New knowledge on the cellular makeup and growth of teeth can expedite developments in regenerative dentistry – a biological therapy for damaged teeth – as well as the treatment of tooth sensitivity. The study, which was conducted by researchers at Karolinska Institutet, is published in Nature Communications.
Teeth develop through a complex process in which soft tissue, with connective tissue, nerves and blood vessels, are bonded with three different types of hard tissue into a functional body part. As an explanatory model for this process, scientists often use the mouse incisor, which grows continuously and is renewed throughout the animal’s life.
Despite the fact that the mouse incisor has often been studied in a developmental context, many fundamental questions about the various tooth cells, stem cells and their differentiation and cellular dynamics remain to be answered.
Using a single-cell RNA sequencing method and genetic tracing, researchers at Karolinska Institutet, the Medical University of Vienna in Austria and Harvard University in the USA have now identified and characterised all cell populations in mouse teeth and in the young growing and adult human teeth.
“From stem cells to the completely differentiated adult cells we were able to decipher the differentiation pathways of odontoblasts, which give rise to dentine – the hard tissue closest to the pulp – and ameloblasts, which give rise to the enamel,” say the study’s last author Igor Adameyko at the Department of Physiology and Pharmacology, Karolinska Institutet, and co-author Kaj Fried at the Department of Neuroscience, Karolinska Institutet. “We also discovered new cell types and cell layers in teeth that can have a part to play in tooth sensitivity.”
Single-cell analysis of human adult and growing teeth
a Scheme of pulp regions isolated for single-cell RNA-seq from adult human molars and apical papillae of growing human molars (dotted regions). b Characterization of cell composition across five adult and two growing human molars using scVI deep learning framework. UMAP dimensionality reduction visualizes similarity of expression profiles of 39,095 single cells. Colors correspond to individual datasets and indicate clustering by cell types. c Characterization of dental cell types in human teeth. Colors demonstrate 17 clusters as defined by leiden clustering. Major clusters are defined by expression of known markers. d Human dental pulp have at least six transcriptionally distinct states. Top color bar reflects colors of clusters shown in c). Top 198 genes enriched in each cluster are shown (maximum to medium expression across clusters is at least four-fold and p value < 10−50, one-way ANOVA test). e, f Identification of apical-like-mouse-incisor regions in the growing apical papilla of human molar shown by the expression of SFRP2 and SMOC2 (immunohistochemistry) in the growing region of apical papilla. g Dividing, MKI67+ cells are positioned in the growing part of the apical papilla. h, i Expression of POSTN shows very regionalized pattern in two main clusters: periodontal ligament (PDL) on the samples from apical papillae (h), but also demarcate the periodontal layers of adult dental pulp previously recognized as a cell-rich and cell-free zones (i). Immunohistochemical POSTN staining. j S100A13 was proposed as a marker of human odontoblasts. This gene is highly overexpressed in one of the subclusters, which is on t-SNE located in the close proximity to dental pulp. S100A13 was proved to be expressed in odontoblasts by immunohistochemistry. (Od. Odontoblasts; PDL periodontal ligament). Scale bars: 50 µm, insets: 250 µm.
Some of the finds can also explain certain complicated aspects of the immune system in teeth, and others shed new light on the formation of tooth enamel, the hardest tissue in our bodies.
“We hope and believe that our work can form the basis of new approaches to tomorrow’s dentistry. Specifically, it can expedite the fast expanding field of regenerative dentistry, a biological therapy for replacing damaged or lost tissue.”
The results have been made publicly accessible in the form of searchable interactive user-friendly atlases of mouse and human teeth. The researchers believe that they should prove a useful resource not only for dental biologists but also for researchers interested in development and regenerative biology in general.
Source – Karolinska Institutet