Tissue engineering, stem cells and cloning: current concepts and changing trends

Functional ectodermal organ regeneration as the next generation of organ replacement therapy

In this decade, substantial progress in the fields of developmental biology and stem cell biology has ushered in a new era for three-dimensional organ regenerative therapy. The emergence of novel three-dimensional cell manipulation technologies enables the effective mimicking of embryonic organ germ formation using the fate-determined organ-inductive potential of epithelial and mesenchymal stem cells. This advance shows great potential for the regeneration of functional organs with substitution of complete original function in situ. Organoids generated from multipotent stem cells or tissue stem cells via establishment of an organ-forming field can only partially recover original organ function owing to the size limitation; they are considered ‘mini-organs’. Nevertheless, they hold great promise to realize regenerative medicine. In particular, regeneration of a functional salivary gland and an integumentary organ system by orthotopic and heterotopic implantation of organoids clearly points to the future direction of organ regeneration research. In this review, we describe multiple strategies and recent progress in regenerating functional three-dimensional organs, focusing on ectodermal organs, and discuss their potential and future directions to achieve organ replacement therapy as a next-generation regenerative medicine.

Advances in Functional Restoration of the Lacrimal Glands

The lacrimal glands produce tears to support a healthy homeostatic environment on the ocular surface. The lacrimal gland dysfunction characteristic of dry eye disease causes ocular discomfort and visual disturbances and in severe cases can result in a loss of vision. The demand for adequate restoration of lacrimal gland function has been intensified due to advances in stem cell biology, developmental biology, and bioengineering technologies. In addition to conventional therapies, including artificial tears, tear alternatives (such as autologous serum eye drops) and salivary gland transplantation, a regenerative medicine approach has been identified as a novel strategy to restore the function of the lacrimal gland. Recent studies have demonstrated the potential of progenitor cell injection therapy to repair the tissue of the lacrimal glands. A current three-dimensional (3D) tissue engineering technique has been shown to regenerate a secretory gland structure by reproducing reciprocal epithelial-mesenchymal interactions during ontogenesis in vitro and in vivo. A novel direct reprogramming method has suggested a possibility to induce markers in the lacrimal gland developmental process from human pluripotent stem cells. The development of this method is supported by advances in our understanding of gene expression and regulatory networks involved in the development and differentiation of the lacrimal glands. Engineering science has proposed a medical device to stimulate tearing and a bio-hybrid scaffold to reconstruct the 3D lacrimal gland structure. In this review, we will summarize recent bioengineering advances in lacrimal gland regeneration toward the functional restoration of the lacrimal glands as a future dry eye therapy.

Application of adult mesenchymal stem cells in bone and vascular tissue engineering

Tissue engineering is a very promising field of regenerative medicine. Life expectancy has been increasing, and tissue replacement is increasingly needed in patients suffering from various degenerative disorders of the organs. The use of adult mesenchymal stem cells (e.g. from adipose tissue or from bone marrow) in tissue engineering seems to be a promising approach for tissue replacements. Clinical applications can make direct use of the large secretome of these cells, which can have a positive influence on other cells around. Another advantage of adult mesenchymal stem cells is the possibility to differentiate them into various mature cells via appropriate culture conditions (i.e. medium composition, biomaterial properties, and dynamic conditions). This review is focused on current and future ways to carry out tissue replacement of damaged bones and blood vessels, especially with the use of suitable adult mesenchymal stem cells as a potential source of differentiated mature cells that can later be used for tissue replacement. The advantages and disadvantages of different stem cell sources are discussed, with a main focus on adipose-derived stem cells. Patient factors that can influence later clinical applications are taken into account.

Stem cells: their source, potency and use in regenerative therapies with focus on adipose-derived stem cells - a review

Stem cells can be defined as units of biological organization that are responsible for the development and the regeneration of organ and tissue systems. They are able to renew their populations and to differentiate into multiple cell lineages. Therefore, these cells have great potential in advanced tissue engineering and cell therapies. When seeded on synthetic or nature-derived scaffolds in vitro, stem cells can be differentiated towards the desired phenotype by an appropriate composition, by an appropriate architecture, and by appropriate physicochemical and mechanical properties of the scaffolds, particularly if the scaffold properties are combined with a suitable composition of cell culture media, and with suitable mechanical, electrical or magnetic stimulation. For cell therapy, stem cells can be injected directly into damaged tissues and organs in vivo. Since the regenerative effect of stem cells is based mainly on the autocrine production of growth factors, immunomodulators and other bioactive molecules stored in extracellular vesicles, these structures can be isolated and used instead of cells for a novel therapeutic approach called "stem cell-based cell-free therapy". There are four main sources of stem cells, i.e. embryonic tissues, fetal tissues, adult tissues and differentiated somatic cells after they have been genetically reprogrammed, which are referred to as induced pluripotent stem cells (iPSCs). Although adult stem cells have lower potency than the other three stem cell types, i.e. they are capable of differentiating into only a limited quantity of specific cell types, these cells are able to overcome the ethical and legal issues accompanying the application of embryonic and fetal stem cells and the mutational effects associated with iPSCs. Moreover, adult stem cells can be used in autogenous form. These cells are present in practically all tissues in the organism. However, adipose tissue seems to be the most advantageous tissue from which to isolate them, because of its abundancy, its subcutaneous location, and the need for less invasive techniques. Adipose tissue-derived stem cells (ASCs) are therefore considered highly promising in present-day regenerative medicine.

