Role of Heterotypic Cellular Interactions in the Regeneration of the Anterior Cruciate Ligament-to-Bone Interface

The anterior cruciate ligament (ACL) inserts into subchondral bone through a fibrocartilage interface with controlled spatial distribution of different cell types and biochemical composition. The function of the interface is to minimize stress concentration and mediate load transfer between the soft and hard tissue. The fibrocartilage interface is however not regenerated post ACL reconstruction surgery using semitendinosus grafts, and the lack of graft-bone integration is one of the primary causes of graft failure. Currently, the mechanism for interface regeneration is not known. To address the challenge of biological fixation and to elucidate the underlying mechanism, our working hypothesis is that interactions between cells derived from soft tissue (fibroblasts) and bone (osteoblast) are important in the fibrochondrogenic differentiation of progenitor cell populations, such as fibroblasts or bone marrow stromal cells (BMSC). To test this hypothesis, we first performed an interface characterization study in order to identify interface-related markers and to establish the baseline parameters for evaluating the fibrochondrogenic differentiation of BMSC. Subsequently, co-culture and tri-culture models mimicking the multi-cellular organization at the native ACL-to-bone interface are designed to determine the effects of heterotypic cellular interactions on the development of fibrocartilage-related markers. These novel multi-culture models allow controlled interactions via paracrine communication as well as cell-cell physical contact. It was observed that co-culture of fibroblasts and osteoblasts lead to phenotypic changes towards the fibrochondrocytic phenotype, and subsequent tri-culture of these two cell types with BMSC induced stem cell differentiation and the expression of fibrochondrocyte-related markers. In addition, the effects of fibroblast-osteoblast interactions are found to be mediated by paracrine communication and are further enhanced by fibroblast-osteoblast physical contact. Furthermore, mechanical loading modulates these cellular interactions in tri-culture. These observations collectively delineate the importance of fibroblast-osteoblast interactions in promoting interface regeneration and the multi-culture models developed in this thesis represent an innovative approach to examine the relevance of heterotypic cellular interactions. Findings from these studies have yielded new insights into the mechanisms governing interface regeneration, and it is anticipated that these critical understandings will facilitate the development of innovative tissue engineering strategies aimed at promoting biological graft fixation and soft tissue-to-bone integration.

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Year

Entry

1999

Buschmann MD, Kim Y-J, Wong M, Frank E, Hunziker EB, Grodzinsky AJ. Stimulation of aggrecan synthesis in cartilage explants by cyclic loading is localized to regions of high interstitial fluid flow1. Arch Biochem Biophys. June 1, 1999;366(1):1-7.

1998

Benjamin M, Ralphs JR. Fibrocartilage in tendons and ligaments: an adaptation to compressive load. J Anat. 1998;193(4):481-494.

1992

Buschmann MD, Gluzband YA, Grodzinsky AJ, Kimura JH, Hunziker EB. Chondrocytes in agarose culture synthesize a mechanically functional extracellular matrix. J Orthop Res. November 1992;10(6):745-758.

1995

Buschmann MD, Gluzband YA, Grodzinsky AJ, Hunziker EB. Mechanical compression modulates matrix biosynthesis in chondrocyte/agarose culture. J Cell Sci. April 1995;108(4):1497-1508.

1997

Chen CS, Mrksich M, Huang S, Whitesides GM, Ingber DE. Geometric control of cell life and death. Science. May 30, 1997;276(5317):1425.