Early effects of X-ray irradiation on rat incisor periodontal ligament morphology and MMP-2 expression

Radiotherapy is an effective treatment for head and neck cancer; however, it also induces alterations in healthy oral tissues. Although late radiation-induced damage has been extensively investigated, the early progression of morphological and extracellular matrix changes in the periodontal ligament (PDL) remains poorly understood. This study investigated early radiation-induced changes in collagen organisation, the type I-to-type III collagen ratio, and matrix metalloproteinase-2 (MMP-2) expression in the lingual PDL of rat incisors. Adult Wistar rats were allocated to four irradiated groups (n = 5 per group) and exposed to a single 15 Gy X-ray dose directed to the head. Animals were euthanized at 4, 9, 14, or 25 days after irradiation. A non-irradiated control group was euthanized on day 4. Left hemimandibles were fixed, decalcified, embedded in paraffin, and sectioned. Histological analyses were performed using hematoxylin and eosin, Masson's trichrome, and Picrosirius Red staining under polarized light. MMP-2 expression was evaluated by immunohistochemistry and quantified by grayscale pixel intensity analysis. Irradiation induced progressive alterations in the lingual periodontal ligament. Fibroblast nuclei exhibited morphological features consistent with cellular injury beginning on day 4, with increasing changes at subsequent time points. These changes were accompanied by progressive disorganisation of collagen bundles, increased interfibrillar spaces, and loss of normal extracellular matrix architecture. Picrosirius Red analysis demonstrated a reduction in the type I-to-type III collagen ratio on days 4, 9, and 14 after irradiation, supporting the morphological changes observed with H.E and Masson's trichrome staining. MMP-2 expression increased significantly on day 4, reached its highest level on day 9, and then progressively decreased on days 14 and 25. A single dose of X-ray irradiation induces early morphological and extracellular matrix alterations in the rat incisor periodontal ligament. The temporal association between increased MMP-2 expression, collagen fibre disorganisation, and a decreased type I-to-type III collagen ratio suggests that MMP-2 is associated with the early extracellular matrix remodelling observed on days 4 and 9 after irradiation. The subsequent decline in MMP-2 expression was accompanied by an apparent partial restoration of collagen fibre organisation on days 14 and 25.