Modern outlook on morphological criteria of organ and tissue decellularization

A rapidly growing development of tissue engineering promotes the increasing interest in the obtainment of various decellularizedtissues and organs. Minimal quality evaluation criteria of obtained tissue engineered constructions have been previously specified. In the discussionpaper the group of authors considers the morphological methods of matrix evaluation applied by various researchers on the model of heart decellularization. The analysis of modern literature and the authors’ own researches have shown that morphological evaluation of decellularization quality has to be complex and should consist of several stages which include both basic and additional evaluation methods.

A Ligation of the Lacrimal Excretory Duct in Mouse Induces Lacrimal Gland Inflammation with Proliferative Cells

The lacrimal gland secretes tear fluids to ocular surface, which plays an indispensable role in maintaining the health of the ocular epithelia and protecting the ocular surface from the external environment. The dysfunction of the lacrimal glands causes dry eye disease due to a reduction in tear volume. The dry eye disease is becoming a popular public disease, for the number of patients is increasing, who have subjective symptom and loss of vision, which affect the quality of life. Inflammatory change in the damaged lacrimal gland has been reported; however, a major challenge is to establish a simple animal model to observe the changes. Here, we demonstrated an injury model by ligating the main excretory duct of the lacrimal gland, which is a simple and stable way to clearly understand the mechanism of lacrimal gland inflammation. We observed the process of injury and proliferation of the lacrimal gland and detected a population of lacrimal gland epithelial cells with proliferation potential which were also nestin-positive cells following duct ligation. This study successfully established an injury model to observe the tissue injury process of the lacrimal gland, and this model will be useful for analysis of the inflammation and proliferation mechanism in the future.

Practical whole-tooth restoration utilizing autologous bioengineered tooth germ transplantation in a postnatal canine model

Whole-organ regeneration has great potential for the replacement of dysfunctional organs through the reconstruction of a fully functional bioengineered organ using three-dimensional cell manipulation in vitro. Recently, many basic studies of whole-tooth replacement using three-dimensional cell manipulation have been conducted in a mouse model. Further evidence of the practical application to human medicine is required to demonstrate tooth restoration by reconstructing bioengineered tooth germ using a postnatal large-animal model. Herein, we demonstrate functional tooth restoration through the autologous transplantation of bioengineered tooth germ in a postnatal canine model. The bioengineered tooth, which was reconstructed using permanent tooth germ cells, erupted into the jawbone after autologous transplantation and achieved physiological function equivalent to that of a natural tooth. This study represents a substantial advancement in whole-organ replacement therapy through the transplantation of bioengineered organ germ as a practical model for future clinical regenerative medicine.

Generation of a Bioengineered Lacrimal Gland by Using the Organ Germ Method

In organogenesis including lacrimal gland development, cell arrangement within a tissue plays an important role. The lacrimal gland develops from embryonic ocular surface epithelium through reciprocal epithelial and mesenchymal interaction, which is organized by interactive regulation of various pathways of signaling molecules. Current development of an in vitro three-dimensional cell manipulation procedure to generate a bioengineered organ germ, named as the organ germ method, has shown the regeneration of a histologically correct and fully functional bioengineered lacrimal gland after engraftment in vivo. This method demonstrated a possibility of lacrimal gland organ replacement to treat dry eye disease, which has been a public health problem leading reduction of visual function. Here, we describe protocols for lacrimal gland germ regeneration using the organ germ method and methods for analyzing the function of the bioengineered lacrimal gland after its transplantation in vivo.

Tooth Organ Bioengineering: Cell Sources and Innovative Approaches

Various treatment approaches for restoring missing teeth are being utilized nowadays by using artificial dental crowns/bridges or the use of dental implants. All aforementioned restorative modalities are considered to be the conventional way of treating such cases. Although these artificial therapies are commonly used for tooth loss rehabilitation, they are still less conservative, show less biocompatibility and fail to restore the natural biological and physiological function. Adding to that, they are considered to be costly due to the risk of failure and they also require regular maintenance. Regenerative dentistry is currently considered a novel therapeutic concept with high potential for a complete recovery of the natural function and esthetics of teeth. Biological-cell based dental therapies would involve replacement of teeth by using stem cells that will ultimately grow a bioengineered tooth, thereby restoring both the biological and physiological functions of the natural tooth, and are considered to be the ultimate goal in regenerative dentistry. In this review, various stem cell-based therapeutic approaches for tooth organ bioengineering will be discussed.

