## 4 Discussion  
This morphological approach to the study of normal and pathological dentin demonstrates the effectiveness of correlative FEI-SEM/TEM analysis. We have established, at an ultrastructural level, the presence of altered dentin and enamel organization in OI patients. Hence, both null hypotheses can be rejected. Dentinogenesis is a highly regulated developmental process that occurs both prenatally and postnatally, during which a layer of predentin is always present between the odontoblasts and the mineralized front [17]. As odontoblasts migrate towards the pulp, they leave behind an extension referred to as the odontoblastic process, which then penetrates the predentin. At this stage, predentin is not mineralized and is composed of a collagen network, which gradually becomes mineralized as the extension of the odontoblastic process results in protein secretion necessary for the mineralization process [18]. With mutations in the genes encoding collagen, dentin defects may occur [13]. The primary dentition is usually more severely affected than permanent teeth, which can be explained by the faster formation of the primary teeth and the fact they are formed in the fetal period during which high collagen expression is present, while the permanent teeth are formed entirely after birth [19]. In some cases, dental changes are the most visible clinical manifestation of OI, and early diagnosis and understanding of the ultrastructural changes found in these teeth are essential for maintaining or improving the oral health of OI patients and their quality of life [20]. The present study demonstrated irregular enamel prisms and unpacked hydroxyapatite crystals. Similar to our findings, [PERSON] et al. reported more irregularly mineralized enamel in patients with OI. An almost nonprismatic layer was observed within the superficial region of the enamel, and more irregular arrangement of the prism pattern was present close to the dentin-enamel junction [16]. Contrary to our study results, [PERSON] et al. reported no difference between OI-affected and healthy enamel [15]. In a more recent study by [PERSON] et al., optical microscopy and confocal laser-scanning microscopy (CLSM) showed normal enamel with regular prismatic structure in OI patients [11]. One possible explanation of the different findings in our study may lie in the facts that children who are more severely affected by OI often have a more severe dental phenotype and that enamel malformations can result from alterations in a gene that is not known or considered to have a role in enamel formation [12; 21]. The range of enamel malformations observed in our SEM analysis is probably compatible with enamel hypomaturation, which may explain the susceptibility of teeth affected by OI-related DGI to severe attrition, or even fractures. The histological evaluation of dentin in patients affected with OI has been of interest, and many methods have been employed in investigating its structure [11; 15; 22; 23]. Regardless of the type of OI (I, III, or IV), distinct pathological changes in dentin have been observed and reported in the literature [11]. The results of our study are in agreement with previous findings and support the observations of irregular dentinal tubules and areas lacking dentin tubules. In previous studies, a reduction in the number of dental tubules and the presence of narrow-diameter and short tubules which did not penetrate through the whole thickness of the dentine were reported [15]. Aberrant dentin with randomly oriented collagen fibers with areas of occluded tubules was also seen using CLSM [11]. The level of ultrastructural dental changes observed in our study is likely due to collagen defects in patients with OI and the dysfunction of odontoblasts [14]. Interestingly, the combined FEI-SEM/TEM approach used in this study allowed us to notice areas of necrosis in some of the dentinal tubules, which, to the best of our knowledge, has not been reported before.