Development of heterodont dentition in house shrew (Suncus murinus)

Abstract

Mammalian heterodont dentition comprises incisors, canines, premolars, and molars. Although there has been intensive research, the patterning of these specific tooth types has not yet been elucidated. In order for the gene expression data to be linked with tooth type determination, it is first necessary to determine precisely the incisor-, canine-, premolar-, and molar-forming regions in the jaw primordia. To accomplish this, we studied dentition development in the house shrew (Suncus murinus), which has retained all the tooth types, using three-dimensional reconstructions from serial histological sections and the Sonic hedgehog (Shh) expression patterns. Before the appearance of morphological signs of odontogenesis, Shh expression localized to the presumptive tooth-forming regions, in which the mesial and distal expression domains corresponded to the incisor- and premolar-forming regions, respectively. The upper incisor region was found to extend across the boundary between the frontonasal and the maxillary processes. The canine-forming regions later appeared in the intermediate portions of the maxillary and the mandibular processes. The molar-forming regions later appeared distal to the initially demarcated tooth-forming regions by secondary extension of the distal ends. The demarcation visualized by the Shh expression pattern in the jaw primordia of the house shrew probably represents the basic developmental pattern of mammalian heterodont dentition.

The role of the dental lamina in mammalian tooth replacement

“…Our observations indicate that the ferret M1 is not initiated from the primary dental lamina, but rather from the deep posterior dental lamina. Similar observations have been made in the shrew (Yamanaka et al, 2007;Järvinen et al, unpublished). These results suggest that molars may be initiated from the same placode as the deciduous premolars, and that premolars and molars may thus belong to the same tooth family and to be initiated from the same field, as commonly assumed (Butler,'39).…”

Morphological variation of the maxillary lateral incisor

“…However, the mouse dentition is highly specialized with a long toothless diastema region instead of canine and premolar teeth, so some care is needed when translating findings to humans. Yamanaka et al [35] used an insectivora (house shrew, Suncus murinus) as a model for mammalian heterodonty because it displays all tooth classes, and they showed that Sonic hedgehog expression localized to the presumptive tooth-forming regions for each tooth class.…”

Evolution and development of the mammalian dentition: Insights from the marsupial Monodelphis domestica

“…Shh and Fgf8 are expressed in the dental lamina in mouse (Bitgood and McMahon, 1995;Kettunen and Thesleff, 1998). Shh is also expressed in the dental lamina of snakes (Buchtova et al, 2008), shrews (Yamanaka et al, 2007), and catsharks (Smith et al, 2009), suggesting a strong conservation at this early stage of tooth development and a role in the generation of replacement teeth.…”

“…Sonic hedgehog (Shh) expression in the EK of mammalian tooth germs has been documented in placental mammals, including mouse (Vaahtokari et al, 1996), vole (Kerä nen et al, 1998), shrew (Yamanaka et al, 2007), and ferret (Jä rvinen et al, 2009). Evidence from wild-type and mutant mice suggests that a functional EK is induced/maintained by signaling between epithelium and mesenchyme by means of fibroblast growth factors (FGFs; Kettunen et al, 2000) and that this signaling is modulated by negative feedback regulators of FGF and other receptor tyrosine kinase (RTK) signaling, encoded by Sprouty genes (Klein et al, 2006).…”