Whole Tooth Regeneration as a Future Dental Treatment

Dental problems caused by dental caries, periodontal disease and tooth injury compromise the oral and general health issues. Current advances for the development of regenerative therapy have been influenced by our understanding of embryonic development, stem cell biology, and tissue engineering technology. Tooth regenerative therapy for tooth tissue repair and whole tooth replacement is currently expected a novel therapeutic concept with the full recovery of tooth physiological functions. Dental stem cells and cell-activating cytokines are thought to be candidate approach for tooth tissue regeneration because they have the potential to differentiate into tooth tissues in vitro and in vivo. Whole tooth replacement therapy is considered to be an attractive concept for next generation regenerative therapy as a form of bioengineered organ replacement. For realization of whole tooth regeneration, we have developed a novel three-dimensional cell manipulation method designated the "organ germ method". This method involves compartmentalisation of epithelial and mesenchymal cells at a high cell density to mimic multicellular assembly conditions and epithelial-mesenchymal interactions in organogenesis. The bioengineered tooth germ generates a structurally correct tooth in vitro, and erupted successfully with correct tooth structure when transplanted into the oral cavity. We have ectopically generated a bioengineered tooth unit composed of a mature tooth, periodontal ligament and alveolar bone, and that tooth unit was engrafted into an adult jawbone through bone integration. Bioengineered teeth were also able to perform physiological tooth functions such as mastication, periodontal ligament function and response to noxious stimuli. In this review, we describe recent findings and technologies underpinning whole tooth regenerative therapy.

Detection of hepatic maturation by Raman spectroscopy in mesenchymal stromal cells undergoing hepatic differentiation

Introduction

Mesenchymal stromal cells (MSCs) are well known for their application potential in tissue engineering. We previously reported that MSCs are able to differentiate into hepatocytes in vitro. However, conventional methods for estimating the maturation of hepatic differentiation require relatively large amounts of cell samples. Raman spectroscopy (RS), a photonic tool for acquisition of cell spectra by inelastic scattering, has been recently used as a label-free single-cell detector for biological applications including phenotypic changes and differentiation of cells and diagnosis. In this study, RS is used to real-time monitor the maturation of hepatic differentiation in live MSCs.

Methods

The MSCs were cultured on the type I collagen pre-coating substrate and differentiated into hepatocytes in vitro using a two-step protocol. The Raman spectra at different time points are acquired in the range 400–3000 cm^–1and analyzed by quantification methods and principle component analysis during hepatic differentiation from the MSCs.

Results

The intensity of the broad band in the range 2800–3000 cm^–1 reflects the amount of glycogen within lipochrome in differentiated hepatocytes. A high correlation coefficient between the glycogen amount and hepatic maturation was exhibited. Moreover, principle component analysis of the Raman spectra from 400 to 3000 cm^–1 indicated that MSC-derived hepatocytes were close to the primary hepatocytes and were distinct from the undifferentiated MSCs.

Conclusions

In summary, RS can serve as a rapid, non-invasive, real-time and label-free biosensor and reflects changes in live cell components during hepatic differentiation. The use of RS may thus facilitate the detection of hepatic differentiation and maturation in stem cells. Such an approach may substantially improve the feasibility as well as shorten the time required compared to the conventional molecular biology methods.

Regeneration and repair of human digits and limbs: fact and fiction

A variety of digit and limb repair and reconstruction methods have been used in different clinical settings, but regeneration remains an item on every plastic surgeon's "wish list." Although surgical salvage techniques are continually being improved, unreplantable digits and limbs are still abundant. We comprehensively review the structural and functional salvage methods in clinical practice, from the peeling injuries of small distal fingertips to multisegmented amputated limbs, and the developmental and tissue engineering approaches for regenerating human digits and limbs in the laboratory. Although surgical techniques have forged ahead, there are still situations in which digits and limbs are unreplantable. Advances in the field are delineated, and the regeneration processes of salamander limbs, lizard tails, and mouse digits and each component of tissue engineering approaches for digit- and limb-building are discussed. Although the current technology is promising, there are many challenges in human digit and limb regeneration. We hope this review inspires research on the critical gap between clinical and basic science, and leads to more sophisticated digit and limb loss rescue and regeneration innovations.

Cell Therapy for Stress Urinary Incontinence

Urinary incontinence (UI) is the involuntary loss of urine and is a common condition in middle-aged and elderly women and men. Stress urinary incontinence (SUI) is caused by leakage of urine when coughing, sneezing, laughing, lifting, and exercise, even standing leads to increased intra-abdominal pressure. Other types of UI also exist such as urge incontinence (also called overactive bladder), which is a strong and unexpected sudden urge to urinate, mixed forms of UI that result in symptoms of both urge and stress incontinence, and functional incontinence caused by reduced mobility, cognitive impairment, or neuromuscular limitations that impair mobility or dexterity. However, for many SUI patients, there is significant loss of urethral sphincter muscle due to degeneration of tissue, the strain and trauma of pregnancy and childbirth, or injury acquired during surgery. Hence, for individuals with SUI, a cell-based therapeutic approach to regenerate the sphincter muscle offers the advantage of treating the cause rather than the symptoms. We discuss current clinically relevant cell therapy approaches for regeneration of the external urethral sphincter (striated muscle), internal urethral sphincter (smooth muscle), the neuromuscular synapse, and blood supply. The use of mesenchymal stromal/stem cells is a major step in the right direction, but they may not be enough for regeneration of all components of the urethral sphincter. Inclusion of other cell types or biomaterials may also be necessary to enhance integration and survival of the transplanted cells.

