Perivascular Stem Cells at the Tip of Mouse Incisors Regulate Tissue Regeneration

Ptip is essential for tooth development via regulating Wnt pathway

OBJECTIVE

Epigenetic regulation plays important role in stem cell maintenance. Ptip was identified as epigenetic regulator, but the role in dental progenitor cells remains unclear.

SUBJECTS AND METHODS

Dental mesenchymal progenitor cells were targeted by Sp7-icre and visualized in mTmG; Sp7-icre mice. The Ptipf/f; Sp7-icre mice were generated and the phenotype of incisors and molars were shown by micro-computerized tomography, scanning electron microscope, hematoxylin & eosin staining, and immunofluorescence. Dental mesenchymal progenitor cells were sorted by fluorescence-activated cell sorting from lower incisors and RNA sequencing was performed.

RESULTS

The Sp7-icre targets dental mesenchymal progenitor cells in incisors and molars. The Ptipf/f; Sp7-icre mice showed spontaneous fractures in the cusp of upper incisors and lower incisors at 3 weeks (w), compensative overgrowth of lower incisors at 1 month (M), and overgrowth extended to the outside at 2 M. The molars showed shortened roots. The functions of odontoblasts and dental mesenchymal progenitor cells were impaired. Mechanically, loss of Ptip activates the Wnt pathway and upregulates the expression of Wls in dental mesenchymal progenitor cells. Also, the regenerative ability of lower incisors was significantly impaired.

CONCLUSION

We first demonstrated that Ptip was crucial for tooth development via regulating Wnt signaling.

Mesenchymal condensation in tooth development and regeneration: a focus on translational aspects of organogenesis

The teeth are vertebrate-specific, highly specialized organs performing fundamental functions of mastication and speech, the maintenance of which is crucial for orofacial homeostasis and is further linked to systemic health and human psychosocial well-being. However, with limited ability for self-repair, the teeth can often be impaired by traumatic, inflammatory, and progressive insults, leading to high prevalence of tooth loss and defects worldwide. Regenerative medicine holds the promise to achieve physiological restoration of lost or damaged organs, and in particular an evolving framework of developmental engineering has pioneered functional tooth regeneration by harnessing the odontogenic program. As a key event of tooth morphogenesis, mesenchymal condensation dictates dental tissue formation and patterning through cellular self-organization and signaling interaction with the epithelium, which provides a representative to decipher organogenetic mechanisms and can be leveraged for regenerative purposes. In this review, we summarize how mesenchymal condensation spatiotemporally assembles from dental stem cells (DSCs) and sequentially mediates tooth development. We highlight condensation-mimetic engineering efforts and mechanisms based on ex vivo aggregation of DSCs, which have achieved functionally robust and physiologically relevant tooth regeneration after implantation in animals and in humans. The discussion of this aspect will add to the knowledge of development-inspired tissue engineering strategies and will offer benefits to propel clinical organ regeneration.

Cav1.2 regulated odontogenic differentiation of NG2+ pericytes during pulp injury

NG2⁺ pericytes, as the possible precursor cells of mesenchymal stem cells (MSCs), have drawn attention due to their ability to differentiate into odontoblasts. Ca_v1.2 is involved in the differentiation process of stem cells, but its role in the differentiation of NG2⁺ pericytes is not clear. The aim of the present study was to examine the role of Ca_v1.2 in the differentiation of NG2⁺ pericytes into odontoblasts. NG2⁺ pericytes were obtained from human dental pulp cells by magnetic-activated cell sorting. During the odontogenic differentiation of NG2⁺ pericytes, the effects of the Ca_v1.2 inhibitors, nimodipine and Ca_v1.2 knockdown shRNA, were analyzed by real-time polymerase chain reaction and alizarin red staining. NG2CreERT2/Rosa26-GFP lineage-tracing mice were established to further investigate the roles of NG2⁺ pericytes and Ca_v1.2 in incisor self-repair after injury in vivo. At 10 min, 1 day, and 3 days after pulp injuries in transgenic mice, NG2-GFP⁺ and Ca_v1.2 immunofluorescence co-staining was performed on the incisors. Nimodipine treatment and Ca_v1.2 knockdown showed similar inhibition of calcium nodule formation and mRNA levels of osteogenic markers (DSPP, DMP1, and Runx2, p < 0.05). NG2⁺ pericytes migrated from their inherent perivascular location to the odontoblast layers after pulp injury. Ca_v1.2 showed a similar response pattern as NG2⁺ pericytes and gradually returned to normal levels. In addition, many co-stained areas of Ca_v1.2 and NG2⁺ pericytes, both near the perivascular and odontoblast layers, were observed. These results indicate that Ca_v1.2 played a vital role in the odontogenic differentiation of NG2⁺ pericytes, and that it might be closely linked to the NG2⁺ pericytes-mediated repair of dental pulp injury in vivo.

