Discussion

The findings of the presented study indicate that the porous sponge-like Col-HA composites have good biocompatibility and biomimetic properties and may be used as scaffolds for bone tissue regeneration. The Col-HA composites with ratios 80:20 and 50:50 supported the attachments and proliferations of mouse MSCs and hPDSCs. These findings indicate that Col-HA composite complexes have strong potentials for bone tissue regeneration.

A biocompatible scaffold is one of the basic ingredients for bone tissue regeneration. The scaffold provides the framework for cell growth and differentiation at the implant sites. An ideal scaffold should facilitate cell attachment and proliferation and have a suitable biodegradation profile and good tissue biocompatibility. The Col-HA composites developed in this study appear to be good candidates for scaffolds that can be used in bone tissue-engineering constructs. The major solid components of human bone are hydroxyapatite [Ca₁₀(PO₄)₆(OH₂)] (a natural ceramic, also found in teeth) and collagen (a natural polymer, also found in skin, cartilage, blood vessel walls, and tendons). The Col-HA composites have been shown to have the potential to mimic and replace skeletal tissues.

In natural bones, carbonate substituted HA crystals are mineralized within small gaps of the collagen fibrils and have been quoted as 50 × 25 × 2–5 nm in length, width, and thickness, respectively. The ideal bone graft materials should mimic the nano-textured environment to the natural bone. Nano-structured biomaterials are identified as less than 100 nm in at least one dimension. The HA crystals or particles produced by the direct precipitation method in this study are nano-meter scaled and resemble those of natural bone. This type of HA is perceived to be beneficial for bone regeneration applications, as it possesses the functional properties that facilitate cell growth and bone formation.

The 3-dimensional (3D) scaffolds provide the necessary support for cells to adhere, proliferate, and in combination with host tissues, produce a mineralized matrix. Various 3D scaffolds have been seeded with osteoblasts or its precursor cells for tissue engineering applications. In the present study, we used 3D collagen-hydroxyapatite composites and MSCs for bone regenerations. The images of the prototype type I collagen without HA and 3 different ratios of Col-HA composites are sponge-like plugs with different density and rigidity. Several methods for synthesis of Col-HA composite exist. We found that the methods developed in the present study using direct precipitation of HA on collagen fibrils, freeze-drying, and thermal cross-linking, consistently produced porous Col-HA composites that are suitable for stem cell and growth factor loading.

Osteogenesis is the mechanism for forming bone directly from osteoblasts. Osteoinduction means that bone graft materials are capable of inducing the transformation of mesenchymal cells into osteoblasts, and thus enhance bone growth. Osteoconduction is the process that permits bone apposition from the existing bone. Both type I collagen and hydroxyapatite are osteoconductive materials. While combined together, they were shown to promote osteogenesis by osteoblasts. Biomimetic collagen-hydroxyapatite composites were also shown to have good biocompatibility and biologic properties in animals and humans.

MSCs possess self-renewing capacity and multilineage differentiation potential. High rate of ex vivo proliferation capacity makes these cells promising therapeutic candidates for many human diseases. Adult human MSCs, isolated from bone marrow or periosteum, have been shown to differentiate into a variety of mesodermal cell lineages, including osteoblasts. Multipotent human MSCs from various oral tissues including periodontium (PDSCs), dental pulp, apical papillae, and exfoliated deciduous teeth have been isolated and characterized. The hPDSCs used in the present study are capable of differentiating into odontoblasts, adipocytes, neural cells, and osteoinductive cells. As hPDSCs have excellent biocompatibility with the Col-HA scaffolds, they may be a suitable resource of cells for maxillofacial and alveolar bone regeneration.