Introduction

Recently, Autogenous transplantation has been successfully performed to repair various hard tissue defects. However, the tissue damage and surgical invasion are often considered to be crucial problems to the patient. It has, thus far, been highly anticipated the development of a cell transplantation technique with a certain artificial scaffold, resulting in a bone or cartilage tissue engineering, for a more secure and safe tissue defect treatment. An anagenetic technique using mesenchymal stem cells (MSCs) has a good affinity between the organism and material, because MSCs can proliferate quickly and differentiate into various tissues [1]. In combination with a three-dimensional culture system, it seems to be possible to develop a tissue block or scaffold consisting of MSCs, which can alter itself to a tissue component equivalent to peripheral tissue. However, an appropriate scaffold has not yet been developed for a three-dimensional culture system of MSCs.

Collagen is the predominant protein in hard tissue, and easily metabolized in the general organs and tissues. In addition, collagen solution has a unique characteristic; i.e. it undergoes gelatinization by adjusting the concentration, pH and temperature [2]. Furthermore, collagen has the ability to promote ossification and subsequent bone formation [3]. Therefore, collagen is considered to be an efficient transplant material for tissue regeneration. Recently, we have established a three-dimensional collagen gel culture system to generate bone and cartilage from MSCs [4]. However, the system requires a long period to generate hard tissues from MSCs. Therefore, the development of additional materials that enable a quick differentiation of MSCs into chondrocytes and osteoblasts is required.

The high expression of hyaluronic acid (HA) during endochondral bone formation is indispensable to bone and cartilage development. HA is a highly and widely distributed over various tissues as a component of extracellular matrices. In fetal tissues, HA is particularly rich [5, 6], but the content decreases during development [6]. Interestingly, HA has been reported to promote the migration and proliferation of MSCs [7, 8], and is associated with the acceleration of wound healing [9]. Furthermore, HA binds to specific cell-surface receptors, such as CD44 and the receptor for HA-mediated motility (RHAMM), and also binds to other matrix molecules, such as collagen and proteoglycans [6, 10-12]. Thus far, we hypothesized that HA exerts certain influences on the differentiation process from MSCs and becomes an appropriate scaffold for the three-dimensional culture system of MSCs.

This study was conducted to examine the differentiation of human MSCs embedded in the HA-collagen hybrid gels into chondrocytes and osteoblasts, and to elucidate the usefulness of HA as an additional scaffold for the three-dimensional collagen gel culture system of MSCs.

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