## 3 Results
### US Exposure Protected Chondrocytes from Ultrastructual Changes Induced by OS  
TEM images from each HCH group were examined to assess the changes in chondrocyte ultrastructure. In the control group chondrocytes had a normal aspect of the nucleus and cytoplasm. Nuclei presented a polymorphic aspect, with a prevalence of euchromatin and with a peripheral ring of heterochromatin. A small amount of endoplasmic reticulum; many \begin{table}
\begin{tabular}{c c c c} \hline \hline
**OS Markers** & **OS Group** & **US-OS Group** & **CG** \\  & **(Mean \(\pm\) SD)** & **(Mean \(\pm\) SD)** & **(Mean \(\pm\) SD)** \\ \hline MDA (nmoL/mg prot) & 11.9 \(\pm\) 1.2 & 5.0 \(\pm\) 0.8 & 7.2 \(\pm\) 0.6 \\ \hline TNF (nmoL/mg prot) & 13.7 \(\pm\) 1.1 & 6.5 \(\pm\) 0.8 & 5.7 \(\pm\) 0.7 \\ \hline \hline \end{tabular}  
* OS—oxidative stress, MDA—malonidiladdehyde, TNF-\(\alpha\)—tumour necrosis factor –\(\alpha\), OS group—oxidative stress group, US-OS group—ultrasounds- oxidative stress group, CG—control group, SD—standard deviation.  
\end{table} 
Table 1: Oxidative stress markers in the three groups. Figure 4: TNF-\(\alpha\) level in control group, OS group, and US-OS group. TNF-\(\alpha\) level increased in OS group compared to control group (\(p\) < 0.05). In US-OS group TNF-\(\alpha\) level decreased compared to OS group (\(p\) < 0.05). (TNF-\(\alpha\)—tumour necrosis factor–\(\alpha\), OS group—oxidative stress group, US-OS group—ultrasounds- oxidative stress group, CG—control group). Figure 3: MDA level in control group, OS group, and US-OS group. MDA level increased in OS group compared to control group (\(p\) < 0.05). In the US-OS group the MDA level decreased compared to the OS group (\(p\) < 0.05). (MDA—malonidiladdehyde, OS group—oxidative stress group, US-OS group—ultrasounds- oxidative stress group, CG—control group). glycogen granules, sometimes crowded in large areas; and secretory vesicles were observed in cytoplasm (Figure 5). In the OS group, we identified two different types of chondrocyte response to OS induced by H\({}_{2}\)O\({}_{2}\) (Figure 6). As a first type of response, in cells with otherwise a quite normal aspect, we identified many electron-transparent vacuoles, as well as a large amount of expanded rough endoplasmic reticulum (RER). Mitochondria displayed a more condensed matrix. Glycogen was present in all cells, but its amount varied in different cells. Some chondrocytes had an excessive amount of fibrillar material, which occupied large areas of the cytoplasm. The fibrillar material was more compact and electron-dense. Lipid droplets appeared in some cells. Nuclei were unchanged as compared to control group. As a second type of chondrocyte response to OS, the same chondrocytes displayed a high degree of vacuolation and even signs characteristic for cell death by necrosis. In this group we identified numerous destroyed cells. In the remaining cells, there were many vacuoles and disorganized organelles in the cytoplasm. Nuclei were ballonized with lysatesd chromatin. In the US-OS group the analysed chondrocytes exposed to US prior to H\({}_{2}\)O\({}_{2}\) exposure were able to conserve their ultrastructure and viability under OS conditions. In this group we did not identify necrotic chondrocytes. In the US-OS group chondrocytes ultrastructure was only slightly changed compared to that in the control group. In this group nuclei had a normal appearance, shape, and chromatin aspect. Mitochondria had a normal matrix, without condensation and a normal aspect of cristae. In the US-OS group the cytoplasm contained a large number of vacuoles, but less compared to the OS group. Rough endoplasmic reticulum was dilated in many cells from the US-OS group, but fibrillar material was generally less than in the OS group, with perinuclear and intracytoplasmic disposition. In some chondrocytes of the US-OS group lysosomes were more numerous than in the OS and control groups, and we also identified autophagosomes (Figure 7).