## 1 Introduction  
Type I collagen is the most abundant collagen of the human body and constitutes 85-90% of the dentin organic matrix. It possesses a triple helix containing two alpha 1 and one alpha 2 polypeptides, which are expressed from _COLIA1_ and _COLIA2_[1]. Mutations in these genes cause osteogenesis imperfecta (OI), an autosomal genetic disorder that affects connective tissues and, more specifically, the proteins involved in the organization of the extracellular matrix. The main symptoms of this disease are attributable to fragile bones, blue sclerae, hearing loss, scoliosis, and dentinogenesis imperfecta (DGI) [2]. OI is usually classified on the basis of clinical and radiologic criteria into four types, each further subdivided based on the presence or absence of the developmental dental defect known as DGI [3].  
DGI is a hereditary dentin disorder characterized by abnormal dentin structure affecting either the primary or both the primary and secondary dentition. DGI is reported to have an incidence of 1 in 6000 to 1 in 8000 and is inherited in a simple autosomal dominant mode, with high penetrance and a low mutation rate [4]. To date, various authors have sought to recommend adequate nomenclature for DGI, but confusion still exists among clinicians dealing with this population of patients [5]. Major advances have been made in the field of molecular genetics [6], and new classification proposals for dental defects have been made based on the knowledge of molecular pathophysiology [7]. However, the most commonly used classification still remains the one proposed by [PERSON], and overall, DGI associated with OI can be regarded as a syndromic form of OI or DGI type I [8]. All other forms of DGI appear to result from mutations in the gene encoding dentin sialophosphoprotein (_DSPP_), suggesting that these conditions are allelic [9]. Table 1 shows a synthesis of proposed classifications for DGI disorders, including aetiology and clinical manifestations.  
Regardless of the classification used, the clinical features of patients with OI-associated DGI remain characteristic and include brownish teeth discoloration, crown fractures, dentin attrition, and late dentition [10]. A previous study demonstrated that there is not always a direct correlation between OI and DGI. In other words, it was found that 62.5% of children with OI displayed clinically visible dental alterations [11]. Overall, it is considered that the prevalence of DGI in OI patients ranges from 8% up to 100% [12]. The primary dentition is usually more severely affected than permanent teeth [13], and, alongside the patients' young age, this further complicates treatment procedures and their outcomes. Therefore, it is of importance to understand the structure of the enamel and dentin at the microscopic level in order to provide adequate oral health care for patients suffering from OL Previous studies [11; 14; 15] have reported pathologically altered dentin and essentially normal enamel in patients suffering from OI. However, some authors [16] did report slight changes in the structure of the enamel in DGI teeth, and these findings may awake suspicion that even the enamel in OI patients may be affected. Thus, the aim of the present study was to clarify the structural and ultrastructural alterations of enamel in the deciduous teeth of patients affected by OI by means of field-emission in-lens scanning electron microscopy (FEI-SEM) analysis and to evaluate the dentin collagen network organization by means of correlative FEI-SEM and TEM analyses. The null hypotheses of the present study were that there are no ultrastructural alterations in the (1) enamel or (2) dentin of the deciduous teeth of OI patients, when compared to deciduous teeth from healthy children.