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Introduction

The right sizes, profile, shade and alignment of teeth controls the appearance of one's simile and establishes one's uniqueness. Moreover, teeth are obviously vital tools for chewing of food and ensuring human beings engage in proper speech. The loss of teeth to common dental ailments such as periodontal disease, dental caries and shock inflicts considerable emotional and monetary burdens to patients; customary methods of teeth replacement have proved unsatisfactory among patients. In this regard, scientists are convinced that replacement of tooth by growing new ones will soon become a usual phenomenon (Lanza, 2004).

Britain and other countries in the United Kingdom countries record a low number of teeth loss compared to places such as the United States and Asia as shown in the graph below.

Instead of using artificial teeth, with this new stem cell research based initiative; UK dental patients may have their missing teeth replaced by implanting a small ball of cells which eventually grow into new teeth. Though a lot of uncertainties and complains surround the field of stem cell research, regenerating teeth naturally has obvious advantages over use of artificial teeth; artificial teeth require the support of a metal framework and a plastic or porcelain cap to function optimally unlike new teeth generated from stem cell research.

Statement

Tooth development and morphology is a very pivotal area of study in understanding the basis in which stem cell research is applied in dentistry. Dentition is a very imperative part of nourishment in vertebrates; tooth shapes frequently played a useful role in evolution of vertebrates. Andreasen, Andreasen, and Andreasen (2007) claim that in human beings, deviations in dentition as well as poor dental practices are not acute conditions in the modern world but they are still challenging problems for many. It has been established that tissue engineering can assist a larger number of people to make their lives better; there are various methods of producing bioartificial tooth implants that can be helpful for future dental use as replacement teeth.

Just like other ectodermal organs, teeth grow through a series of mutual signaling procedures between the epithelium and the mesenchyme. The informative capability exchanges between these tissues. Tooth development can be grouped into three phases: initiation, morphogenesis and terminal differentiation. The primary morphological indication during the growth of all ectodermal organs is an epithelial condensation at the initiation stage causing creation of an epithelial placode. The dental lamina is an ectodermal edge that extends the length of the emergent maxilla and mandible. Tooth growth begins at definite locations along the dental lamina while the mesenchyme packs into the epithelium.

Mesenchyme cells that generate tissues of bones, the face, cartilage and teeth are obtained from cranial neural crest. The neural crest cells drift as specific streams from the frontal neural lobe to occupy different regions of the embryo. Therefore, the cells that are involved in tooth formation come from the caudal midbrain and rostral hindbrain. The individual fortunes of crest cells within a stream are detailed by external signals they receive at their destinations. The cranial neural crest cells are intended for participating in tooth growth by signals they accept from oral epithelium. The oral epithelium is consequently the spring of the inductive signals for the odontogenesis.

The instructive ability to establish morphogenesis switches from epithelium to mesenchyme at the bud stage is paramount. For many of these instructive signals originating from either epithelium or mesenchyme timing and intensity are of supreme significance, and many of the signals are only transiently generated by one tissue before being generated by another. Signaling proteins that are characteristic of all major gene families of development and differentiation factors take part in controlling tooth development. Such factors include members of fibroblast growth factor, tumour neurosis factor, hedgehog, Notch and transforming growth factor.

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Separation and recognition of stem cells is the primary step in studying the latent capability of stem cell mediated remedy (Meyer, Meyer, Handschel, & Wiesmann, 2009). Postnatal stem cells have been separated from a variety of tissues and recent studies have effectively isolated dental pulp stem cells from fully grown dental tissue of extracted adult teeth. Analogous to other mesenchymal stem cells, dental pulp stem cells have the ability to produce clonogenic cells. Dental pulp stem cells are able to discriminate into osteo/ondogenic cells, neural cells and adipose tissue cells. Nevertheless, the assets of multi-prospective separation of dental pulp stem cells continue unconfirmed.