Odontogenic MSC Heterogeneity: Challenges and Opportunities for Regenerative Medicine

Cellular heterogeneity refers to the genetic and phenotypic differences among cells, which reflect their various fate choices, including viability, proliferation, self-renewal probability, and differentiation into different lineages. In recent years, research on the heterogeneity of mesenchymal stem cells has made some progress. Odontogenic mesenchymal stem cells share the characteristics of mesenchymal stem cells, namely, good accessibility, low immunogenicity and high stemness. In addition, they also exhibit the characteristics of vasculogenesis and neurogenesis, making them attractive for tissue engineering and regenerative medicine. However, the usage of mesenchymal stem cell subgroups differs in different diseases. Furthermore, because of the heterogeneity of odontogenic mesenchymal stem cells, their application in tissue regeneration and disease management is restricted. Findings related to the heterogeneity of odontogenic mesenchymal stem cells urgently need to be summarized, thus, we reviewed studies on odontogenic mesenchymal stem cells and their specific subpopulations, in order to provide indications for further research on the stem cell regenerative therapy.

SM22α-lineage niche cells regulate intramembranous bone regeneration via PDGFRβ-triggered hydrogen sulfide production

Bone stromal cells are critical for bone homeostasis and regeneration. Growing evidence suggests that non-stem bone niche cells support bone homeostasis and regeneration via paracrine mechanisms, which remain to be elucidated. Here, we show that physiologically quiescent SM22α-lineage stromal cells expand after bone injury to regulate diverse processes of intramembranous bone regeneration. The majority of SM22α-lineage cells neither act as stem cells in vivo nor show their expression patterns. Dysfunction of SM22α-lineage niche cells induced by loss of platelet-derived growth factor receptor β (PDGFRβ) impairs bone repair. We further show that PDGFRβ-triggered hydrogen sulfide (H₂S) generation in SM22α-lineage niche cells facilitates osteogenesis and angiogenesis and suppresses overactive osteoclastogenesis. Collectively, these data demonstrate that non-stem SM22α-lineage niche cells support the niche for bone regeneration with a PDGFRβ/H₂S-dependent regulatory mechanism. Our findings provide further insight into non-stem bone stromal niche cell populations and niche-regulation strategy for bone repair.

Dental Pulp Stem Cell Heterogeneity: Finding Superior Quality "Needles" in a Dental Pulpal "Haystack" for Regenerative Medicine-Based Applications

Human dental pulp stem/stromal cells (hDPSCs) derived from the permanent secondary dentition are recognised to possess certain advantageous traits, which support their potential use as a viable source of mesenchymal stem/stromal cells (MSCs) for regenerative medicine-based applications. However, the well-established heterogeneous nature of hDPSC subpopulations, coupled with their limited numbers within dental pulp tissues, has impeded our understanding of hDPSC biology and the translation of sufficient quantities of these cells from laboratory research, through successful therapy development and clinical applications. This article reviews our current understanding of hDPSC biology and the evidence underpinning the molecular basis of their heterogeneity, which may be exploited to distinguish individual subpopulations with specific or superior characteristics for regenerative medicine applications. Pertinent unanswered questions which still remain, regarding the developmental origins, hierarchical organisation, and stem cell niche locations of hDPSC subpopulations and their roles in hDPSC heterogeneity and functions, will further be explored. Ultimately, a greater understanding of how key features, such as specific cell surface, senescence and other relevant genes, and protein and metabolic markers, delineate between hDPSC subpopulations with contrasting stemness, proliferative, multipotency, immunomodulatory, anti-inflammatory, and other relevant properties is required. Such knowledge advancements will undoubtedly lead to the development of novel screening, isolation, and purification strategies, permitting the routine and effective identification, enrichment, and expansion of more desirable hDPSC subpopulations for regenerative medicine-based applications. Furthermore, such innovative measures could lead to improved cell expansion, manufacture, and banking procedures, thereby supporting the translational development of hDPSC-based therapies in the future.

