## 4 Discussion  
### Tooth Development and Why Tooth Anomalies of Number, Size and Structure Are Connected  
Generally speaking, tooth development is an intricate mechanism that involves multiple factors, both genetic and environmental, and in order to fully comprehend the picture  
Figure 8: The CBCT scan of the maxilla, confirming the more severe bone recession on the left side. Figure 7: CBCT scan of the maxilla. of developmental anomalies of teeth we must look into the mechanisms and explanations that, in time, were given for these processes. In 1945, [PERSON] [32] modified Butler's Field Theory for the human dentition, which focused on the evolution and development of dentition and tried to explain different patterns of agenesis describing four morphological fields (incisors, canines, premolars and molars). The more mesial tooth in each field was proposed to be the more stable tooth and was rarely missing, while the teeth at the end of each field were more unstable and hence, were more frequently absent [33]. Indeed, most studies have shown that the most affected teeth by agenesis are the lower second premolars, the upper lateral incisors and the third molars, i.e., the most distal element of a field [21; 26; 34; 35]. [PERSON] puts forward the concept of an odontogenic homeobox to explain how different teeth are initiated in different parts of the oral cavity in response to molecular cues and the expression of specific groups of homeobox genes [36]. [PERSON] proposed that the previous models could all be incorporated into a single concept to explain dental patterning [37] and [PERSON] [21] also proposed a multifactorial model that encompasses genetic, epigenetic and environmental causes and also that there is an underlying continuous distribution of tooth and size, with thresholds determining hypodontia and supernumeraries. Hence, both hypodontia and supernumerary anomalies have to be considered, because they are interlinked [21; 38]. Going forward with his explanations, [PERSON] suggests that even if a single mutation of one gene or one major environmental insult has been identified in a patient with a dental anomaly, a careful phenotype analysis will often reveal variation between affected individuals in the same family, between dentitions in the same individual and even between different teeth in the same dentition. Thus, the same gene or environmental factor, depending on all other intricate mechanisms that happen during the dental development, will have a different phenotype expression [21]. At the same time, same or very similar clinical phenotype of whether anomalies of tooth number or structure, can arise from different aetiologies, both environmental and genetic. More than three hundred genes have been found to take part in the tooth development, while, at the same time, crucial processes need to take place between the mesenchymal and epithelial cells, and disturbances of these factors will determine the presence or absence, the size and the shape of the individual tooth [21]. Studies on mice presenting with supernumerary teeth show that these supernumeraries may develop from vestigial tooth buds that did not undergo apoptosis as they would normally have done. They also presented with an aberrant thickening of the oral epithelium [21]. As the development of number, size and structure of teeth is interconnected, so are their anomalies and therefore, they appear clinically together in the same dentition. Is Hypodontia More Frequent in Present-Day Populations and Were Supernumeraries More Frequent in the Past? The introduction of more processed foods over time, is believed to have had an impact on the size of human jaws and masticatory system, as less and less effort is required for mastication, leading to smaller jaws and possibly fewer teeth, as humans evolve [39]. Another theory hypothesizes that the last tooth of each class are \"vestigous bodies\" that became obsolete during the evolution process [33].