Functional tooth regenerative therapy: tooth tissue regeneration and whole-tooth replacement

Oral and general health is compromised by irreversible dental problems, including dental caries, periodontal disease and tooth injury. Regenerative therapy for tooth tissue repair and whole-tooth replacement is currently considered a novel therapeutic concept with the potential for the full recovery of tooth function. Several types of stem cells and cell-activating cytokines have been identified in oral tissues. These cells are thought to be candidate cell sources for tooth tissue regenerative therapies because they have the ability to differentiate into tooth tissues in vitro and in vivo. Whole-tooth replacement therapy is regarded as an important model for the development of an organ regenerative concept. A novel three-dimensional cell-manipulation method, designated the organ germ method, has been developed to recapitulate organogenesis. This method involves compartmentalisation of epithelial and mesenchymal cells at a high cell density to mimic multicellular assembly conditions and epithelial-mesenchymal interactions. A bioengineered tooth germ can generate a structurally correct tooth in vitro and erupt successfully with the correct tooth structure when transplanted into the oral cavity. We have ectopically generated a bioengineered tooth unit composed of a mature tooth, periodontal ligament and alveolar bone, and that tooth unit was successfully engrafted into an adult jawbone through bone integration. Such bioengineered teeth were able to perform normal physiological tooth functions, such as developing a masticatory potential in response to mechanical stress and a perceptive potential for noxious stimuli. In this review, we describe recent findings and technologies underpinning tooth regenerative therapy.

Functional lacrimal gland regeneration by transplantation of a bioengineered organ germ

The lacrimal gland has a multifaceted role in maintaining a homeostatic microenvironment for a healthy ocular surface via tear secretion. Dry-eye disease, which is caused by lacrimal gland dysfunction, is one of the most prevalent eye diseases that cause corneal epithelial damage and results in significant loss of vision and a reduction in the quality of life. Here we demonstrate orthotopic transplantation of bioengineered lacrimal gland germs into adult mice with an extra-orbital lacrimal gland defect, a mouse model that mimics the corneal epithelial damage caused by lacrimal gland dysfunction. The bioengineered lacrimal gland germs and harderian gland germs both develop in vivo and achieve sufficient physiological functionality, including tear production in response to nervous stimulation and ocular surface protection. This study demonstrates the potential for bioengineered organ replacement to functionally restore the lacrimal gland.

Lacrimal glands maintain a healthy corneal epithelium but are dysfunctional for example in dry-eye disease. Here, the authors transplant bioengineered lacrimal and harderian gland germs into mice, where they connect to the host duct and nervous system and restore lacrimal gland function.

Development and prospects of organ replacement regenerative therapy

Current approaches for the development of regenerative therapies have been influenced by our understanding of embryonic development, stem cell biology, and tissue engineering technology. The ultimate goal of regenerative therapy is to develop fully functioning bioengineered organs to replace lost or damaged organs that result from disease, injury, or aging. Almost all organs including ectodermal organs, such as teeth, hair, salivary glands, and lacrimal glands, arise from organ germs induced by reciprocal epithelial-mesenchymal interactions in the developing embryo. A novel concept to generate a bioengineered organ is to recreate organogenesis and thereby develop fully functioning bioengineered organs from the resulting bioengineered organ germ generated via 3-dimensional cell manipulation using immature stem cells in vitro. We have previously developed a bioengineering method for forming a 3-dimensional organ germ in the early developmental stages, termed the "bioengineered organ germ method." Recently, we reported fully functioning bioengineered tooth replacements after transplantation of a bioengineered tooth germ or a mature tooth unit comprising the bioengineered tooth and periodontal tissues. This concept could be adopted to generate not only teeth but also bioengineered hair follicles, salivary glands, and lacrimal glands. These studies emphasize the potential for bioengineered organ replacement in future regenerative therapies. In this review, we will summarize the strategies and the recent progress of research and development for the establishment of organ replacement regenerative therapies.

Tissue engineering for plastic surgeons: a primer

A central tenet of reconstructive surgery is the principle of "replacing like with like." However, due to limitations in the availability of autologous tissue or because of the complications that may ensue from harvesting it, autologous reconstruction may be impractical to perform or too costly in terms of patient donor-site morbidity. The field of tissue engineering has long held promise to alleviate these shortcomings. Scaffolds are the structural building blocks of tissue-engineered constructs, akin to the extracellular matrix within native tissues. Commonly used scaffolds include allogenic or xenogenic decellularized tissue, synthetic or naturally derived hydrogels, and synthetic biodegradable nonhydrogel polymeric scaffolds. Embryonic, induced pluripotent, and mesenchymal stem cells also hold immense potential for regenerative purposes. Chemical signals including growth factors and cytokines may be harnessed to augment wound healing and tissue regeneration. Tissue engineering is already clinically prevalent in the fields of breast augmentation and reconstruction, skin substitutes, wound healing, auricular reconstruction, and bone, cartilage, and nerve grafting. Future directions for tissue engineering in plastic surgery include the development of prevascularized constructs and rationally designed scaffolds, the use of stem cells to regenerate organs and tissues, and gene therapy.

Molecular regulatory mechanism of tooth root development

The root is crucial for the physiological function of the tooth, and a healthy root allows an artificial crown to function as required clinically. Tooth crown development has been studied intensively during the last few decades, but root development remains not well understood. Here we review the root development processes, including cell fate determination, induction of odontoblast and cementoblast differentiation, interaction of root epithelium and mesenchyme, and other molecular mechanisms. This review summarizes our current understanding of the signaling cascades and mechanisms involved in root development. It also sets the stage for de novo tooth regeneration.

