Characterization of a three-dimensional mucosal equivalent: similarities and differences with native oral mucosa

Collagen: The Superior Material for Full-Thickness Oral Mucosa Tissue Engineering

BACKGROUND

Tissue engineering has significantly progressed in developing full-thickness oral mucosa constructs designed to replicate the natural oral mucosa. These constructs serve as valuable in vitro models for biocompatibility testing and oral disease modeling and hold clinical potential for replacing damaged or lost oral soft tissue. However, one of the major challenges in tissue engineering of the oral mucosa is the identification of an appropriate scaffold with optimal porosity, interconnected porous networks, biodegradability, and biocompatibility. These characteristics facilitate cell migration, nutrient delivery, and vascularization. Various biomaterials have been investigated for constructing tissue-engineered oral mucosa models; collagen has demonstrated superior outcomes compared with other materials.

HIGHLIGHT

This review discusses the different types of tissue-engineered oral mucosa developed using various materials and includes articles published between January 2000 and December 2022 in PubMed and Google Scholar. The review focuses on the superiority of collagen-based scaffolds for tissue engineering of oral mucosa, explores in vitro applications, and discusses potential clinical applications.

CONCLUSION

Among the various scaffold materials used for engineering the connective tissue of the oral mucosa, collagen-based scaffolds possess excellent biological properties, offering high-quality oral mucosa constructs and high resemblance to the native human oral mucosa in terms of histology and expression of various differentiation markers.

Oral mucosa equivalents, prevascularization approaches, and potential applications

BACKGROUND

Oral mucosa equivalents (OMEs) have been used as in vitro models (eg, for studies of human oral mucosa biology and pathology, toxicological and pharmacological tests of oral care products), and clinically to treat oral defects. However, the human oral mucosa is a highly vascularized tissue and implantation of large OMEs can fail due to a lack of vascularization. To develop equivalents that better resemble the human oral mucosa and increase the success of implantation to repair large-sized defects, efforts have been made to prevascularize these constructs.

PURPOSE

The aim of this narrative review is to provide an overview of the human oral mucosa structure, common approaches for its reconstruction, and the development of OMEs, their prevascularization, and in vitro and clinical potential applications.

STUDY SELECTION

Articles on non-prevascularized and prevascularized OMEs were included, since the development and applications of non-prevascularized OMEs are a foundation for the design, fabrication, and optimization of prevascularized OMEs.

CONCLUSIONS

Several studies have reported the development and in vitro and clinical applications of OMEs and only a few were found on prevascularized OMEs using different approaches of fabrication and incorporation of endothelial cells, indicating a lack of standardized protocols to obtain these equivalents. However, these studies have shown the feasibility of prevascularizing OMEs and their implantation in animal models resulted in enhanced integration and healing. Vascularization in tissue equivalents is still a challenge, and optimization of cell culture conditions, biomaterials, and fabrication techniques along with clinical studies is required.

Mucus-producing 3D cell culture models

In vitro cell-based models have been used for a long time since they are normally easily obtained and have an advantageous cost-benefit. Besides, they can serve a variety of ends, from studying drug absorption and metabolism to disease modeling. However, some in vitro models are too simplistic, not accurately representing the living tissues. It has been shown, mainly in the last years, that fully mimicking a tissue composition and architecture can be paramount for cellular behavior and, consequently, for the outcomes of the studies using such models. Because of this, 3D in vitro cell models have been gaining much attention, since they are able to better replicate the in vivo environment. In this review we focus on 3D models that contain mucus-producing cells, as mucus can play a pivotal role in drug absorption. Being frequently overlooked, this viscous fluid can have an impact on drug delivery. Thus, the aim of this review is to understand to which extent can mucus affect mucosal drug delivery and to provide a state-of-the-art report on the existing 3D cell-based mucus models.

