Ectoderm, endoderm, and the evolution of heterodont dentitions

Abstract

Summary: Mammalian dentitions consist of different shapes/types of teeth that are positioned in different regions of the jaw (heterodont) whereas in many fish and reptiles all teeth are of similar type (homodont). The process by which heterodont dentitions have evolved in mammals is not understood. In many teleosts teeth develop in the pharynx from endoderm (endodermal teeth), whereas mammalian teeth develop from the oral ectoderm indicating that teeth can develop (and thus possibly evolve) via different mechanisms. In this article, we compare the molecular characteristics of pharyngeal/foregut endoderm with the molecular characteristics of oral ectoderm during mouse development. The expression domains of Claudin6, Hnf3b, a-fetoprotein, Rbm35a, and Sox2 in the embryonic endoderm have boundaries overlapping the molar tooth-forming region, but not the incisor region in the oral ectoderm. These results suggest that molar teeth (but not incisors) develop from epithelium that shares molecular characteristics with pharyngeal endoderm. This opens the possibility that the two different theories proposed for the evolution of teeth may both be correct. Multicuspid (eg. molars) having evolved from the externalization of endodermal teeth into the oral cavity and monocuspid (eg. incisors) having evolved from internalization of ectodermal armour odontodes of ancient fishes. The two different mechanisms of tooth development may have provided the developmental and genetic diversity on which evolution has acted to produce heterodont dentitions in mammals. genesis 48:382-389, 2010. V V C 2010 Wiley-Liss, Inc.

Expression of Sox genes in tooth development

“…Sox2 expression has been shown in tooth development (Ohazama et al ., 2010; Juuri et al ., 2012, 2013; Zhang et al ., 2012). Sox2 is expressed in tooth epithelium at the initiation stage (E10.5 and E11.5; Fig.…”

“…Members of the Sox gene family show dynamic and diverse expression patterns during development and mutation analyses in humans and mice provide evidence that they play multiple roles during development (Pevny and Lovell-Badge 1997, Hosking and Koopman 2008, Wegner 1999, Oommen et al ., 2012). Sox2 has been shown to be expressed in rodent tooth germs including the incisor cervical loops (Ohazama et al ., 2010; Juuri et al ., 2012, 2013; Zhang et al ., 2012). The expression of other members of Sox family in tooth development however remains unstudied.…”

Genetic analyses in Lake Malawi cichlids identify new roles for Fgf signaling in scale shape variation

“…refs. 58,59 ), we are largely ignorant about how such differences evolve at the genetic/genomic level. In addition, birds and mammals can exhibit dramatic variation in the distribution of feathers and hair, respectively, across the body, and some progress has been made in identifying the genetic variants that mediated such variation 60,61 .…”

Development and evolution of the unique cetacean dentition

“…Our understanding of how morphologically-distinct tooth classes and complex tooth crown shapes develop in mammals is largely derived from research on mice (Munne et al, 2010; Ohazama et al, 2010a; Ohazama et al, 2010b). The first morphological sign of tooth development begins with a thickening of oral epithelium along the maxillary and mandibular arches early in the embryonic period before individual teeth can be distinguished (Leche, 1892; Leche, 1895; Orban, 1980; Ruch et al, 1984).…”

“…The first morphological sign of tooth development begins with a thickening of oral epithelium along the maxillary and mandibular arches early in the embryonic period before individual teeth can be distinguished (Leche, 1892; Leche, 1895; Orban, 1980; Ruch et al, 1984). During this developmental stage, functional experiments on mice indicate that gene expression in the oral epithelium is crucial for controlling the shape and hence tooth class of individual teeth (Kollar & Baird, 1969; Sharpe, 1995; Tucker & Sharpe, 1999; Cobourne & Sharpe, 2003; Catón & Tucker, 2009; Ohazama et al, 2010b). Distinct gene networks are expressed in regions along the oral epithelium and induce areas to form either incisor teeth with simple, unicuspid crowns or more complex, multicuspid occlusal patterns typical of molar teeth.…”