Therapeutic Potential of Stem Cells in Regenerative Maxillofacial Surgery – A Review

Introduction

Stem cells are the cells having self-renewing or clonogenic ability that can convert into multiple cells or the non-specialized, generic cells which can make exact copies of itself and can differentiate to produce specialized cells for various tissues of body. These are the basic cells in multicellular organism and have the ability to convert into broad range of adult cells. Two important characteristics include: a) Self renewal b) Totipotency. This unique property of stem cells is very attractive for the researchers in regeneration therapy.^1,2 Stem cells based on biologic properties can be categorized as – a) Pluripotent stem cells, b) Multipotent stem cells.

During development, the pluripotent stem cells get differentiated to specialized multipotent stem cells that in due course forms tissues of the body. Multipotent stem cells get divided to form more restricted specialized cells.³ Stem cells has the potential to build every tissue in human body, thus can be of great therapeutic use in future for tissue repair and regeneration. These cells can grow in different ways to form organs in the body. Stem cells self-renew by undergoing division and give rise to a daughter cell and a progenitor cell (which later achieves differentiation).

Differentiation types:

1) Direct differentiation: Specific cell developed in a multistep pathway.

2) Trans differentiation: Conversion of one cell type to different cell type.

3) Dedifferentiation: Unipotent stem cell gets converted to multipotent stem cell.

4) Cell fusion: Stem cell fuses with somatic cell.⁴

Classification of stem cells based on source

Early/ Embryonic stem cells: Embryonic stem cells are obtained five days after development from the blastocyst’s inner cell mass. It differentiates into germ layers (ectoderm, endoderm, mesoderm) and is capable of multipotential differentiation. Limitations for the use of embryonic stem cell is due to the ethical issues, or the possibility of tumorigenesis. Immune rejections are more common with embryonic stem cells.⁵

Late/ Adult stem cells: These are the multipotent, postnatal stem cells. Adult stem cells contain Hematopoietic, Mesenchymal and Neuronal stem cells. It is obtained after birth from umbilical cord and placenta and also from specific mature tissues of body. These types of cells can be obtained from different tissues such as brain tissue, amniotic fluid, bone marrow, cornea, dental pulp, adipose tissue etc. These cells multiply by cell division to regenerate damaged cells and tissues. Immune rejection reactions and teratoma formation is rare with adult stem cells.⁶

Induced Pluripotent stem cells: This is an emerging concept. About three to four genes found in stem cells are injected into donor cells using proper vectors. Stem cells from exfoliated deciduous teeth, Stem cells from apical papilla, Dental pulp stem cells, Mucosal stem cells, Periodontally derived stem cells have higher regenerative efficacy when induced with four genetic factors such as- Oct3/4, Sox2, Klf4, C-MYC. These can be utilized to generate periodontal tissues, oral mucosa, salivary gland, jaw bones, and teeth.⁷

Scaffold

The material used to carry stem cells while its application to close the defect or in regeneration of organ/tissue is known as scaffold. It can be of various shapes, biomaterials and of different patterns. It can be natural or artificial and degradable or non-degradable. Commonly used materials for scaffold includes polyglycolic acid, polyethylene terephthalate, fibrin, alginate, polylactic acid, collagen etc.

Stem Cell Culture

Culturing the stem cells can be done by transferring the stem cells obtained from their original sources to a lab culture dish containing nutrient broth as the culture media. Cells divides and multiply and spread all over the media plate. They are then removed and plated in other fresh culture dish. This process of sub culturing and replating is repeated number of times. Once the cell line (family of constantly dividing cells) is formed, the original stem cell yields millions of similar stem cells.⁸

Stem Cell Preservation

For the research purpose and for the medical application of therapies based on stem cells, preservation of stem cells is more important. The ability to conserve cells allows complete safety and quality testing before use as well as movements of cells during collection, processing and medical administration. There are various methods of preservation of stem cells but Cryopreservation is the most common technique used. The steps involved include:

