There is a debate about the high incidence of histological change in dental follicles associated with an impacted third molar. This argument extends to include not only a long-standing symptomatic third molar but also the left or overlooked follicle after mandibular third molar odontectomy. The present study investigated the expression of cell cycle regulatory proteins in epithelial components of dental follicles associated with partial bony/soft-tissue impacted and full bony impacted third molars in order to find an answer for the hypothesis that the position of the impacted tooth may affect the cellular activities of the surrounding dental follicular tissues. The present research has selected three trustworthy immunohistochemical markers to achieve a reliable answer to this suggestion 14–17.
The choice of the markers used in this study, PCNA, cyclin D1, and p21, was based on their roles in regulating cellular proliferation, cell cycle progression, and cell kinetics. Cell proliferation plays an important role in several biological and pathological events. Cellular detection of PCNA is considered a reliable marker of cell proliferation. Cyclin D1 stimulates the cell cycle. It controls cell cycle transit from G1 to S phase by binding Cdks 4 and 6 and is thus considered a biological marker to predict cell cycle progression. p21 is the product of a suppressor gene that acts as an inhibitor of complexes formed by cyclins D, E, and A and their respective Cdks and thus functions as a regulator of cell cycle progression at G1 18–22.
In the present study, PCNA-positive cells were detected in the reduced epithelium of the dental follicles and in the basal/suprabasal cell layers of the stratified squamous epithelium. Although a statistically significant difference was not detected between the two groups, the high expression levels of PCNA in the epithelial components in both groups suggested a high proliferation rate in the dental follicular tissues. These results of high PCNA expression are consistent with those of Cabbar et al.4, who investigated the proliferative potential of dental follicles of asymptomatic impacted third molar teeth using Ki-67 and mini-chromosome maintenance protein 2 proliferation markers. They indicated that the odontogenic epithelium in follicular tissues of asymptomatic impacted third molars proliferates actively and may be an indicator of the differentiation potential of dental follicles. On the basis of these observations, they strongly supported the prophylactic removal of impacted third molars.
Expression of p21 was greater in group 2 than in group 1. It appears that the increased p21 expression may be a compensatory mechanism to the high PCNA expression and it represents a cellular attempt to balance the high cellular proliferation rate. Previous studies examining the expression of the p21 protein in tooth germs and follicular tissues showed different expression levels during distinct developmental stages 23,24. Kumamoto et al. 25 reported that most odontogenic epithelial cells of human tooth germs at the initial stage of crown mineralization were reactive for the p21 protein. They suggested that the cellular differentiation of the odontogenic epithelium correlates with p21 expression. On the basis of their findings, it may also be concluded in this study that the increased p21 expression was related to the high degree of differentiation of the epithelial components in the dental follicular tissues.
The current study revealed that most of the dental follicular tissues exhibited decreased immunohistochemical staining for cyclin D1 as defined by the absence or decreased intensity of staining in the epithelial cells. In fact, these results correlate well with the p21 results, wherein it is expected that the level of cyclin D1 would decrease as the level of p21 expression increases. In addition, no significant difference in cyclin D1 expression between the two groups was identified. Although one might expect a higher cyclin D1 expression similar to the high PCNA expression, this was not the case. The absence of cyclin D1 in mitotic cells is probably due to the inactivation of this protein at the end of S phase and due to its short half-life 20,26.
This study showed that there was high PCNA expression in the epithelial components of the dental follicular tissues of both groups, confirming the highly proliferative nature of these tissues. There have been no previous studies comparing the expression of cell cycle proteins in relation to the depth of the impacted tooth. The significant difference observed in the level of p21 expression between dental follicles surrounding full bony and partial bony/soft-tissue impacted mandibular third molars suggests the possibility of differences in cell cycle regulation and cellular activities. Further studies using additional markers and techniques are needed to confirm the finding of this preliminary investigation. The obtained data will clarify the role of the impacted tooth and its relation to the degree of cellular proliferation in the overall evaluation of risk factor associated with deeply seated impacted teeth and their associated follicular tissue.
The findings of the present study have elucidated the possible deleterious behavior of cells of the dental follicle, particularly those associated with a deeply impacted mandibular third molar. Oral and Maxillofacial Surgeon should pay attention to this effect whether to monitor asymptomatic impacted mandibular third molar or during surgery to excise completely the residual follicle.
There are no conflicts of interest.
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