Oral epithelial stem cells in tissue maintenance and disease: the first steps in a long journey

Abstract

The identification and characterization of stem cells is a major focus of developmental biology and regenerative medicine. The advent of genetic inducible fate mapping techniques has made it possible to precisely label specific cell populations and to follow their progeny over time. When combined with advanced mathematical and statistical methods, stem cell division dynamics can be studied in new and exciting ways. Despite advances in a number of tissues, relatively little attention has been paid to stem cells in the oral epithelium. This review will focus on current knowledge about adult oral epithelial stem cells, paradigms in other epithelial stem cell systems that could facilitate new discoveries in this area and the potential roles of epithelial stem cells in oral disease.

Figure 1

Oral mucosa in Mus musculus. (a) Diagram of H&E-stained buccal mucosa collected from a 12-week-old C57BL/6 female mouse. In this photo the basal, spinous, granular and cornified layers are all present. Rete ridges and dermal papilla can also be identified. Unlike humans, the buccal mucosa in C57BL/6 mice is keratinized; in general, the location and type of keratinization within the oral cavity differs among mammalian species. (b) 7 µm H&E-stained sections from intraoral sites. All surfaces of the oral epithelium in C57BL/6 mice, unlike humans, appear to be keratinized. H&E, hematoxylin and eosin.

Figure 2

CRE recombinase technology. The CRE recombinase enzyme was identified in the P1 bacteriophage, where it recognizes and recombines 34 base-pair DNA sequences called loxP sites.^, LoxP sites consist of two 13 base-pair palindromic DNA sequences separated by an eight-base spacer region. When two loxP sites are oriented in the same direction on a strand of DNA, the CRE recombinase can recombine them such that the intervening DNA will be removed from the genome. Transgenic mice have been developed that harbor genes flanked by loxP sites (‘floxed' genes). When bred with mice that express a tissue specific CRE recombinase (i.e., a CRE whose expression is controlled by a specific promoter that is only active in a particular tissue), floxed gene expression can be completely abrogated in very specific cell populations. Recently, newer mouse models have been created that allow for temporal control of Cre expression. CRE recombinases fused to mutant ERs have been developed that no longer bind endogenous estrogens at physiologic levels, but instead are only activated by binding tamoxifen or its active metabolite 4-hydroxy-tamoxifen. In the absence of tamoxifen, the Cre-ER construct is sequestered in the cytoplasm (a). When tamoxifen binds the ER domain of the fusion protein, the CRE recombinase translocates to the nucleus, where it removes floxed genes from the genome. Some transgenic fluorescent reporters are constructed such that they are inhibited from being transcribed by floxed transcriptional STOP elements (aka lox-stop-lox or LSL elements). When the Cre-ER construct is activated and enters the nucleus, it can remove this STOP sequence, which will allow the fluorescent reporter to be expressed, which in this example is RFP (b). ER, estrogen receptor; RFP, red fluorescent protein.

Figure 3

The invariant asymmetry and neutral drift models. (a) The invariant asymmetry model in the interfollicular epidermis proposes that a self-renewing stem cell gives rise to transit amplifying cells, which then give rise to differentiating keratinocytes in discrete cellular territories called epidermal proliferation units. If stem cells in this model are labeled using GIFM, then the overall number of clones (groups of labeled cells (b)) along with the number of basal cells per clone would be expected to reach a maximum size over time (c, d). (e) In the neutral drift model, one or more stem/progenitor cell populations may be present and cell division results in one of three outcomes: two additional stem/progenitor cells, a stem/progenitor cell and a differentiating keratinocyte, or two differentiating keratinocytes. These divisions occur in a stochastic (random) manner, and thus if these stem/progenitor cells are labeled using GIFM, then clones of various sizes will result (f). However, with time, the overall number of clones will decrease due to random chance whereas the number of basal cells in surviving clones will increase linearly with time (g, h). Figure modified from Klein et al. GIFM, genetic inducible fate mapping.