Dental Epithelial Stem Cells Research Articles

In vitro differentiation of dental epithelial progenitor cells through epithelial–mesenchymal interactions

In developing teeth, dental epithelial progenitor cells differentiate through sequential and reciprocal interactions with neural-crest-derived mesenchyme. However, the molecular mechanisms involved in cell differentiation are not well understood. Continuously growing teeth are useful in the study of differentiation of dental progenitor cells. In rat lower incisors, ameloblasts originate from the dental epithelial adult stem cell compartment referred to as the 'apical bud'. To elucidate the mechanism of ameloblast differentiation, we designed a primary culture system and confirmed the differentiation of dental epithelial cells through interaction with mesenchymal cells. Cytokeratin was used as a marker for epithelial cells, nerve growth factor receptor p75 for inner enamel epithelial (IEE) cells, and ameloblastin for ameloblasts. The apical bud cells could only differentiate into IEE cells and, within 10 days, into ameloblasts expressing ameloblastin in the presence of dental papilla cells. Interestingly, the IEE cells could proliferate transiently and differentiate into ameloblasts in the presence or absence of dental papilla cells. These results suggest that apical bud cells can enter the ameloblast cell lineage through interaction with mesenchymal cells. IEE cells, on the other hand, are already committed to differentiate into ameloblasts. This culture system is useful in future studies of ameloblast differentiation.

New perspectives on tooth development and the dental stem cell niche.

Adult stem cells have the capacity to self-renew and differentiate along multiple lineages in addition to contributing to ongoing tissue maintenance and regeneration after injury. They reside in specific locations called stem cell niches. In biology of the tooth, the discovery of dental epithelial stem cells in continuously growing teeth has been a recent breakthrough. The niche for the adult stem cells of these teeth is formed at the region of the apical end in tooth development. The region possesses a commonly specialized histological structure for the maintenance of adult stem cells and the production of various progenitor cells producing dental tissues. The molecular signals regulating the maintenance and cell fate decision of adult stem cells, such as Notch1, Lunatic fringe, fibroblast growth factor (FGF)-10, are expressed in the epithelial structure and the surrounding mesenchyme. Based on histological and molecular biological studies, we propose a new concept that the eternal tooth buds producing various dental progeny are formed at the apical end in the development of continuously growing teeth, and coin a new term of "apical bud" for indicating this specialized epithelial structure. Furthermore, the relationship between signaling centers and the expression of FGF-10 mRNA as the determinant of morphogenesis is discussed with an emphasis on tooth and limb development, taking note that the expression pattern of FGF-10 is an important key for understanding the mechanisms for the diversity of cusp patterns and between continuous and limited growth.

Cadmium adaptation in the lung – a double-edged sword?

This review article discusses the major cellular and molecular responses characterizing pulmonary adaptation to cadmium (Cd) that may ultimately contribute to Cd carcinogenesis. Hallmarks of Cd adaptation include hyperplasia and hypertrophy of type II alveolar epithelial stem cells, an inflammatory response involving polymorphonuclear leukocytes, and the increased gene and protein expression of several resistance factors. The most prominent biochemical change is associated with Cd-induced up-regulation of metallothionein, a cysteine-rich, metal-binding protein that sequesters Cd and also possesses considerable free radical scavenging ability. Increased levels of glutathione (GSH) and induction of enzymes involved with both the synthesis of GSH (γ-glutamylcysteine synthetase regulatory and catalytic subunits) and its metabolism (GSH S-transferases) also constitute important components of the pulmonary adaptive response. Enhancement of several important cellular defense systems in response to Cd exposure may, at first, appear to be beneficial. However, recent evidence suggests that the Cd-adaptive phenotype could have deleterious consequences and may represent a double-edged sword. It has been discovered that Cd-adapted alveolar epithelial cells have a reduced ability to repair DNA damage due, in part, to the inhibition of two base excision repair enzymes (8-oxoguanine-DNA glycosylase and endonuclease III). Cells with genetic aberrations resulting from unrepaired DNA lesions would normally be removed from the lung by apoptosis. However, another study has demonstrated that apoptotic cell death, following an oxidant challenge, is significantly attenuated in Cd-adapted cells compared to non-adapted counterparts. Suppressed apoptosis could leave pre-neoplastic or neoplastic cells alive, favor their clonal expansion, and ultimately promote tumor development. The presence of superior antioxidant defenses would also be expected to increase the resistance of these tumors to chemotherapeutic agents.

Localization of putative stem cells in dental epithelium and their association with Notch and FGF signaling.

The continuously growing mouse incisor is an excellent model to analyze the mechanisms for stem cell lineage. We designed an organ culture method for the apical end of the incisor and analyzed the epithelial cell lineage by 5-bromo-2'-deoxyuridine and DiI labeling. Our results indicate that stem cells reside in the cervical loop epithelium consisting of a central core of stellate reticulum cells surrounded by a layer of basal epithelial cells, and that they give rise to transit-amplifying progeny differentiating into enamel forming ameloblasts. We identified slowly dividing cells among the Notch1-expressing stellate reticulum cells in specific locations near the basal epithelial cells expressing lunatic fringe, a secretory molecule modulating Notch signaling. It is known from tissue recombination studies that in the mouse incisor the mesenchyme regulates the continuous growth of epithelium. Expression of Fgf-3 and Fgf-10 were restricted to the mesenchyme underlying the basal epithelial cells and the transit-amplifying cells expressing their receptors Fgfr1b and Fgfr2b. When FGF-10 protein was applied with beads on the cultured cervical loop epithelium it stimulated cell proliferation as well as expression of lunatic fringe. We present a model in which FGF signaling from the mesenchyme regulates the Notch pathway in dental epithelial stem cells via stimulation of lunatic fringe expression and, thereby, has a central role in coupling the mitogenesis and fate decision of stem cells.