Dental Pulp Stem Cells Derived From Adult Human Third Molar Tooth: A Brief Review

The fields of regenerative medicine and stem cell-based tissue engineering have the potential of treating numerous tissue and organ defects. The use of adult stem cells is of particular interest when it comes to dynamic applications in translational medicine. Recently, dental pulp stem cells (DPSCs) have been traced in third molars of adult humans. DPSCs have been isolated and characterized by several groups. DPSCs have promising characteristics including self-renewal capacity, rapid proliferation, colony formation, multi-lineage differentiation, and pluripotent gene expression profile. Nevertheless, genotypic, and phenotypic heterogeneities have been reported for DPSCs subpopulations which may influence their therapeutic potentials. The underlying causes of DPSCs’ heterogeneity remain poorly understood; however, their heterogeneity emerges as a consequence of an interplay between intrinsic and extrinsic cellular factors. The main objective of the manuscript is to review the current literature related to the human DPSCs derived from the third molar, with a focus on their physiological properties, isolation procedures, culture conditions, self-renewal, proliferation, lineage differentiation capacities and their prospective advances use in pre-clinical and clinical applications.

The Role of Dendritic Cells during Physiological and Pathological Dentinogenesis

The dental pulp is a soft connective tissue of ectomesenchymal origin that harbors distinct cell populations, capable of interacting with each other to maintain the vitality of the tooth. After tooth injuries, a sequence of complex biological events takes place in the pulpal tissue to restore its homeostasis. The pulpal response begins with establishing an inflammatory reaction that leads to the formation of a matrix of reactionary or reparative dentin, according to the nature of the exogenous stimuli. Using several in vivo designs, antigen-presenting cells, including macrophages and dendritic cells (DCs), are identified in the pulpal tissue before tertiary dentin deposition under the afflicted area. However, the precise nature of this phenomenon and its relationship to inherent pulp cells are not yet clarified. This literature review aims to discuss the role of pulpal DCs and their relationship to progenitor/stem cells, odontoblasts or odontoblast-like cells, and other immunocompetent cells during physiological and pathological dentinogenesis. The concept of “dentin-pulp immunology” is proposed for understanding the crosstalk among these cell types after tooth injuries, and the possibility of immune-based therapies is introduced to accelerate pulpal healing after exogenous stimuli.

Special AT-rich sequence-binding protein 2 (Satb2) synergizes with Bmp9 and is essential for osteo/odontogenic differentiation of mouse incisor mesenchymal stem cells

OBJECTIVES

Mouse incisor mesenchymal stem cells (MSCs) have self-renewal ability and osteo/odontogenic differentiation potential. However, the mechanism controlling the continuous self-renewal and osteo/odontogenic differentiation of mouse incisor MSCs remains unclear. Special AT-rich sequence-binding protein 2 (SATB2) positively regulates craniofacial patterning, bone development and regeneration, whereas SATB2 deletion or mutation leads to craniomaxillofacial dysplasia and delayed tooth and root development, similar to bone morphogenetic protein (BMP) loss-of-function phenotypes. However, the detailed mechanism underlying the SATB2 role in odontogenic MSCs is poorly understood. The aim of this study was to investigate whether SATB2 can regulate self-renewal and osteo/odontogenic differentiation of odontogenic MSCs.

MATERIALS AND METHODS

Satb2 expression was detected in the rapidly renewing mouse incisor mesenchyme by immunofluorescence staining, quantitative RT-PCR and Western blot analysis. Ad-Satb2 and Ad-siSatb2 were constructed to evaluate the effect of Satb2 on odontogenic MSCs self-renewal and osteo/odontogenic differentiation properties and the potential role of Satb2 with the osteogenic factor bone morphogenetic protein 9 (Bmp9) in vitro and in vivo.

RESULTS

Satb2 was found to be expressed in mesenchymal cells and pre-odontoblasts/odontoblasts. We further discovered that Satb2 effectively enhances mouse incisor MSCs self-renewal. Satb2 acted synergistically with the potent osteogenic factor Bmp9 in inducing osteo/odontogenic differentiation of mouse incisor MSCs in vitro and in vivo.

CONCLUSIONS

Satb2 promotes self-renewal and osteo/odontogenic differentiation of mouse incisor MSCs. Thus, Satb2 can cooperate with Bmp9 as a new efficacious bio-factor for osteogenic regeneration and tooth engineering.