Orchestrating cell/material interactions for tissue engineering of surgical implants

Research groups are currently recognising a critical clinical need for innovative approaches to organ failure and agenesis. Allografting, autologous reconstruction and prosthetics are hampered with severe limitations. Pertinently, readily available 'laboratory-grown' organs and implants are becoming a reality. Tissue engineering constructs vary in their design complexity depending on the specific structural and functional demands. Expeditious methods on integrating autologous stem cells onto nanoarchitectured 3D nanocomposites, are being transferred from lab to patients with a number of successful first-in-man experiences. Despite the need for a complete understanding of cell/material interactions tissue engineering is offering a plethora of exciting possibilities in regenerative medicine.

An in situ forming collagen-PEG hydrogel for tissue regeneration

There are limited options for surgeons to repair simple or complex tissue defects due to injury, illness or disease. Consequently, there are few treatments for many serious ailments, including neural-related injuries, myocardial infarction and focal hyaline cartilage defects. Tissue-engineered scaffolds offer great promise for addressing these wide-ranging indications; however, there are many considerations that need to be made when conceptualizing a product. For many applications, an in situ forming scaffold that could completely fill defects with complex geometries, adhere to adjacent tissues and foster cell proliferation would be ideal. Additionally, the scaffold would preferably have tailored mechanical properties similar to native tissues and highly controllable gelation kinetics, and would not require an external trigger, such as ultraviolet light, for gelation. We have developed a unique injectable hydrogel system composed of collagen and multi-armed poly(ethylene glycol) (PEG) that meets all of these criteria. The collagen component enables cellular adhesion and permits enzymatic degradation, while the multi-armed PEG component has amine-reactive chemistry that also binds proteins/tissue and is hydrolytically degradable. We have characterized the mechanical properties, swelling, degradation rates and cytocompatibility of these novel hydrogels. The hydrogels demonstrated tunable mechanics, variable swelling and suitable degradation profiles. Cells adhered and proliferated to near confluence on the hydrogels over 7 days. These data suggest that these collagen and PEG hydrogels exhibit the mechanical, physical and biological properties suitable for use as an injectable tissue scaffold for the treatment of a variety of simple and complex tissue defects.

Generation of a bioengineered tooth by using a three-dimensional cell manipulation method (organ germ method)

The arrangement of cells within a tissue plays an essential role in organogenesis, including tooth development. Organ morphogenesis and physiological functions induced by three-dimensional tissue organization are well known to be regulated by the proper spatiotemporal organization of various signaling molecules, including cytokines, extracellular matrix proteins, and adhesion molecules. Development of a three-dimensional cell manipulation technology to create a bioengineered organ germ, designated as the organ germ method, enabled the generation of a structurally correct and fully functional bioengineered tooth in vivo. This method is expected to be utilized as a valuable technique for analyzing gene and protein functions during organogenesis. Here, we describe protocols for tooth germ reconstitution using the organ germ method and methods for analyzing tooth development in vitro and in vivo.

Functional tooth regeneration using a bioengineered tooth unit as a mature organ replacement regenerative therapy

Donor organ transplantation is currently an essential therapeutic approach to the replacement of a dysfunctional organ as a result of disease, injury or aging in vivo. Recent progress in the area of regenerative therapy has the potential to lead to bioengineered mature organ replacement in the future. In this proof of concept study, we here report a further development in this regard in which a bioengineered tooth unit comprising mature tooth, periodontal ligament and alveolar bone, was successfully transplanted into a properly-sized bony hole in the alveolar bone through bone integration by recipient bone remodeling in a murine transplantation model system. The bioengineered tooth unit restored enough the alveolar bone in a vertical direction into an extensive bone defect of murine lower jaw. Engrafted bioengineered tooth displayed physiological tooth functions such as mastication, periodontal ligament function for bone remodeling and responsiveness to noxious stimulations. This study thus represents a substantial advance and demonstrates the real potential for bioengineered mature organ replacement as a next generation regenerative therapy.

Kidney spheroids recapitulate tubular organoids leading to enhanced tubulogenic potency of human kidney-derived cells

Cell-based approaches utilizing autologous human renal cells require their isolation, expansion in vitro, and reintroduction back into the host for renal tissue regeneration. Nevertheless, human kidney epithelial cells (hKEpCs) lose their phenotype, dedifferentiate, and assume the appearance of fibroblasts after relatively few passages in culture. We hypothesized that growth conditions may influence hKEpC phenotype and function. hKEpCs retrieved from human nephrectomy tissue samples showed the ability to reproducibly form kidney spheres when grown in suspension culture developed in nonadherent conditions. Genetic labeling and time-lapse microscopy indicated, at least in part, the aggregation of hKEpCs into 3D spheroids rather than formation of pure clonally expanded spheres. Characterization of hKEpC spheroids by real-time polymerase chain reaction and FACS analysis showed upregulation of some renal developmental and "stemness" markers compared with monolayer and mostly an EpCAM(+)CD24(+)CD133(+)CD44(+) spheroid cell phenotype. Oligonucleotide microarrays, which were used to identify global transcriptional changes accompanying spheroid formation, showed predominantly upregulation of cell matrix/cell contact molecules and cellular biogenesis processes and downregulation of cell cycle, growth, and locomotion. Accordingly, hKEpC spheroids slowly proliferated as indicated by low Ki-67 staining, but when grafted in low cell numbers onto the chorioallantoic membrane (CAM) of the chick embryo, they exclusively reconstituted various renal tubular epithelia. Moreover, efficient generation of kidney spheroids was observed after long-term monolayer culture resulting in reestablishment of tubulogenic capacity upon CAM grafting. Thus, generation of a tubular organoid in hKEpC spheroids may provide a functional benefit for kidney-derived cells in vivo.

