Epithelial Dlx-2 Homeogene Expression and Cementogenesis

Abstract

SUMMARYThe Dlx-2 (distal-less gene) homeoprotein transcription factor controls early tooth development but has not been studied during the late stages of biomineralization. Transgenic mice containing a Dlx-2/LacZ reporter construct were used to map the Dlx-2 expression pattern in cementoblasts, the dental cells most closely related to bone cells and therefore suggested to be uniquely positioned osteoblasts. During initial root formation, marked expression of Dlx-2 was evident in molar and incisor root epithelium, whereas dental papilla and follicle were negative. Dlx-2 was expressed in this epithelium from the apical loop to the area of its disruption. During acellular cementum formation in both incisors and molars, Dlx-2 expression was observed in the majority of differentiated cementoblasts from the apical region to the erupting zones. During cellular cementum formation, the presence of which characterizes growth-limited molars, Dlx-2 expression was restricted to the innermost cementoblasts and entrapped cementocytes. These data further support the hypothesis of a complex origin and fate of cementum-forming cells, as previously suggested by the expression patterns of a set of mesenchymal and epithelial markers, notably ameloblastin as shown here. Dlx-2 expression might constitute a landmark of cementoblast subpopulations of epithelial origin.

Role of Hertwig's epithelial root sheath cells in tooth root development

“…The HERS cells were subcloned by stringent selection (culturing in different growth medium favoring epithelial over mesenchymal growth). At present, this method is the only available technique to purify this subpopulation of cells as there are no definitive markers of HERS cells with the exception of ameloblastin and the transcription factor Dlx-2, which marks early HERS cells (Lezot et al, 2000). However, we are very confident that HERS cells were selected and the purity and identity of these cells was initially established by morphologic and biochemical criteria.…”

“…However, recent studies suggest that not all HERS cells migrate away; a few of them undergo apoptosis, and some of them remain on the root surface (Kaneko et al, 1999). Furthermore, previous analysis of the expression of Dlx-2 (using a lac Z reporter) and ameloblastin (using antibodies) indicate that some HERS cell express both markers, whereas others express only one, suggesting that there may be more than one cell type present in this tissue (Lezot et al, 2000).…”

Hertwig's epithelial root sheath cells do not transform into cementoblasts in rat molar cementogenesis

“…There have been several in vivo (Webb et al, 1996;Bosshardt andNanci, 1998, 2004;Lésot et al, 2000) and in vitro experiments (Thomas, 1995;Zeichner-David et al, 2003;Sonoyama et al, 2007) that support the epithelial-mesenchymal transformation of the epithelial sheath cells. Webb et al (1996) Epithelial sheath cells were reported to surround collagen fibril bundles in cytoplasmic concave structures and to develop intracellular organelles for protein synthesis and secretion in rat molars and porcine teeth (Bosshardt and Nanci, 2004).…”

“…However, the epithelial cells in question could change shape by thickening of fibril bundles and synthesize protein(s) other than cementum matrix proteins (discussed later). Lésot et al (2000) examined the expression of Dlx-2 (Drosophia distal-less homeobox gene) in root formation in transgenic mice containing a Dlx-2/LacZ reporter construct. During the initial root formation in incisors and molars, the epithelial root sheath expressed Dlx-2, whereas dental follicle cells did not.…”

“…These findings led Lésot et al (2000) to suggest cementoblast subpopulations of epithelial origin. However, this opinion could be caused by a misidentification of epithelial cells remaining on the root as cementoblasts and of embedded epithelial cells as cementocytes.…”