Distinct Expression Patterns of Cxcl12 in Mesenchymal Stem Cell Niches of Intact and Injured Rodent Teeth

Specific stem cell populations within dental mesenchymal tissues guarantee tooth homeostasis and regeneration throughout life. The decision between renewal and differentiation of stem cells is greatly influenced by interactions with stromal cells and extracellular matrix molecules that form the tissue specific stem cell niches. The Cxcl12 chemokine is a general marker of stromal cells and plays fundamental roles in the maintenance, mobilization and migration of stem cells. The aim of this study was to exploit Cxcl12-GFP transgenic mice to study the expression patterns of Cxcl12 in putative dental niches of intact and injured teeth. We showed that endothelial and stromal cells expressed Cxcl12 in the dental pulp tissue of both intact molars and incisors. Isolated non-endothelial Cxcl12⁺ dental pulp cells cultured in different conditions in vitro exhibited expression of both adipogenic and osteogenic markers, thus suggesting that these cells possess multipotent fates. Taken together, our results show that Cxcl12 is widely expressed in intact and injured teeth and highlight its importance as a key component of the various dental mesenchymal stem cell niches.

Unveiling diversity of stem cells in dental pulp and apical papilla using mouse genetic models: a literature review

The dental pulp, a non-mineralized connective tissue uniquely encased within the cavity of the tooth, provides a niche for diverse arrays of dental mesenchymal stem cells. Stem cells in the dental pulp, including dental pulp stem cells (DPSCs), stem cells from human exfoliated deciduous teeth (SHEDs) and stem cells from apical papilla (SCAPs), have been isolated from human tissues with an emphasis on their potential application to regenerative therapies. Recent studies utilizing mouse genetic models shed light on the identities of these mesenchymal progenitor cells derived from neural crest cells (NCCs) in their native conditions, particularly regarding how they contribute to homeostasis and repair of the dental tissue. The current concept is that at least two distinct niches for stem cells exist in the dental pulp, e.g., the perivascular niche and the perineural niche. The precise identities of these stem cells and their niches are now beginning to be unraveled thanks to sophisticated mouse genetic models, which lead to better understanding of the fundamental properties of stem cells in the dental pulp and the apical papilla in humans. The new knowledge will be highly instrumental for developing more effective stem cell-based regenerative therapies to repair teeth in the future.

Gli1+ Periodontium Stem Cells Are Regulated by Osteocytes and Occlusal Force

Teeth are attached to alveolar bone by the periodontal ligament (PDL), which contains stem cells supporting tissue turnover. Here, we identified Gli1+ cells in adult mouse molar PDL as multi-potential stem cells (PDLSCs) giving rise to PDL, alveolar bone, and cementum. They support periodontium tissue turnover and injury repair. Gli1+ PDLSCs are surrounding the neurovascular bundle and more enriched in the apical region. Canonical Wnt signaling is essential for their activation. Alveolar bone osteocytes negatively regulate Gli1+ PDLSCs activity through sclerostin, a Wnt inhibitor. Blockage of sclerostin accelerates the PDLSCs lineage contribution rate in vivo. Sclerostin expression is modulated by physiological occlusal force. Removal of occlusal force upregulates sclerostin and inhibits PDLSCs activation. In summary, Gli1+ cells are the multipotential PDLSCs in vivo. Osteocytes provide negative feedback to PDLSCs and inhibit their activities through sclerostin. Physiological occlusal force indirectly regulates PDLSCs activities by fine-tuning this feedback loop.

Comparison of osteogenic and dentinogenic potentials of mice incisor and molar pulps in vitro

OBJECTIVE

In the present study, we compared the in vitro osteogenic and dentinogenic potential of pulp cells from incisors and molars.

DESIGN

Primary pulp cultures were established from DSPP-Cerulean/DMP1-Cherry and BSP-GFPtpz reporter mouse lines and processed for various assays.

RESULTS

Our results showed marked differences in dentinogenic and osteogenic potentials of primary cultures from unerupted molars and incisors isolated from 5 to 7 days old pups. While primary cultures from both incisors and molars differentiated into odontoblasts and osteoblasts, cultures from molars differentiated into more DSPP-Cerulean + cells (∼5.5 %) compared to incisor cultures (∼0.7 %) at Day 14 and appear to be more committed to the odontogenic lineage. On the other hand, cultures from incisors show more differentiation into BSP-GFPtpz + cells (∼25 %) compared to molar cultures (∼16 %) and were more committed to the osteogenic lineage. Data were analyzed by Student's t-test and statistical significance was set at P ≤ 0.05.