An Oral-mucosa-on-a-chip sensitively evaluates cell responses to dental monomers

Knowledge of human gingival cell responses to dental monomers is critical for the development of new dental materials. Testing standards have been developed to provide guidelines to evaluate biological functionality of dental materials and devices. However, one shortcoming of the traditional testing platforms is that they do not recapitulate the multi-layered configuration of gingiva, and thus cannot evaluate the layer-specific cellular responses. An oral mucosa-chip with two cell layers was previously developed as an alternative platform to assess the oral mucosa responses to dental biomaterials. The mucosa-chip consists of an apical keratinocyte layer attached to a fibroblast-embedded collagen hydrogel through interconnecting pores in a three-microchannel network. Here, cell responses in the mucosa-chip were evaluated against 2-hydroxyethyl methacrylate (HEMA), a common monomer used in restorative and aesthetic dentistry. The response of mucosal cell viability was evaluated by exposing the chip to HEMA of concentrations ranging from 1.56 to 25 mM and compared to cells in conventional well-plate monoculture. The co-cultured cells were then stained and imaged with epifluorescence and confocal microscopy to determine the layer-specific responses to the treatment. Mucosa-chips were demonstrated to be more sensitive to assess HEMA-altered cell viability than well-plate cultures, especially at lower doses (1.56 and 6.25 mM). The findings suggest that the mucosa-chip is a promising alternative to traditional platforms or assays to test a variety of biomaterials by offering a multi-layered tissue geometry, accessible layer-specific information, and higher sensitivity in detecting cellular responses.

Keratin expression in gingival tissue and primary cultured gingival keratinocytes: Are there differences?

OBJECTIVE

To investigate whether the process of primary gingival keratinocytes culture obtained from normal human gingiva modifies the expression of keratins (K) 10, K14, and K19.

DESIGN

Human gingival fragments were collected from healthy individuals in the same oral site. One part of the samples underwent an immunohistochemistry assay for K10, K14, and K19. The labeling in the epithelium was quantified using a semiautomated method. Another part was used for primary gingival keratinocytes isolation and growth in two-dimensional culture. These cells were also stained for K10, K14, and K19 using immunofluorescence and immunocytochemistry. Positive cells were counted, and the nuclei and cytoplasmatic labeling areas were quantified.

RESULTS

In the gingival tissue, a higher expression was found for K14 versus K10 (p < 0.001); K19 was negative in all samples. In gingival keratinocytes culture, K14 (89.6 %) had the highest expression with significant differences in relation to K10 (76.9 %, p < 0.01) and K19 (9.9 %, p < 0.01). The cells positive for K14 exhibited larger nuclei in comparison with K10 (p < 0.05) and K19 (p < 0.05), suggesting a more undifferentiated phenotype. K19 cells showed the largest cytoplasmatic labeling in relation to K10- (p < 0.05) and K14-positive (p < 0.05) cells.

CONCLUSION

The process of growth in culture of gingival keratinocytes maintained the expression pattern of K10 and K14 observed in gingival tissues. However, this method induces the expression of K19, suggesting a potential transformation of the keratin network presented in the gingival keratinocytes during the formation of a monolayer in vitro. This reflects the dynamics of cell differentiation.

Photobiomodulation of inflammatory-cytokine-related effects in a 3-D culture model with gingival fibroblasts

The aim of this study was to assess the effects of IL-6 and IL-8 cytokines on human gingival fibroblasts (HGF) cultured in a 3-D model and the possible photobiomodulation (PBM) of such effects by low-level laser therapy. In complete culture medium (DMEM), HGF from a healthy patient were seeded in a type I collagen matrix inserted into 24-well plates. After 5 days of incubation, the cytokines were added or not to serum-free DMEM, which was applied to the cell-enriched matrices. Then, PBM was performed: three consecutive irradiations using LaserTable diode device (780 nm, 0.025 W) at 0.5 J/cm² were delivered or not to the cells. Twenty-four hours after the last irradiation, cell viability and morphology, gene expression, and synthesis of inflammatory cytokines and growth factors were assessed. The histological evaluation demonstrated that, for all groups, matrices presented homogeneous distribution of cells with elongated morphology. However, numerous cytokine-exposed cells were rounded. IL-6 and IL-8 decreased cell viability, synthesis of VEGF, and gene expression of collagen type I. PBM enhanced cell density in the matrices and stimulated VEGF expression, even after IL-6 challenge. Reduced TNF-α synthesis occurred in those cells subjected to PBM. In conclusion, PBM can penetrate collagen matrix and stimulate HGF, highlighting the relevance of this research model for further phototherapy studies and in vitro biomodulation of the healing process.