Micromanipulation of culture niche permits long-term expansion of dental pulp stem cells--an economic and commercial angle

Stem cells isolated from dental pulp possess the capacity for self-renewal and the potential for multi-lineage differentiation. However, dental pulp stem cells have different characteristics in terms of their culture conditions. The success of stem cells culture is governed by its micro-environmental niche. Therefore, we studied the effects of culture niche on long-term expansion of dental pulp stem cells in terms of cell morphology, growth kinetics, senescence pattern, cell surface marker expression differentiation capacity, and seeding plating density of dental pulp stem cells in four different, widely used media composition Among the various basal media tested, α-minimum essential media and knock out-minimum essential media supplemented with 10% fetal bovine serum were found to be the most optimal media composition in preserving the phenotypic characteristics and differentiation potential for prolonged periods as compared with DMEM-F12 and DMEM-LG. Plating density has been shown to affect overall yield. As a conclusion, the adoption of an appropriate culture system significantly improved cell yield, thus enabling the attainment of sufficient yields for therapeutic applications economizing in terms of cost of production and minimizing seeding cell density for maximum yield.

Engineering hydrogels as extracellular matrix mimics

Extracellular matrix (ECM) is a complex cellular environment consisting of proteins, proteoglycans, and other soluble molecules. ECM provides structural support to mammalian cells and a regulatory milieu with a variety of important cell functions, including assembling cells into various tissues and organs, regulating growth and cell-cell communication. Developing a tailored in vitro cell culture environment that mimics the intricate and organized nanoscale meshwork of native ECM is desirable. Recent studies have shown the potential of hydrogels to mimic native ECM. Such an engineered native-like ECM is more likely to provide cells with rational cues for diagnostic and therapeutic studies. The research for novel biomaterials has led to an extension of the scope and techniques used to fabricate biomimetic hydrogel scaffolds for tissue engineering and regenerative medicine applications. In this article, we detail the progress of the current state-of-the-art engineering methods to create cell-encapsulating hydrogel tissue constructs as well as their applications in in vitro models in biomedicine.

High-throughput reconstitution of epithelial-mesenchymal interaction in folliculoid microtissues by biomaterial-facilitated self-assembly of dissociated heterotypic adult cells

The aim of this study was to develop a method for efficient production of folliculoid keratinocyte-dermal papilla (DP) microtissues to facilitate epithelial-mesenchymal interaction. The behavior of DP cells and adult keratinocytes from hairless skin on poly(ethylene-co-vinyl alcohol) (EVAL) surface was investigated. Keratinocytes, poorly adherent both to substrate and between homotypic cells, become suspended disperse cells after homotypic cell seeding. Seeded simultaneously, keratinocytes and DP cells are able to aggregate into spheroidal microtissues. Dynamical analysis shows that DP cells act as a carrier in the process due to the heterotypic intercellular adhesion. DP cells attach faster to EVAL and start to aggregate. Keratinocytes adhere to DP cells and are then carried by DP cells to form initial hybrid aggregates. Due to the high motility of DP cells, these hybrid aggregates move collectively as clusters and merge into larger spheroids which subsequently detach from the substratum and can be easily collected. Compared with random cell distribution in spheroids generated in hanging drops, these hybrid spheroids have a preferential compartmented core-shell structure: an aggregated DP cell core surrounded by a keratinocyte shell. In addition to ameliorated DP signature gene expression, keratinocytes show down-regulated epidermal terminal differentiation and enhanced follicular differentiation. Functionally, these microtissues are able to grow hairs in vivo. This work sheds light on the complex effects and dynamics of cell-cell and cell-substratum interaction in the patterning of heterotypic cells into tissue forms and is of potential to be applied to mass generation of other epithelial organ primordia in vitro.

Fully functional bioengineered tooth replacement as an organ replacement therapy

Current approaches to the development of regenerative therapies have been influenced by our understanding of embryonic development, stem cell biology, and tissue engineering technology. The ultimate goal of regenerative therapy is to develop fully functioning bioengineered organs which work in cooperation with surrounding tissues to replace organs that were lost or damaged as a result of disease, injury, or aging. Here, we report a successful fully functioning tooth replacement in an adult mouse achieved through the transplantation of bioengineered tooth germ into the alveolar bone in the lost tooth region. We propose this technology as a model for future organ replacement therapies. The bioengineered tooth, which was erupted and occluded, had the correct tooth structure, hardness of mineralized tissues for mastication, and response to noxious stimulations such as mechanical stress and pain in cooperation with other oral and maxillofacial tissues. This study represents a substantial advance and emphasizes the potential for bioengineered organ replacement in future regenerative therapies.