The discovery on dental pulp stem cells may offer a potential for exploiting them for dentin and pulp tissues restoration. Human teeth do not go through the kind of remodeling that is seen in other mineral-containing tissues such as bones; such tissues adjust to maintain structural integrity. Once a tooth sprouts up, dentinal harm caused by mechanical shock, disclosure to chemicals or contagious processes, stimulates the formation of reparative dentin that is although structurally poorly organized, but performs as an obstacle to the dental pulp with restricted aptitude. According to Bronzino (2006) it was found out that bone morphonogenetic protein-7 is able to arouse tertiary dentin development when applied to newly cut dentin. This almost certainly takes place via an osteo/ondontogenic introduction property of BMP-7 protein because BMP-7 fibroblasts had the capacity of expressing osteo/ondontogenic properties and develop bone tissue. Dental pulp stem cells were also able to form reparative dentin arrangement on the surfaces of ordinary human dentin.

However, it appears that dental pulp stem cells display diminished and modified in vivo odontogenic ability when stacked on the face of human dentin. Albeit the reason is still unclear, it may be linked to the microenvironment that houses in vivo separation of dental pulp stem cells. In the modern times, it was established that autogenous transplantation of dental pulp stem cells was able to excite dentin development on the surgically removed pulp. This discovery suggests that combination treatment using stem cells and growth may progress stem cell-mediated dentin renewal.

The dental papilla bears the pulp tissue, which is composed of fibroblasts, lymphatic ducts, blood vessels, nerves and odontoblasts. The odontoblasts are basically cells obtained from mesenchymal cells in the dental papilla bordering the interior enamel epithelium. Useful odontoblasts show polarized columnar morphology that alters into a resting state and become small and flat after prime dentin development. However, odontoblasts stay useful all over their life and can generate secondary dentin if shock is gentle. The dental follicle emerges as a transient arrangement during teeth morphogenesis. It is the source of three main types of cells: fibroblasts, cementoblasts and osteoblasts. Osteoblasts generate bone around the lower part of teeth while cementoblasts produce cementum which is fastened to the surface of the root. Fibroblasts generate collagen which generates periodontal ligaments. Periodontal ligaments act as shock absorbers, as springs of feeling, and are considered the main momentum for the tooth eruption process. The multifaceted arrangement that includes the periodontal ligaments, nearby cementum and alveolar bone is referred to as periodontium.

Although dental neural crest-derived cells are observed to preserve innate plural potency, it is still uncertain if teeth contain true multi-potent stem cells or the potentiality of dental precursor cells is rather restricted. The idea of stem cell as it relates to adult tissues like tooth can be explained as constricted and it is frequently more suitable to name these cell numbers precursor cells, but as per the accepted principle the idiom stem cell remains in motion. It has long been discovered that reproduction of dentin after tooth damage is attained by fresh odontoblasts that materialize close to the injured location.

Classification of multiplying cells with thymidine following injury has exposed disparity in classification depending on location related to the shock site.

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The subsisting odontoblast layer and the cavernous pulp layers did not show any label, but some perivascular classification suggests that progenitor cells are situated around the vessels. The buildup of labeled cells with time proposed that there is a steady movement of cells from deep tissue in the pulp to the margin. This information supports the concept that undifferentiated mesenchymal cells in the pulp have the aptitude to distinguish into cells similar to odontoblast which are in charge of new dentin development after a dental injury has occurred. It is still unclear if these cells are the reminder of earlier neural crest cells or they are somehow coupled with blood vessels created during organogenesis, or even become drawn to the damaged site from the flowing precursors most probably resulting from bone marrow.

One of the techniques to deal with these questions is to separate precursor cell populations from the pulp and typify them (Polak, Mantalaris & Harding, 2008). Indeed, numerous populations of dental cells with elevated propagation potential have been spotted both in animal models and in human beings. Human dental pulp cells resulting from emergent third molars have been cultured under mineralization-improving conditions and shown to form odontoblast-like cells that assemble dentin-like arrangement in vitro and express nestin. Other studies have shown that human dental pulp from adult teeth and peeled deciduous teeth has dental pulp stem cells. These cells have the capacity to undergo multi-potent differentiation; this phenomenon links them to mesenchymal cells found in the bone marrow. Both kinds of cells express mesenchymal cell indicators CD146 and STRO-1. In fact, dental pulp stem cells generate the dentin when transplanted into host animals, while bone marrow mesenchymal stem cells fail to take part in dentine regeneration in vivo.