CONCLUSION

Since in the dental pulp, MSCs are the primary source of progenitors capable of giving rise to osteoblasts and odontoblasts, our results provide evidence for differences in the commitment of MSCs in molars and incisors to the odontogenic and osteogenic lineages, respectively.

Molecular and cellular mechanisms of tooth development, homeostasis and repair

The tooth provides an excellent system for deciphering the molecular mechanisms of organogenesis, and has thus been of longstanding interest to developmental and stem cell biologists studying embryonic morphogenesis and adult tissue renewal. In recent years, analyses of molecular signaling networks, together with new insights into cellular heterogeneity, have greatly improved our knowledge of the dynamic epithelial-mesenchymal interactions that take place during tooth development and homeostasis. Here, we review recent progress in the field of mammalian tooth morphogenesis and also discuss the mechanisms regulating stem cell-based dental tissue homeostasis, regeneration and repair. These exciting findings help to lay a foundation that will ultimately enable the application of fundamental research discoveries toward therapies to improve oral health.

Regulatory mechanisms of jaw bone and tooth development

Jaw bones and teeth originate from the first pharyngeal arch and develop in closely related ways. Reciprocal epithelial-mesenchymal interactions are required for the early patterning and morphogenesis of both tissues. Here we review the cellular contribution during the development of the jaw bones and teeth. We also highlight signaling networks as well as transcription factors mediating tissue-tissue interactions that are essential for jaw bone and tooth development. Finally, we discuss the potential for stem cell mediated regenerative therapies to mitigate disorders and injuries that affect these organs.

Activation of αSMA expressing perivascular cells during reactionary dentinogenesis

AIM

To examine the contribution of perivascular cells expressing αSMA to reactionary dentinogenesis.

METHODOLOGY

An inducible, Cre-loxP in vivo fate-mapping approach was used to examine the contribution of the descendants of cells expressing the αSMA-CreERT2 transgene to reactionary dentinogenesis in mice molars. Reactionary dentinogenesis was induced by experimental mild injury to dentine without pulp exposure. The Student's t test was used to determine statistical significance at *P ≤ 0.05.

RESULTS

The lineage tracing experiments revealed that mild injury to dentine first led to activation of αSMA-tdTomato+ cells in the entire pulp chamber. The percentage of areas occupied by αSMA-tdTomato+ in injured (7.5 ± 0.7%) teeth were significantly higher than in teeth without injury (2 ± 0.5%). After their activation, αSMA-tdTomato+ cells migrated towards the site of injury, gave rise to pulp cells and a few odontoblasts that became integrated into the existing odontoblast layer expressing Col2.3-GFP and Dspp.

CONCLUSION

Mild insult to dentine activated perivascular αSMA-tdTomato+ cells giving rise to pulp cells as well as a few odontoblasts that were integrated into the pre-existing odontoblast layer.

Autotherapies: Enhancing Endogenous Healing and Regeneration

The promise of tissue engineering and regenerative medicine to reduce the burden of disease and improve quality of life are widely acknowledged. Traditional tissue engineering and regenerative medicine approaches rely on generation of tissue constructs in vitro for subsequent transplantation or injection of exogenously manipulated cells into a host. While promising, few such therapies have succeeded in clinical practice. Here, we propose that recent advances in stem cell and developmental biology, immunology, bioengineering, and material sciences, position us to develop a new generation of in vivo regenerative medicine therapies, which we term autotherapies. Autotherapies are strategies based on optimizing endogenous tissue responses and capitalizing on manipulation of stem cell niches and endogenous tissue microenvironments to enhance tissue healing and regeneration.

Regulation of Mesenchymal Stem to Transit-Amplifying Cell Transition in the Continuously Growing Mouse Incisor

In adult tissues and organs with high turnover rates, the generation of transit-amplifying cell (TAC) populations from self-renewing stem cells drives cell replacement. The role of stem cells is to provide a renewable source of cells that give rise to TACs to provide the cell numbers that are necessary for cell differentiation. Regulation of the formation of TACs is thus fundamental to controlling cell replacement. Here, we analyze the properties of a population of mesenchymal TACs in the continuously growing mouse incisor to identify key components of the molecular regulation that drives proliferation. We show that the polycomb repressive complex 1 acts as a global regulator of the TAC phenotype by its direct action on the expression of key cell-cycle regulatory genes and by regulating Wnt/β-catenin-signaling activity. We also identify an essential requirement for TACs in maintaining mesenchymal stem cells, which is indicative of a positive feedback mechanism.

