone tissue structures surrounding the middle ear cavity, including the bony external auditory canal and the ossicles.

CONCLUSION

These findings may support the existence of a perilymphatic barrier of specialized bone and suggest that even a single episode of acute infection may alter properties of ossicular chain conduction. The authors conclude that acute otitis media is accompanied by massive and progressing net osteoneogenesis, already evident at 3 days and peaking 3 months after inoculation, followed by some degree of normalization. This is conceivably in support of the environmental theory of mastoid pneumatization, claiming inflammatory disease as the cause of a sclerotic mastoid.

Inhibition of bone resorption and stimulation of formation by mechanical loading of the modeling rat ulna in vivo

During normal growth of the rat ulna, bone is resorbed from the medial periosteal surface. This occurs as part of the modeling process by which the bone achieves its adult shape. By attaching strain gauges to the ulnae of rats in vivo, we measured the strains imposed on that surface of the bone during normal locomotion. We then applied mechanical loads to the ulnae of other rats in vivo for 6 consecutive days, inducing strains approximately double those we had measured. Fluorochromes were given on the 1st and 5th days. The histology of the medial ulnar periosteal surface was correlated with the amount of fluorochrome incorporation and tartrate resistant acid phosphatase (TRAP) activity in serial sections. In the nonloaded ulnae, the surfaces were lined with bone resorbing cells. Corresponding areas of the loaded bones were lined with osteoid and osteoblasts. There was insignificant label incorporation in the nonloaded bones but almost continuous label incorporation in the corresponding regions of the loaded bones, which was significantly different from the nonloaded bones. TRAP activity of the periosteal cells in the loaded bones was significantly less than in the nonloaded limbs. It is widely acknowledged that loading induces bone formation, and this implies that it also has the ability to inhibit resorption. However, to date there has been little direct evidence for the inhibition of resorption in vivo by mechanical loading. The changes we have observed are similar to the sequence of cellular events that occur during the reversal phase of bone remodeling, in which osteoclastic resorption ceases and osteoblasts are recruited and begin formation. This model may help increase understanding of that process.

Differential osteoclast activation in endochondral and intramembranous bone

Osteoclast activation within the temporal bone leads to hearing loss in a variety of pathologic conditions, including otosclerosis, osteogenesis imperfecta, Paget's disease of bone, and chronic otitis media. Although endochondral (EC) bone is remodeled in these diseases, it is relatively more resistant to remodeling when compared to intramembranous (IM) bone. In this study, localized bone resorption and remodeling were induced on the inner surface of the middle ear by pressurization with air to 10 mm Hg above atmospheric. Osteoclast surface increased in IM bone, but not in EC bone. This study supports the concept that there may be fundamental differences between EC and IM bone and that EC is more resistant to pressure-induced localized resorption than IM bone.

Distribution of bone remodeling units in the otic capsule of the rabbit. A semiquantitative morphometric study

Distribution of bone remodeling units (BRU) in relation to the perilymphatic space was studied in undecalcified temporal bones from adult rabbits labeled in vivo with bone-seeking fluorochromes. Based on recordings of focal bone formation, relative densities of BRUs inside concentric tissue zones around the inner ear spaces were estimated. Zonal densities of BRUs were found to decline towards the perilymphatic space, lending further support to the existence of a local inner ear mechanism in control of capsular bone tissue dynamics. The possible nature of this mechanism is considered briefly with special reference to inner ear electromechanic activity.

Remodeling patterns in the bony otic capsule of the dog

Quantum-type bone remodeling was studied in undecalcified temporal bones and long bone specimens from adult mongrel dogs by means of combined microradiography, osteofluorochrome labeling, and osteoid staining. The overall turnover rate of capsular bone was estimated to be low compared to that of extracapsular compact bone. Evolving secondary osteons were distributed centrifugally relative to the perilymphatic space. Moreover, a zone of bone tissue enclosing perilymphatic spaces was found completely devoid of evolving secondary osteons. Throughout the otic capsule, segmentary perivascular foci of resorption and formation were present. These findings indicate that remodeling of capsular bone is subject to sustained local inhibition. The significance of this conclusion is considered with special reference to otosclerosis.