Besides mineralization, dental pulp stem cells and stem cells obtained from human peeled deciduous teeth can express neural indicators and have the prospective to differentiate into adipocytes. Compared with dental pulp stem cells, stem cells from human peeled teeth show higher propagation rates and augmented population replications. Stem cells from peeled human teeth can also form globular aggregations, which link them to neural precursors. Based on these remarks it has been proposed that stem cells from peeled human teeth are different from dental pulp stem cells. When dental pulp stem cells are entrenched subcutaneously into immune-compromised mice, dentin pulp like structure is formed rather than a lamella bone even though bone marrow stem cells form bone. In sequence, stem cells from peeled human teeth generated both dentin and bone though not mature dentin pulp complexes when they were involved in transplantation in vivo. It has been hypothesizes that both dental pulp stem cells and stem cells from peeled human teeth could have a portion of cells that can be expressed as bone fide stem cells. Another exclusive population of stem cells called root apical papilla stem cells has been separated from the root apex of the developing human third molar. It has been established that these types of stem cells are able differentiate into adipocytes and odontoblasts. 

The noticeable heterogeneity of dental pulp stem cells and stem cells from human peeled teeth is not an exclusive characteristic of these cells. For instance, replica strains of bone marrow stem cells revealing the aptitude to redevelop bone in vivo can still display non-matching indicator expression. The usefulness of dental pulp stem cells is strengthened by the fact that they can be preserved in such a way that their multi-potential differentiation ability is preserved. Moreover, it has been established that under suitable conditions, dental pulp stem cells have greater qualities to bone marrow stem cells in encouraging immune tolerance to T-cells. One extra appealing characteristic of dental pulp stem cells is their capability to draw and preserve growth of neurons. This is perhaps due to the ability of dental pulp to perform as a sensory organ. An intimate association with neurons is conceivably established by the neural-crest origins of dental pulp. Immortalization of mouse dental pulp stem cells has been attained by limiting dilution heredity. The cells preserved several transcription details for example dental matrix protein-1, syaloprotein and other types of gene combinations.

Similar to dental pulp, the supplementary tissues of teeth can also regenerate to a definite extent following a gentle trauma. Earlier studies have established splitting cells emerging from wounded periodontal ligaments as heterogeneous cells similar to fibroblasts derived from the environs of blood vessels, though of hematopoietic origin. As in the instance of dental pulp, undifferentiated perivascular progenitors were proposed as the origin of cells for restoration. The isolation of periodontal ligament arrangement stem cells has drawn significant exertion. Numerous groups accounted for such cell populations being separated. Multi-potent progenitors from human periodontal ligaments have been characterized after clone selection and magnetic activated cell categorization with STRO-1.

Periodontal stem cells are typically STRO-1 and CD146 positive. Under distinct culture conditions, periodontal stem cells are multi-potent and display differntaiation ability into cells analogous to cementoblasts, adipocytes and fibroblasts. When periodontal stem cells are transferred into the host mice' cementum, periodontal like structures are generated. Fascinatingly, periodontal stem cells can be separated from the solid frozen human primary tissue. Cells separated from frozen samples have been confirmed to maintain their stem cell characteristics and tissue regeneration ability. Gene expression outlining of periodontal stem cells has also been tried.  The eminent expression of early growth response-1, osteoprotegerin, elastin and IGF binding protein-3 was established.

A primordial subpopulation of periodontal stem cells has been separated from adult rat periodontal by implementing the method of neurosphere development, proposing that periodontal ligaments contain neural-crest produced precursors. These cells express neural crest indicators including Slug, Sox2, Twist, Nestin and Sox 9. Under differentiation circumstances these cells were found to make neuron-like cells proposing that they are factual neural-crest stem cells. Another subpopulation that has been typified in periodontal stem cells is the side population selected by the expression of the ABCG-2 carrier due to dye elimination and fluorescence categorization.  Periodontal stem cells have been established to restore periodontal ligaments in rats when cultured cell layers were transplanted into earlier produced periodontal imperfections of molar teeth.