Comparison of two digestion strategies on characteristics and differentiation potential of human dental pulp stem cells

OBJECTIVE

This study aimed to compare the behavior of dental pulp stem cells (DPSCs) after isolation using solutions containing either collagenase/dispase or collagenase alone.

DESIGN

DPSCs were isolated by two digestion methods (collagenase/dispase or collagenase alone) from human third molars. Immunophenotypic features were confirmed by flow cytometry for cell markers STRO-1, cluster of differentiation (CD) 146, CD45, and collagen type-I. The proliferation potential of cells was evaluated by 5-bromo-2'-deoxyuridine (brdU) incorporation assay, and finally they were assessed for multi-lineage differentiation potential. Data were analyzed using one-way analysis of variance and independent t-tests.

RESULTS

DPSCs isolated by either method showed similar levels of STRO-1, CD45, and collagen type-I and similar incorporation of brdU (P > 0.05). However, DPSCs obtained by collagenase I/dispase treatment had significantly higher numbers of CD146⁺ cells and osteogenic and chondrogenic capacities compared to those obtained by treatment with collagenase I alone (P < 0.05). On the other hand, more STRO-1+/CD164-DPSCs were found in the collagenase alone group with higher adipogenic potential.

CONCLUSIONS

Different enzyme solutions gave rise to different populations of DPSCs. Dispase enhanced isolation of CD146⁺ DPSCs probably by disrupting the basement membranes of blood vessels and releasing DPCSs embedded in the perivascular niche. Furthermore, the differentiation potential of DPSCs was influenced by the change in enzyme solution.

Biology Explaining Tooth Repair and Regeneration: A Mini-Review

The tooth is an intricate composition of precisely patterned, mineralized matrices and soft tissues. Mineralized tissues include enamel (produced by the epithelial cells called ameloblasts), dentin and cementum (produced by mesenchymal cells called odontoblasts and cementoblasts, respectively), and soft tissues, which include the dental pulp and the periodontal ligament along with the invading nerves and blood vessels. It was perceived for a very long time that teeth primarily serve an esthetical function. In recent years, however, the role of healthy teeth, as well as the impact of oral health on general well-being, became more evident. Tooth loss, caused by tooth decay, congenital malformations (tooth agenesis), trauma, periodontal diseases, or age-related changes, is usually replaced by artificial materials which lack many of the important biological characteristics of the natural tooth. Human teeth have very low to almost absent regeneration potential, due to early loss of cell populations with regenerative capacity, namely stem cells. Significant effort has been made in recent decades to identify and characterize tooth stem cells, and to unravel the developmental programs which these cells follow in order to generate a tooth.

Digit Tip Regeneration: Merging Regeneration Biology with Regenerative Medicine

Regeneration Biology is the study of organisms with endogenous regenerative abilities, whereas Regenerative Medicine focuses on engineering solutions for human injuries that do not regenerate. While the two fields are fundamentally different in their approach, there is an obvious interface involving mammalian regeneration models. The fingertip is the only part of the human limb that is regeneration-competent and the regenerating mouse digit tip has emerged as a model to study a clinically relevant regenerative response. In this article, we discuss how studies of digit tip regeneration have identified critical components of the regenerative response, and how an understanding of endogenous regeneration can lead to expanding the regenerative capabilities of nonregenerative amputation wounds. Such studies demonstrate that regeneration-incompetent wounds can respond to treatment with individual morphogenetic agents by initiating a multi-tissue response that culminates in structural regeneration. In addition, the healing process of nonregenerative wounds are found to cycle through nonresponsive, responsive and nonresponsive phases, and we call the responsive phase the Regeneration Window. We also find the responsiveness of mature healed amputation wounds can be reactivated by reinjury, thus nonregenerated wounds retain a potential for regeneration. We propose that regeneration-incompetent injuries possess dormant regenerative potential that can be activated by targeted treatment with specific morphogenetic agents. We believe that future Regenerative Medicine-based-therapies should be designed to promote, not replace, regenerative responses. Stem Cells Translational Medicine 2018;7:262-270.