A method to improve cellular content for corporal tissue engineering

We have previously shown that penile corporal structures engineered using autologous cells are able to achieve erection, penetration, and ejaculation. However, fully functional corpora could not be engineered because of the limited cellular content present within the corporal tissue construct. In this study, we investigated whether a dynamic seeding approach would improve cellularity within the corporal tissue construct and thereby restore normal erectile function. Corporal cells were either statically or dynamically seeded on acellular corporal tissue matrices and maintained in a bioreactor system. After 48 h, the cell-matrix complexes were implanted subcutaneously in athymic mice and analyzed for cell attachment, survival, and distribution using histological and molecular techniques. Native tissues and matrices without cells served as controls. The seeded cells attached and proliferated within the sinusoidal walls of the matrices. After completing the seeding, the DNA and cellular content of the dynamically seeded matrices reached 71% of normal corpora, whereas the statically seeded matrices reached 39% of normal corpora. These findings were confirmed histologically, biochemically, and using scanning electron microscopy. This study demonstrates that dynamic cell attachment, using a bioreactor system, leads to the formation of morphologically and biochemically improved corporal tissue, which may be useful for penile reconstruction.

In vitro generation of three-dimensional renal structures

End-stage renal disease is currently being treated effectively by transplantation. However, increasing demand and donor shortage make this treatment challenging. Recent advances in cell-based therapies have provided potential opportunities to alleviate the current challenges of donor shortage. In this study we developed a system to generate renal structures in vitro using primary kidney cells. This system involves the cultivation of expanded primary renal cells in a three-dimensional collagen-based culture system. After one week of growth, individual renal cells began to form renal structures resembling tubules and glomeruli. Histologically, these structures show phenotypic resemblance to native kidney structures. The reconstituted tubules stained positively for Tamm-Horsfall protein, which is expressed in the thick ascending limb of Henle's Loop and distal convoluted tubules. These results show that renal structures can be reconstituted in a three-dimensional culture system, which may eventually be used for renal cell therapy applications.

Potential for access to embryonic-like cells from human umbilical cord blood

All too often media attention clouds the reality that there are many types of stem cell. The embryos, bone marrow and umbilical cord blood (UCB) are the three most used sources. However, despite what it would appear, embryonic stem cells have not been the first to yield life-saving cures at present. Faster routes to clinical intervention have been using adult stem cells that can be sourced from bone marrow and from cord blood, and that are readily accessible and are more ethically acceptable to the general public. Both these non-embryonic sources have been able to provide sufficient numbers of cells to allow development of clinical translational protocols. Bone marrow-derived cells have been used successfully in myocardial infarct therapy where relining by endothelial tissue has allowed limited reperfusion to damaged cardiac tissue. UCB have also demonstrated significant success for around 20 years in haematotransplantation. With a global human population in excess of 6 billion, births thus UCB, remain the largest untouched source of stem cells available every year. UCB also provide a distinct advantage over other adult stem cells due to the length of the telomere and also due protected immunological status of the developing neonatal environment. The total mutation load in the UCB populations is clearly likely to be significant less than in adult tissues.

Tooth Regenerative Therapy, Approached from Organogenesis

Regenerative medicine is expected to be a novel therapeutic system in this century [1-3]. The human body consists of 200 cell species generated from immature stem cells. In the 1990s, a treatment transplanting hematopoietic stem cells to replace all blood cells was established and successfully cured leukemia [4]. With this as a model, stem cell transplantation therapy is being developed to restore the partial loss of organ function [5, 6]. The ultimate goal of regenerative medicine is to replace loss or damaged organs with artificial organs, so-called organ replacement therapy. Technical development to produce “tissues” made of a single cell species modeled on skin, bone, heart muscle, and cornea is advancing, but little development of organs per se has been attempted. In the sections that follow, we discuss why and explain how we are trying with the problems of “tooth regeneration.”

Tunica repair with acellular bladder matrix maintains corporal tissue function

Penile conditions, such as Peyronie's disease or tumor resection may require surgical reconstruction of the tunica albuginea. Various materials have been proposed, as a biomaterial for tunica albuginea repair, however, little functional data are available. We examined the applicability and functional outcome of a collagen-based matrix derived from the bladder (acellular bladder matrix (ABM)), as a biomaterial for tunica repair. Biocompatibility testing was performed on the matrix, which included mitochondrial metabolic activity, cell viability and apoptosis. Approximately 50% of the dorsal penile tunica albuginea was replaced with the collagen-based matrix patch after surgical removal in 24 New Zealand White rabbits. Cavernosometry and cavernosography were performed. The animals were killed 1, 2 and 3 months after surgery for analyses. The matrix showed excellent biocompatibility. All animals implanted with the matrix survived without any noticeable untoward effects. There was no evidence of inflammation or infection at the time of retrieval. Cavernosometry of the implanted animals demonstrated normal intracavernosal pressures with visual erections. Cavernosography of the repaired corpora showed a normal anatomical configuration. Biomechanical analysis of the retrieved matrices demonstrated similar tensile strengths as native tunica. Histologically, there was only a minimal inflammatory response, which gradually decreased over time. These results show that ABM is biocompatible, durable and effective when used as a tunica substitute. The matrix may be useful as an off-the-shelf biomaterial for patients requiring tunica albuginea repair.