1. Pre- freeze processing

2. Introduction of cryo-preservation solution

3. Freezing protocol

4. Storage condition

5. Thawing condition

6. Post thaw assessment

Human Embryonic Stem Cell [hESCs]: Colonies of hESCs placed in vitrification solution composed of DMSO (dimethyl sulfoxide) + EG + 0.5 mol/L sucrose. These colonies are loaded into straws and plunged into liquid nitrogen.⁹

Stem Cell Storage

As the clinical need of stem cells for the treatment and prevention of diseases is growing, the need for proper storage of stem cells is also increasing. Long-term stem cells storage in cell banks is required. Vials are needed for transportation and storage to prevent contamination. Stem cells need to be stored at a temperature that is at least below the glass transition temperature, so as to arrest the molecular process. This can be achieved by storing stem cells in liquid phase or vapour phase of liquid nitrogen and also by mechanical refrigeration which can deliver stable temperature of about 135 degrees which is reliable. As reported by studies, there was no significant difference between liquid nitrogen storage and storage by mechanical refrigeration method for about five years. Fluctuating temperature can be damaging. Each method of preservation and storage of stem cells can be damaging if not performed properly. Continuous storage monitoring system is an absolute requirement.¹⁰

Stem Cells in Maxillofacial Region

Replacement of tissues/ organs in oral and maxillofacial region is difficult because of the delicate functioning of oral tissues like chewing, swallowing, facial expressions as well as articulation and due to the complex anatomical structures formed by hard and soft tissues. These are three-dimensional structures; therefore, the essential requirement to regenerate tissues of this region include – stem cells, growth factors and biomimetic materials. Using stem cell therapy, regeneration of oral and maxillofacial organs and tissues has been possible in recent times.

Stem cells from maxillofacial region include mesenchymal stem cells that can be derived from:

- Dental Pulp Stem Cells (DPSCs)

- Stem Cells from Exfoliated Deciduous Teeth (SHED)

- Periodontal Ligament Stem Cells (PDLSCs)

- Stem Cells from Apical Papilla (SCAP)

- Dental Follicle Progenitor Cells (DFPCs)

- Oral Mucosa derived Stem Cells

- Periosteum derived Stem Cells

- Salivary gland derived Stem Cells¹¹

Advantages of stem cells from maxillofacial region are – possibility of long duration cryopreservation, good interaction with growth factors, high amount of plasticity.

Applications of Stem Cells in Maxillofacial Surgery

Craniofacial defect: They can be used in regeneration of bone and in correction of large craniofacial defects caused due to trauma, tumour resection, or cyst enucleation. Human mandibular bony defect treated with dental pulp stem cells (DPSCs) and collagen sponge biocomplexes using stem cells derived from adipose in a seven year old girl for calvarial defect due to a severe head injury has been reported. Thus, the stem cell regenerative therapy technique has given a new ray of hope for difficult reconstructive procedures.¹²

Soft tissue reconstruction: Soft tissues reconstruction is very important in maxillofacial region as there is loss of soft tissue during trauma or during operative surgical procedures. Use of flaps or grafts cause donor site morbidity. Stem cells of mesenchymal origin can be used for soft tissue reconstruction when exposed to adipogenic medium. These cells with proper scaffold can be used for regeneration and reconstruction of lost soft tissues.¹³

Temporomandibular joint (TMJ): TMJ has complex functions and anatomy, being composed of multiple tissues like bone, ligaments, muscles, synovial membrane, cartilages etc. It is difficult to reproduce the required cellular components of this joint in appropriate place. Stem cells have been identified in TMJ disc and condyle having their origin from neural crest cells. Using three major elements i.e stem cells, bioactive molecule and scaffold, it has become more promising to construct a bioengineered TMJ replacement capable of performing physiologic functional requirements of this joint.¹⁴

Alveolar bone augmentation and implants: Regeneration of alveolar bone defects by osteogenic stem cells can enhance osseointegration in dental implant treatment cases. Osteogenic stem cells in implant osteotomy site forms superior bone by improving vascularity at local site facilitating hard tissue augmentation. Stem cell therapy involving autogenic growth factor rich plasma along with inorganic bone has been employed successfully in sinus floor elevation. It was effective in the formation of new vascularised bone.¹⁵