3D printed tissue engineered model for bone invasion of oral cancer

Recent advances in three-dimensional printing technology have led to a rapid expansion of its applications in tissue engineering. The present study was designed to develop and characterize an in vitro multi-layered human alveolar bone, based on a 3D printed scaffold, combined with tissue engineered oral mucosal model. The objective was to incorporate oral squamous cell carcinoma (OSCC) cell line spheroids to the 3D model at different anatomical levels to represent different stages of oral cancer. Histological evaluation of the 3D tissue model revealed a tri-layered structure consisting of distinct epithelial, connective tissue, and bone layers; replicating normal oral tissue architecture. The mucosal part showed a well-differentiated stratified oral squamous epithelium similar to that of the native tissue counterpart, as demonstrated by immunohistochemistry for cytokeratin 13 and 14. Histological assessment of the cancerous models demonstrated OSCC spheroids at three depths including supra-epithelial level, sub-epithelial level, and deep in the connective tissue-bone interface. The 3D tissue engineered composite model closely simulated the native oral hard and soft tissues and has the potential to be used as a valuable in vitro model for the investigation of bone invasion of oral cancer and for the evaluation of novel diagnostic or therapeutic approaches to manage OSCC in the future.

Characterization of Multilayered Tissue-Engineered Human Alveolar Bone and Gingival Mucosa

Advances in tissue engineering have permitted assembly of multilayered composite tissue constructs for potential applications in the treatment of combined hard and soft tissue defects and as an alternative in vitro test model to animal experimental systems. The aim of this study was to develop and characterize a novel three-dimensional combined human alveolar bone and gingival mucosal model based on primary cells isolated from the oral tissues. Bone component of the model was engineered by seeding primary human alveolar osteoblasts into a hydroxyapatite/tricalcium phosphate scaffold and culturing in a spinner bioreactor. The engineered bone was then laminated, using an adhesive tissue sealant, with tissue-engineered gingival mucosa consisting of air/liquid interface-cultured normal human gingival keratinocytes on oral fibroblast-populated collagen gel scaffold. Histological characterization revealed a structure consisting of established epithelial, connective tissue and bone layers closely comparable to normal oral tissue architecture. The mucosal component demonstrated a mature epithelium undergoing terminal differentiation similar to that characteristic of native buccal mucosa, as confirmed using cytokeratin 13 and cytokeratin 14 immunohistochemistry. Ultrastructural analysis confirmed the presence of desmosomes and hemidesmosomes in the epithelial layer, a continuous basement membrane, and newly synthesized collagen in the connective tissue layer. Quantitative polymerase chain reaction (qPCR) assessment of osteogenesis-related gene expression showed a higher expression of genes encoded collagen I (COL1) and osteonectin (ON) compared with osteocalcin (OC), osteopontin (OP), and alkaline phosphatase (ALP). Enzyme-linked immunosorbent assay quantification of COL1, ON, and OC confirmed a pattern of secretion, which paralleled the model's gene expression profile. We demonstrate in this study that, replicating the anatomical setting between oral mucosa and the underlying alveolar bone is feasible and the developed model showed characteristics similar to those of normal tissue counterparts. This trilayered model therefore offers great scope as an advanced and anatomically representative tissue-engineered alternative to animal models.

Development and Characterization of In Vitro Human Oral Mucosal Equivalents Derived from Immortalized Oral Keratinocytes

Tissue-engineered oral mucosal equivalents (OME) are being increasingly used to measure toxicity, drug delivery, and to model oral diseases. Current OME mainly comprise normal oral keratinocytes (NOK) cultured on top of a normal oral fibroblasts-containing matrix. However, the commercial supply of NOK is limited, restricting widespread use of these mucosal models. In addition, NOK suffer from poor longevity and donor-to-donor variability. Therefore, we constructed, characterized, and tested the functionality of OME based on commercial TERT2-immortalized oral keratinocytes (FNB6) to produce a more readily available alternative to NOK-based OME. FNB6 OME cultured at an air-to-liquid interface for 14 days exhibited expression of differentiation markers cytokeratin 13 in the suprabasal layers and cytokeratin 14 in basal layer of the epithelium. Proliferating cells were restricted to the basal epithelium, and there was immuno-positive expression of E-cadherin confirming the presence of established cell-to-cell contacts. The histology and expression of these structural markers paralleled those observed in the normal oral mucosa and NOK-based models. On stimulation with TNFα and IL-1, FNB6 OME displayed a similar global gene expression profile to NOK-based OME, with increased expression of many common pro-inflammatory molecules such as chemokines (CXCL8), cytokines (IL-6), and adhesion molecules (ICAM-1) when analyzed by gene array and quantitative PCR. Similarly, pathway analysis showed that both FNB6 and NOK models initiated similar intracellular signaling on stimulation. Gene expression in FNB6 OME was more consistent than NOK-based OME that suffered from donor variation in response to stimuli. Mucosal equivalents based on immortalized FNB6 cells are accessible, reproducible and will provide an alternative animal experimental system for studying mucosal drug delivery systems, host-pathogen interactions, and drug-induced toxicity.

