The influence of functional pinealectomy and exogenous melatonin application on healing of a burr hole in adult rat calvaria: a histological and immunohistochemical study

Background: Even today, repair of the cranial defects still represents a significant challenge in neurosurgery and various options have been used for their reconstruction to date. However, there are very few studies investigating the effects of exogenous administration of melatonin (MEL) as an agent that promotes bone regeneration. The goal of this study was to investigate the effects of functional pinealectomy (Px) and exogenous MEL administration on the bone repair properties and surrounding connective tissue alterations in a rat calvaria model. Materials and methods: The total of 30 adult female Wistar-Albino rats was randomly divided into three groups (n = 10): control group (CO; 12 h light/12 h dark exposure), functional Px group (24 h light exposure, light-induced functional Px), and Px+MEL group (light-induced Px + MEL, 20 mg/kg/day for 12 weeks). Critical-sized burr-hole defects (diameter: 3.0 mm) were surgically created by a single operator in the calvarium of all rats, using an electric drill. Animals in Px+MEL group received MEL 20 mg/kg/day for 12 weeks. At the end of the study, bone healing and connective tissue alterations surrounding drilled defect area in the rat calvaria were determined in haematoxylin-eosin-stained and Mallory Azan slices applied in anti-bone sialoprotein. Image Pro Express 4.5 programme was used for histomorphometric calculation of areas of new bone and fibrotic tissue. Normality control was performed by Shapiro-Wilk test. Variance homogeneities were examined by Shapiro-Wilk and Levene tests; Tukey HSD test was used as a post hoc method since there was no homogeneity problem. All hypothesis tests were performed at the 0.05 significance level. Results: Histological analysis showed that the bone repair process in the Px+MEL group was similar to that of the CO group, whereas the functional Px group showed a delay. Histomorphometrically, it was found that the Px group had the largest hole diameter and the most fibrotic scar area, although no binary statis- tical significance was found between the CO and Px+MEL groups (p = 0.910).In terms of vascularisation, it was observed that the most vascular structure was found in the Px+MEL group among the scar tissue and ossification areas, while the vascularisation was the least in the Px group (p < 0.001). Conclusions: Our findings revealed that bone repair process was impaired in functional Px group, but exogenous MEL replacement was able to restore this response. Thus, it is concluded that utilisation of MEL may improve the bone repair in calvarial defects. (Folia Morphol 2022; 81, 2: 271-279)