Atala and Lanza (2007) claim that the dental follicle splits the ability to regenerate with the periodontal ligament. Human dental follicle progenitor cells can be acquired from human third molars and are typified by their plastic attachment in culture and expression of indicators like Notch-1 and Nestin. These cells are regarded as cells with the capability to differentiate into periodontal like arrangements, bone and cementum. Further scrutiny has revealed that the presence of heterogeneous cell populations in developing dental follicles after investigation of their mineralization properties in vitro and the growth factor and matrix protein gene expression patterns from numerous cloned cell lines treated under identical culture conditions. Human dental follicle stem cells obtain cementoblast properties when transplanted into immune-deficient mice. Plentiful achievements have been reported in generating immortalized periodontal stem cells and dental follicle stem cells. Strains of genetically altered mice expressing SV40 large T-antigen have previously been utilized to develop cementoblasts and periodontal stem cells. Bovine cementoblast progenitors have been altered with Bmi-1 to come up with an uninterrupted cell line. Mouse periodontal stem cells were immortalized by bringing in human papilloma virus type 16 E6 genes lacking the PDZ-domain. Human periodontal stem cells were generated after a genetic shift of TERT and SV40 outsized T-antigen.

In human beings, ectoderm-generated epithelial cells take part in the construction of the tooth crown during tooth morphogenesis and maturation, but once distinguished into ameloblasts they stop reproducing.

Yen and Sharpe (2008) claim that normally it is conventional that epithelium cannot redevelop after it reaches the mature stage of growth. Nevertheless in other animals, particularly in rodents, incessantly growing teeth present a perfect model for studying renewal of dental epithelium. Mouse incisors and incessantly developing molars in some mammals show restocking populations of enamel organs comprising of a core stellate reticulum, stratum inter-medium and adjacent enamel epithelial cells. An epithelial stem cell slot in a mouse incisor is situated at its labial apical part which is referred to as a cervical loop.

In a bid to purify cells with more morphogenetic potential, STRO-1 positive cells from dental pulps and bone marrows of juvenile rats were selected. After recombining the two kinds of cells with epithelial bud cells, utilizing absorbable gelatin sponge as a transporter, the pellets were entrenched into renal capsules of rats. Dentin-enamel arrangements of correct shape were seen to be created only with dental pulp-derived cells. Bone marrow cells in association with tooth epithelium fail to generate ameloblast differentiation, even though they did attain uncharacteristic dentinogenesis. This finding proposes that selecting the STRO-1 positive population of stem cells may perhaps be insensible. Most probably this population is devoid of signals that can compel epithelial differentiation to take place, though they can act in response to epithelial signals cells themselves.

So far no epithelial equivalent has been acknowledged that can serve for bioartificial tooth production apart from tooth epithelium (Yu, Shi, & Jin, 2008). Trans-differentiation of epithelia has been reported for other purposes. For instance, oral mucosa epithelium has been utilized with success to produce cornea. It is critical to ascertain a method for doing a transplant of bioartificial tooth germs into the mouth. To find out whether it would be possible to implant tooth germs successfully into oral mucosa using in a method that allows them to continue growing, mouse cap-stage molar tooth germs were surgically cut apart and implanted into the soft tissue of the diastema. In less than a month or so the tooth germs were found to have grown into normal sized teeth. Even bioengineered teeth implanted in a mouse's diastema have been discovered to grow into well developed teeth with dentin, periodontal tissue roots, enamel but no reports of eruption have been revealed. The figures below show replacement of a lost tooth by use of dental implant.

Conclusion     

Dental stem cells are effortlessly available sources of mesenchymal stem cells that have a variety of applications in clinical practices. Currently, the sufficient conditions for the maturation of the pre-formed tooth germs can only be attained by implanting them into suitable hosts and therefore further advancements is needed in methods of organotypic culturing of dental cell groupings without delay. Even with these challenges tooth tissue transplantation continues to be promising; animal models have elucidated its validity and this concept may soon be a recognized dental solution to loss of teeth.

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