Effects Of Soft-diet Feeding On The Cell Proliferation And Cell Apoptosis Of The Condylar Cartilage In Growing Rat

Condylar cartilage is a basic development center of mandible, and is very sensitive to the mechanical stress. As a secondary cartilage, condylar cartilage needs functional load to maintain the normal development. Appropriate masticatory force is necessary to promote the growth of condylar cartilage. The relationship between changed masticatory force and development of condylar cartilage has drawn considerable attention. Reduced masticatory force can inhibite the growth of condyle, but the mechanism of which is not clear. Both cell proliferation and cell apoptosis are important in condyle development. And Bcl-2 gene family plays a crucial role in regulation of cartilage cell apoptosis. However, the research about the change of cell proliferation, cell apoptosis and the expression of Bcl-2 gene family in the condition of low masticatory force is scarce. Therefore, the present study is desigened to investigate the relation between low masticatory force induced by powder diet feeding and cell proliferation and cell apoptosis in growing rat.Part I. Effects of soft-diet feeding on histomorphology of the mandibularcondyle in growing ratObjective: To estimate the effect of soft-diet feeding on the size of the mandibular condyle and the condylar cartilage thickness in rats soon after weaning.Methods: Forty 18 days old rats were randomly divided into 2 groups. One group was fed with the normal hard diet; the other group was fed with powder soft diet. The experimental period was 4 weeks. At 4w, 5w, 6wand 7w, the rats were weighed and sacrificed. One condyle of each rat was dissected. After HE staining,the length of condyle and the thickness of condylar cartilage were measured under the light microscope. The cartilage dissected from the other condyle was used for RT-PCR in part III. Results:1. During the experiment period, the body weight of both groups increased markedly. There was no significant difference in body weight between two groups.2. Form 4w to 7w, the length of condyle of both groups increased markedly. Compared with the hard-diet group, the length of condyle of soft-diet group was significantly smaller.3. Significant difference was found in the cartilage thickness of two groups, soft-diet group having a thinner cartilage.Conclusion: The soft-diet feeding hasn't influenced the body weight, however, it leads to decreased growth of the mandibular condyle and changes in the thickness of the cartilage.Part II. Effects of soft-diet feeding on cell proliferation and cell apoptosis ofthe condylar cartilage in growing ratObjective: To investigate the effect of soft-diet feeding on activity of cell proliferation and cell apoptosis of the condylar cartilage in growing rat.Methods: Immunohistochemical staining was used to qualitively examine the PCNA positive cell, TUNEL was chosen to detect the apoptotic cell. Results:1. PCNA positive cell mainly located in proliferative zone and hypertrophic zone. From 4w to 7w, the number of proliferating cell increased gradually in both groups. There was no significant difference between two groups.2. Apoptotic cell mainly located in the deeper proliferative zone and hypertrophic zone. During the experiment period, the number of apoptotic cell in condylar cartilage of hard-diet groups was on the same level. Howere,the number decreased markedly in soft-diet groups. At 4, 5, 6w, the number ofapoptotic cell of soft-diet groups were significantly more than the hard-dietgroups.Conclusion: The soft-diet feeding has no significant effect on proliferation of cell in cartilage. But, it leads to increased number of apoptotic cell in cartilage. It suggests that decreased masticatory force induced by soft-diet feeding promote the maturation and apoptosis of cell in cartilage.Part III. Effects of soft-diet feeding on expression of Bax and Bcl-2 in thecondylar cartilage of growing ratObjective: To observe the effect of soft-diet feeding on the expression of bax and bcl-2 of the condylar cartilage in growing rats.Methods: Immunohistochemical staining was used to examine the expression of Bax and Bcl-2 proteins in cartilage, and the expression of Bax and Bcl-2 mRNA of the whole cartilage were analyzed with RT-PCR. Results:1. The expression levels of both Bax protein and Bcl-2 protein of hard-diet group increased markly in first three weeks, and decreased at 7w. Compared with the hard-diet group, the expression of Bax protein in soft-diet group was downregulated at 6w and 7w. The intensity of staining of Bcl-2 protein in soft-diet group was lower than the hard-diet group during the experiment period. At 4w and 5w, the ratio between Bax and Bcl-2 in soft-diet group was higher than that of hard-diet group.2. From 4w to 7w after birth, the level of Bax and Bcl-2 mRNA in condylar cartilage in hard-diet group decreased gradually. The expression of Bax mRNA in soft-diet group was significantly higher than the hard-diet group at 7w, however, no significant differences were observed between soft-diet group and hard-diet group in the expression of Bcl-2 mRNA. The ratio between Bax mRNA and Bcl-2 mRNA in soft-diet group was higher than thatof hard-diet group at 6w and 7w.Conclusion: The results indicate that Bax and Bcl-2 take part in regulating the cell apoptosis of condylar cartilage under soft-diet feeding.