Tongue regeneration: It has been reported that myoblast progenitor cells carried in collagen gel can be implanted in hemi-glossectomized tongue which would lead to successful myogenesis by muscle regeneration.¹⁶

Salivary gland regeneration: Radiotherapy given to cancers involving the head and neck region leads to impairment in salivary gland function or occurrence of xerostomia. The treatment is only limited to saliva substitutes and sialogogues. Regeneration of salivary gland is now possible mainly by one of the following two approaches:

1) Stem cells application in the damaged tissue region

2) Use of tissue engineering technique for the construction of artificial salivary gland.¹⁷

Facial muscle regeneration: Various methods for regeneration of facial muscles are being researched. Injection of platelet-rich plasma, stem cells and growth factors have been employed with success but while using this technique, proper understanding of the mechanism in the proliferation and differentiation of stem cells and in muscle regeneration is a must.¹⁸

Stem Cell Banking: Process of storing stem cells obtained from patients for future stem cell-based regeneration therapy is known as stem cell banking. The stem cell tissue samples are immersed in specific vials containing nutrient broth as media for transport. It should reach the storage bank before 40 hours. There they are processed, cultured and stored for future use.¹⁸

Discussion

Although we went through all the reviews that focused on dental stem cells, we could find only two with respect to dental stem cells role and clinical application in regenerative aspect.19,20 According to Daltoé FP et al., in their review, it has been concluded that most of the reclaimed studies regarding DPSCs and SHED claim their effectiveness in regeneration in clinical application and bone tissue repair; however, there are few studies supporting the role of stem cells in non-dental tissue functional repair such as cartilages, muscles, blood vessels, neurons, etc.^19,20 In contrast, in a review by de Souza et al., a promising role of human immature dental pulp stem cells (hIDPSCs) for treating systemic diseases, such as Parkinson’s disease and as lupus erythematous, as shown in animal models was discussed.²⁰ These findings show that although these studies were published within a similar span, there are discrepancies in the conclusions among the previously published systematic reviewers.

Challenges faced by oral surgeons in application of stem cells

Major challenge faced by an oral surgeon in bone tissue engineering is due to the vascularization of the graft implemented. Survival of the graft requires speedy and sufficient vascularization. As the oxygen amount is limited to ~150-200μm diffusion distance from a blood vessel supply, the cells lying beyond this suffer from hypoxia.²¹ Under these circumstances, mesenchymal stem cells fail to survive, for not being able to adapt their glucose consumption and thus do not own the necessary glycolytic reserves required to maintain their metabolism for three days and more.²² This underlines the significance of nutrients and oxygen required for survival of cell and energy-related cellular metabolism. Neovascularization along with efficient supply of blood is a prerequisite.

Further exploration in applications of cellular bone tissue engineering should be focused on enhancement of vascularization, since it is a prerequisite for successful bone regeneration. Another challenge is due to the discrepancies in the way of harvesting the human mesenchymal stem cells and whether they are to be directly applied i.e isolated without cultivation or should be cultured ex vivo, along with the donor-dependent variability regarding the potency of bone formation. Thus, further exploration is needed to regulate the use and production of stem cells for their therapeutic applications.

Conclusion

Future is going to be more regenerative based, promising treatment of diseases and regeneration of lost tissues and organs using stem cell therapy. Stem cells have a bright future in therapeutics. Stem cells obtained from all sources have immense medical applications. Therapy using stem cells has given a positive hope to patients where the patient’s own cells can be used for the treatment of disease. Oral and maxillofacial stem cells are easily accessible and can be stored for longer period having mesenchymal potential to differentiate into distinctive types of cells. Although there is no doubt that stem cells have unlimited application in dental and medical fields, proper use of stem cells, proper identification and clinical differentiation are must to avoid its deleterious effects.

Source(s) of support

Nil

Conflicting Interest

None