Mesenchymal-myeloid interaction in the regulation of immunity

Several studies have demonstrated how different cell types of mesenchymal and myeloid origin can independently exhibit immunoregulatory activities. In response to inflammatory cues, they transcribe a molecular repertoire that restores the tissue microenvironment to what it was before the injury. There is accumulating evidence that stromal and myeloid-derived cells do not act independently but that the establishment of a cross-talk between them is a fundamental requirement. Stromal cells, prompted by inflammatory molecules, orchestrate and initiate myeloid cell recruitment and their functional reprogramming. Once instructed, myeloid cells effect the anti-inflammatory activity or, if alternatively required, enhance immune responses. The cross-talk plays a fundamental role in tissue homeostasis, not only to regulate inflammation, but also to promote tissue regeneration and cancer progression.

A quiescent cell population replenishes mesenchymal stem cells to drive accelerated growth in mouse incisors

The extent to which heterogeneity within mesenchymal stem cell (MSC) populations is related to function is not understood. Using the archetypal MSC in vitro surface marker, CD90/Thy1, here we show that 30% of the MSCs in the continuously growing mouse incisor express CD90/Thy1 and these cells give rise to 30% of the differentiated cell progeny during postnatal development. In adulthood, when growth rate homeostasis is established, the CD90/Thy1+ MSCs decrease dramatically in number. When adult incisors are cut, the growth rate increases to rapidly re-establish tooth length and homeostasis. This accelerated growth rate correlates with the re-appearance of CD90/Thy+ MSCs and re-establishment of their contribution to cell differentiation. A population of Celsr1+ quiescent cells becomes mitotic following clipping and replenishes the CD90/Thy1 population. A sub-population of MSCs thus exists in the mouse incisor, distinguished by expression of CD90/Thy1 that plays a specific role only during periods of increased growth rate.

Pericytes in Tissue Engineering

Pericytes have crucial roles in blood-brain barrier function, blood vessel function/stability, angiogenesis, endothelial cell proliferation/differentiation, wound healing, and hematopoietic stem cells maintenance. They can be isolated from fetal and adult tissues and have multipotential differentiation capacity as mesenchymal stem cells (MSCs). All of these properties make pericytes as preferred cells in the field of tissue engineering. Current developments have shown that tissue-engineered three-dimensional (3D) systems including multiple cell layers (or types) and a supporting biological matrix represent the in vivo environment better than those monolayers on plastic dishes. Tissue-engineered models are also more ethical and cheaper systems than animal models. This chapter describes the role of pericytes in tissue engineering for regenerative medicine.

Dental mesenchymal stem cells

Mammalian teeth harbour mesenchymal stem cells (MSCs), which contribute to tooth growth and repair. These dental MSCs possess many in vitro features of bone marrow-derived MSCs, including clonogenicity, expression of certain markers, and following stimulation, differentiation into cells that have the characteristics of osteoblasts, chondrocytes and adipocytes. Teeth and their support tissues provide not only an easily accessible source of MSCs but also a tractable model system to study their function and properties in vivo In addition, the accessibility of teeth together with their clinical relevance provides a valuable opportunity to test stem cell-based treatments for dental disorders. This Review outlines some recent discoveries in dental MSC function and behaviour and discusses how these and other advances are paving the way for the development of new biologically based dental therapies.

The pathological structure of the perivascular niche in different microvascular patterns of glioblastoma

The perivascular niche is critical for intercellular communication between resident cell types in glioblastoma (GBM), and it plays a vital role in maintaining the glioma stem cell (GSC) microenvironment. It is shown in abundant research that different microvascular patterns exist in GBM; and it can be implied that different microvascular patterns are associated with different pathological structures in the perivascular niche. However, the pathological structure of the perivascular niche is still not clear. Here, we investigated the distribution and biological characteristics of different microvascular pattern niches (MVPNs) in GBM by detecting the expression of CD34, CD133, Nestin, α-SMA, GFAP and CD14 in the perivascular niche using multiple -fluorescence. The four basic microvascular patterns are microvascular sprouting (MS), vascular cluster (VC), vascular garland (VG), and glomeruloid vascular proliferation (GVP). By analyzing the proportion of the area of each marker in four types of formations, the results indicated that the expression of CD34, CD133 and Nestin in MS and VC was significantly lower than that in VG and GVP (P<0.05). Furthermore, the results showed that α-SMA expression different in the MS, VC, VG and GVP (P<0.05). However, the expression of GFAP and CD14 in each type of formation exhibited no significant difference (P>0.05). According to the area distributions of different markers, we mapped four precise simulation diagrams to provide an effective foundation for the accurate simulation of glioblastoma in vitro.

