Morphofunctional compensation of masseter muscles in unilateral posterior crossbite patients

Abstract

Unilateral posterior crossbite is a widespread, asymmetric malocclusion characterized by an inverse relationship of the upper and lower buccal dental cusps, in the molar and premolar regions, on one side only of the dental arch. Patients with unilateral posterior crossbite exhibit an altered chewing cycles and the crossbite side masseter results to be less active with respect to the contralateral one. Few studies about morphological features of masticatory muscle in malocclusion disorders exist and most of these have been performed on animal models. The aim of the present study was to evaluate morphological and protein expression characteristics of masseter muscles in patients affected by unilateral posterior crossbite, by histological and immunofluorescence techniques. We have used antibody against PAX-7, marker of satellite cells, and against α-, β-, γ-, δ-, ε- and ζ-sarcoglycans which are transmembrane glycoproteins involved in sarcolemma stabilization. By statistical analysis we have evaluated differences in amount of myonucley between contralateral and ipsilateral side. Results have shown: i) altered fibers morphology and atrophy of ipsilateral muscle if compared to the contralateral one; ii) higher number of myonuclei and PAX-7 positive cells in contralateral side than ipsilateral one; iii) higher pattern of fluorescence for all tested sarcoglycans in contralateral side than ipsilateral one. Results show that in unilateral posterior crossbite hypertrophic response of contralateral masseter and atrophic events in ipsilateral masseter take place; by that, in unilateral posterior crossbite malocclusion masticatory muscles modify their morphology depending on the function. That could be relevant in understanding and healing of malocclusion disorders; in fact, the altered balance about structure and function between ipsilateral and contralateral muscles could, long-term, lead and/ or worsen skeletal asymmetries.

Figure 1.

Compound panel of hematoxylin-eosin images of contralateral (A, B) and ipsilateral (C, D) masseter muscles. A) Low magnification of contralateral muscle showing normal and rectilinear fibers morphology. B) High magnification of contralateral muscle showing high number of nuclei within the fibers and along sarcolemma. C) Low magnification of ispilateral muscle showing non-healthy convolute fibers morphology, some atrophic fibers (arrowhead). D) Image of ipsilateral muscle showing the loss of contractile elements within the fiber (asterisk).

Figure 2.

Compound panel of three images of immunofluorescence reactions performed in contralateral masseter muscle. A) Longitudinal section of masseter where it is possible to observe rectilinear morphology of fibers and the presence of high number of satellite cells, PAX-7 positive (red channel) which are located between sarcolemma and basal lamina; nuclei are evidenced by DAPI (blue channel); it is also possible to observe the presence of some PAX-7 positive cells within the fibers (arrowhead). B) Transversal section of masseter muscle: presence of high number of PAX-7 positive cells around sarcolemma (red channel). C) Image of single localization reaction for alpha-sarcoglycan: the protein has been detected uniformly along the fibers (red channel) and also in the satellite cells (arrowhead). Bars: 20 μm

Figure 3.

Compound panel of three images of immunofluorescence reactions performed in ipsilateral masseter muscle. A) Longitudinal section of masseter where it is possible to observe an altered, non-rectilinear morphology of fibers and the presence of a lower number of satellite cells PAX-7 positive (red channel) along sarcolemma; nuclei are evidenced by DAPI (blue channel). B) Transversal section of masseter muscle: presence of a very low number of PAX-7 positive cells around sarcolemma (red channel). C) Image of single localization reaction for alpha-sarcoglycan: the protein has not been detected uniformly along the fibers and its fluorescence pattern is almost absent (red channel); satellite cells were positive for alpha-sarcoglycan. Bars: 20 μm

Figure 4.

The graphic based on the results obtained by a count of myonuclei within 200 fibers both ipsilateral and contralateral muscle revealed an higher number of myonuclei in contralateral side than ipsilateral one.