A large pool of actively cycling progenitors orchestrates self-renewal and injury repair of an ectodermal appendage

Abstract

The classical model of tissue renewal posits that small numbers of quiescent stem cells (SCs) give rise to proliferating transit-amplifying cells before terminal differentiation. However, many organs house pools of SCs with proliferative and differentiation potentials that diverge from this template. Resolving SC identity and organization is therefore central to understanding tissue renewal. Here, using a combination of single-cell RNA sequencing (scRNA-seq), mouse genetics and tissue injury approaches, we uncover cellular hierarchies and mechanisms that underlie the maintenance and repair of the continuously growing mouse incisor. Our results reveal that, during homeostasis, a group of actively cycling epithelial progenitors generates enamel-producing ameloblasts and adjacent layers of non-ameloblast cells. After injury, tissue repair was achieved through transient increases in progenitor-cell proliferation and through direct conversion of Notch1-expressing cells to ameloblasts. We elucidate epithelial SC identity, position and function, providing a mechanistic basis for the homeostasis and repair of a fast-turnover ectodermal appendage.

Single-Cell RNA-Sequencing From Mouse Incisor Reveals Dental Epithelial Cell-Type Specific Genes

“…Results indicated IEE and OEE or SI and SR to possibly have common genes and roles during tooth development to some extent. This finding agreed with the previous single-cell analysis of the cervical loop in incisors ( Sharir et al, 2019 ). Further, we identified Pttg1 , Cldn10 , Atf3 , and Krt15 as novel markers of dental epithelial cell type ( Figure 3 ).…”

“…Therefore, the dental epithelium is a non-regenerative tissue that limits the observation of the developmental trajectory of cell fate. Rodent incisors have stem cell niche in their roots, referred to as a cervical loop, and grow throughout their life ( Harada et al, 2006 ; Sharir et al, 2019 ).…”

“…This model provides a new perspective on the development of dental epithelium. Single-cell RNA sequencing (scRNA-seq) is a powerful tool to clarify the heterogeneity of developing cell types; recent scRNA-seq studies in tooth development have clarified some characteristics of dental epithelial cell types ( Landin et al, 2012 , 2015 ; Sharir et al, 2019 ; Takahashi et al, 2019 ). However, there has been no report yet that covers the perspective of transcriptome profiles in dental epithelium differentiation, especially amelogenesis using scRNA-seq.…”

Dental cell type atlas reveals stem and differentiated cell types in mouse and human teeth

“…Thus, the progenitor area might rely on functional diversification of different stem cells with a stemness gradation. Such diversity of epithelial progenitor cell subtypes might also reflect the remarkable plasticity noted by earlier studies 35,46 .…”

Single-Cell RNA Sequencing of Calvarial and Long-Bone Endocortical Cells

“…The ability to interrogate the transcriptomes of large numbers of individual cells has led to the discovery of previously unrecognized cell types (8,9) and has detected changes in cellular diversity in response to genetic or environmental perturbation. (38)(39)(40) Recovering individual cells from skeletal tissue is more challenging than from cultured cells or from other tissues whose cells can be easily separated by brief digestion and/or physical disruption. For this reason, in vitro studies of osteoblast differentiation and mineralization typically expand cells in culture and then chemically induce matrix mineralization.…”