Tissue engineering of oral mucosa: a shared concept with skin

Tissue-engineered oral mucosa, in the form of epithelial cell sheets or full-thickness oral mucosa equivalents, is a potential solution for many patients with congenital defects or with tissue loss due to diseases or tumor excision following a craniofacial cancer diagnosis. In the laboratory, it further serves as an in vitro model, alternative to in vivo testing of oral care products, and provides insight into the behavior of the oral mucosal cells in healthy and pathological tissues. This review covers the old and new generation scaffold types and materials used in oral mucosa engineering; discusses similarities and differences between oral mucosa and skin, the methods developed to reconstruct oral mucosal defects; and ends with future perspectives on oral mucosa engineering.

Biological evaluation of enamel sealants in an organotypic model of the human gingiva

OBJECTIVES

Various sealant materials have been suggested to decrease decalcification during orthodontic treatment. However, only a few in vitro studies on the cytotoxicity of resinous pit and fissure sealants have been published, and to the best of our knowledge no similar studies are available for the enamel sealants used in orthodontics. Therefore, we aimed to characterize the possible adverse effects of enamel sealants, especially on the gingival epithelium.

METHODS

Organotypic cultures of the human gingival mucosa were used to assess the possible impact of six enamel sealants. Differentiation and apoptosis were determined by immunofluorescent staining. The pro-inflammatory cytokines IL-1β and IL-6 were quantified by ELISA. Cytotoxicity was measured using MTS assays in monolayer cultures of human gingival fibroblasts. Leaching of monomers from enamel sealants was quantified using HPLC.

RESULTS

The differentiation of the organotypic gingival mucosa remained unaffected. All under-cured and several standard-cured sealants (Light Bond™ Sealant, Light Bond™ Filled Sealant, and L.E.D. Pro Seal®) significantly induced apoptosis in the organotypic model. Light Bond™ Sealant, Light Bond™ Filled Sealant, and L.E.D. Pro Seal® caused a significant induction of pro-inflammatory cytokines. Reducing curing time had an influence on cytotoxicity in monolayer cultures of primary human oral cells. All resin-based sealants leached monomers.

SIGNIFICANCE

Enamel sealants might exert adverse effects on the gingival epithelium. Due to the vicinity of the enamel sealant to the gingival epithelium, and the large surface area of applied sealants, these materials should be carefully applied and sufficiently cured.

Tissue-engineered mucosa is a suitable model to quantify the acute biological effects of ionizing radiation

The aim of this study was to evaluate the suitability of tissue-engineered mucosa (TEM) as a model for studying the acute effects of ionizing radiation (IR) on the oral mucosa. TEM and native non-keratinizing oral mucosa (NNOM) were exposed to a single dose of 16.5Gy and harvested at 1, 6, 24, 48, and 72h post-irradiation. DNA damage induced by IR was determined using p53 binding protein 1 (53BP1), and DNA repair was determined using Rad51. Various components of the epithelial layer, basement membrane, and underlying connective tissue were analyzed using immunohistochemistry. The expression of cytokines interleukin-1β (IL-1β) and transforming growth factor beta 1 (TGF-β1) was analyzed using an enzyme-linked immunosorbent assay. The expression of DNA damage protein 53BP1 and repair protein Rad51 were increased post-irradiation. The expression of keratin 19, vimentin, collage type IV, desmoglein 3, and integrins α6 and β4 was altered post-irradiation. Proliferation significantly decreased at 24, 48, and 72h post-irradiation in both NNOM and TEM. IR increased the secretion of IL-1β, whereas TGF-β1 secretion was not altered. All observed IR-induced alterations in TEM were also observed in NNOM. Based on the similar response of TEM and NNOM to IR we consider our TEM construct a suitable model to quantify the acute biological effects of IR.