Wnt-dependent de novo hair follicle regeneration in adult mouse skin after wounding

The mammalian hair follicle is a complex 'mini-organ' thought to form only during development; loss of an adult follicle is considered permanent. However, the possibility that hair follicles develop de novo following wounding was raised in studies on rabbits, mice and even humans fifty years ago. Subsequently, these observations were generally discounted because definitive evidence for follicular neogenesis was not presented. Here we show that, after wounding, hair follicles form de novo in genetically normal adult mice. The regenerated hair follicles establish a stem cell population, express known molecular markers of follicle differentiation, produce a hair shaft and progress through all stages of the hair follicle cycle. Lineage analysis demonstrated that the nascent follicles arise from epithelial cells outside of the hair follicle stem cell niche, suggesting that epidermal cells in the wound assume a hair follicle stem cell phenotype. Inhibition of Wnt signalling after re-epithelialization completely abrogates this wounding-induced folliculogenesis, whereas overexpression of Wnt ligand in the epidermis increases the number of regenerated hair follicles. These remarkable regenerative capabilities of the adult support the notion that wounding induces an embryonic phenotype in skin, and that this provides a window for manipulation of hair follicle neogenesis by Wnt proteins. These findings suggest treatments for wounds, hair loss and other degenerative skin disorders.

The development of a bioengineered organ germ method

To bioengineer ectodermal organs such as teeth and whisker follicles, we developed a three-dimensional organ-germ culture method. The bioengineered tooth germ generated a structurally correct tooth, after both in vitro organ culture as well as transplantation under a tooth cavity in vivo, showing penetration of blood vessels and nerve fibers. Our method provides a substantial advance in the development of bioengineered organ replacement strategies and regenerative therapies.

Craniofacial tissue engineering by stem cells

Craniofacial tissue engineering promises the regeneration or de novo formation of dental, oral, and craniofacial structures lost to congenital anomalies, trauma, and diseases. Virtually all craniofacial structures are derivatives of mesenchymal cells. Mesenchymal stem cells are the offspring of mesenchymal cells following asymmetrical division, and reside in various craniofacial structures in the adult. Cells with characteristics of adult stem cells have been isolated from the dental pulp, the deciduous tooth, and the periodontium. Several craniofacial structures--such as the mandibular condyle, calvarial bone, cranial suture, and subcutaneous adipose tissue--have been engineered from mesenchymal stem cells, growth factor, and/or gene therapy approaches. As a departure from the reliance of current clinical practice on durable materials such as amalgam, composites, and metallic alloys, biological therapies utilize mesenchymal stem cells, delivered or internally recruited, to generate craniofacial structures in temporary scaffolding biomaterials. Craniofacial tissue engineering is likely to be realized in the foreseeable future, and represents an opportunity that dentistry cannot afford to miss.

Mechanical stimulation induces morphological and phenotypic changes in bone marrow-derived progenitor cells within a three-dimensional fibrin matrix

One of the major limitations in tissue engineering is cell sourcing. Multipotent progenitor cells appear to have many promising features for that purpose. Mechanical stimulation is known to play an important role in determining cell phenotype. The aim of this work was to investigate the effects of cyclic stretch on rat bone marrow derived progenitor cell (BMPC) morphology and smooth muscle-directed differentiation within a three-dimensional fibrin matrix. BMPCs were suspended in a fibrin gel, pipetted into the trough of Flexcell Tissue-Train plates, and stimulated with 10% longitudinal cyclic stretch at 1 Hz for 6 days. Unconstrained (stress- and strain-free) and static anchored (constrained but not stretched) samples were used as controls. Stress filament area per cell was increased in the stretched samples compared to static anchored and free-float controls. Cells in the free float controls were randomly aligned, while they aligned parallel to the direction of the stress or strain in the other groups. Immunofluorescence suggested an increased expression of smooth muscle markers (smooth muscle alpha actin and h1-calponin) in both stretched and constrained control samples, but not in unconstrained controls. Qualitative assessment suggested that collagen production was increased in both mechanically stimulated samples. Proliferation was inhibited in stretched samples compared to the constrained controls. This work suggests an ability of rat BMPCs to differentiate toward a smooth-muscle-cell-like lineage when exposed to biomechanical stimulation in a three-dimensional model. The observation that the constrained samples induced changes in BMPCs suggests that stress alone may be stimulatory, but addition of cyclic stretch appears to augment the responses.

Biomaterials: a primer for surgeons

Biomaterials offer the surgeon a powerful set of clinical tools for patient treatment and are found in virtually every instrument, device, implant, or piece of equipment in the operating room. In fact, surgeons have historically driven clinical application of biomaterials and stand uniquely positioned to contribute to the ongoing development of biomaterials. Having an understanding of the materials available and their basic properties can contribute to better and more effective outcomes. This article provides an overview of the biomaterials field. It begins with a definition and abbreviated history of the field, highlighting its clinical roots. An introduction to the four material classifications--metals, polymers, ceramics, and composites--is then presented, providing the reader with basic properties of each group and examples of materials. Sections on nanotechnology and tissue engineering also briefly describe development within the field. Finally, the evolution of treatments for pectus excavatum and congenital diaphragmatic hernias are presented, highlighting the role of biomaterials. While providing a primer of the field, this paper shows the broad interdisciplinary reach of material science in surgery and suggests sources for further investigations.