Axin2-expressing cells differentiate into reparative odontoblasts via autocrine Wnt/β-catenin signaling in response to tooth damage

In non-growing teeth, such as mouse and human molars, primary odontoblasts are long-lived post-mitotic cells that secrete dentine throughout the life of the tooth. New odontoblast-like cells are only produced in response to a damage or trauma. Little is known about the molecular events that initiate mesenchymal stem cells to proliferate and differentiate into odontoblast-like cells in response to dentine damage. The reparative and regenerative capacity of multiple mammalian tissues depends on the activation of Wnt/β-catenin signaling pathway. In this study, we investigated the molecular role of Wnt/β-catenin signaling pathway in reparative dentinogenesis using an in vivo mouse tooth damage model. We found that Axin2 is rapidly upregulated in response to tooth damage and that these Axin2-expressing cells differentiate into new odontoblast-like cells that secrete reparative dentine. In addition, the Axin2-expressing cells produce a source of Wnt that acts in an autocrine manner to modulate reparative dentinogenesis.

Heterogeneity and Developmental Connections between Cell Types Inhabiting Teeth

Every tissue is composed of multiple cell types that are developmentally, evolutionary and functionally integrated into the unit we call an organ. Teeth, our organs for biting and mastication, are complex and made of many different cell types connected or disconnected in terms of their ontogeny. In general, epithelial and mesenchymal compartments represent the major framework of tooth formation. Thus, they give rise to the two most important matrix–producing populations: ameloblasts generating enamel and odontoblasts producing dentin. However, the real picture is far from this quite simplified view. Diverse pulp cells, the immune system, the vascular system, the innervation and cells organizing the dental follicle all interact, and jointly participate in transforming lifeless matrix into a functional organ that can sense and protect itself. Here we outline the heterogeneity of cell types that inhabit the tooth, and also provide a life history of the major populations. The mouse model system has been indispensable not only for the studies of cell lineages and heterogeneity, but also for the investigation of dental stem cells and tooth patterning during development. Finally, we briefly discuss the evolutionary aspects of cell type diversity and dental tissue integration.

αSMA-Expressing Perivascular Cells Represent Dental Pulp Progenitors In Vivo

The goal of this study was to examine the contribution of perivascular cells to odontoblasts during the development, growth, and repair of dentin using mouse molars as a model. We used an inducible, Cre-loxP in vivo fate-mapping approach to examine the contributions of the descendants of cells expressing the αSMA-CreERT2 transgene to the odontoblast lineage. In vivo lineage-tracing experiments in molars showed the contribution of αSMA-tdTomato⁺ cells to a small number of newly formed odontoblasts during primary dentinogenesis. Using an experimental pulp exposure model in molars to induce reparative dentinogenesis, we demonstrate the contribution of αSMA-tdTomato⁺ cells to cells secreting reparative dentin. Our results demonstrate that αSMA-tdTomato⁺ cells differentiated into Col2.3-GFP⁺ cells composed of both Dspp⁺ odontoblasts and Bsp⁺ osteoblasts. Our findings identify a population of mesenchymal progenitor cells capable of giving rise to a second generation of odontoblasts during reparative dentinogenesis. This population also makes a small contribution to odontoblasts during primary dentinogenesis.

The Nervous System Orchestrates and Integrates Craniofacial Development: A Review

Development of a head is a dazzlingly complex process: a number of distinct cellular sources including cranial ecto- and endoderm, mesoderm and neural crest contribute to facial and other structures. In the head, an extremely fine-tuned developmental coordination of CNS, peripheral neural components, sensory organs and a musculo-skeletal apparatus occurs, which provides protection and functional integration. The face can to a large extent be considered as an assembly of sensory systems encased and functionally fused with appendages represented by jaws. Here we review how the developing brain, neurogenic placodes and peripheral nerves influence the morphogenesis of surrounding tissues as a part of various general integrative processes in the head. The mechanisms of this impact, as we understand it now, span from the targeted release of the morphogens necessary for shaping to providing a niche for cellular sources required in later development. In this review we also discuss the most recent findings and ideas related to how peripheral nerves and nerve-associated cells contribute to craniofacial development, including teeth, during the post- neural crest period and potentially in regeneration.