Effect of unloading history on the proliferation in PDL cells

Objectives:Occlusal hypofunction has been reported to induce atrophic changes in the periodontal ligament (PDL). The unloading condition is likely detected by PDL cells and affects cell proliferation and production of several extracellular matrix proteins and growth factors. However, it is not well known whether the effects of past mechanical unloading history are sustained. Some papers reported that the past mechanical loading history was retained as a memory in bone marrow cells and muscle cells. In the present study, to determine the effect of past mechanical unloading experience in PDL, cell proliferation of PDL was analyzed after the certain time of unloading and subsequent rest period.

Methods:To generate the unloading condition in molars, metal bite plates and caps were attached to the upper and lower incisors of 6-weeks-old male wistar rats. After 3 or 7 days unloading, upper and lower molars (M1 and M2) were extracted from 3 rats and PDL cells were isolated with serial collagenase/trypsin digestion (3 samples each). First fraction (up to 50 min) was used for the analysis to minimize the contamination of cementoblasts. The cells were cultured in alfa-MEM with 10% FBS and 1% antibiotics for 4 days, following 4 X 104 cells were seeded to 35 mm. After 3 days of culture, cells were subjected to proliferation assay with 5-Bromo-2'-deoxy-uridine (BrdU) labeling. The data were analyzed by logistic regression.

Results:Rest period of 7 days after the hypofunction, BrdU positive cell ratio was significantly lower than that of controls, 3.3% and 3.9%, 3 and 7 days respectively, while control showed 9.6% (p<0.01).

Conclusion:The results clearly showed that the PDL cells retain the previous mechanical unloading history at least 7 days in vitro. It is suggested that the optimal occlusal loading is crucial for periodontal ligament tissue maintenance.