There is an ever-increasing clinical need for the regeneration and replacement of tissue to replace soft tissue lost due to trauma, disease and cosmetic surgery. A potential alternative to the current treatment modalities is the use of tissue engineering applications using mesenchymal stem cells that have been identified in many tissue including the infrapatellar fat pad. In this study, stem cells isolated from the infrapatellar fat pad were characterised to ascertain their origin, and allowed to undergo adipogenic differentiation to confirm multilineage differentiation potential.

The infrapatellar fat pad was obtained from total knee replacement for osteoarthritis. Cells were isolated and expanded in monolayer culture. Cells at passage 2 stained strongly for CD13, CD29, CD44, CD90 and CD105 (mesenchymal stem cell markers). The cells stained poorly for LNGFR and STRO1 (markers for freshly isolated bone marrow derived stem cells), and sparsely for 3G5 (pericyte marker). Staining for CD34 (haematopoetic marker) and CD56 (neural and myogenic lineage marker) was negative.

For adipogenic differentiation, cells were cultured in adipogenic inducing medium consisting of basic medium with 10ug/ml insulin, 1uM dexamthasone, 100uM indomethacin and 500uM 3-isobutyl-1-methyl xanthine. By day 16, many cells had lipid vacuoles occupying most of the cytoplasm. On gene expression analyses, the cells cultured under adipogenic conditions had almost a 1,000 fold increase in expression of peroxisome proliferator-activated receptor gamma-2 (PPAR gamma-2) and 1,000,000 fold increase in expression of lipoprotein lipase (LPL). Oil red O staining confirmed the adipogenic nature of the observed vacuoles and showed failure of staining in control cells.

Our results show that the human infrapatellar fat pad is a viable potential autogeneic source for mesenchymal stem cells capable of adipogenic differentiation as well as previously documented ostegenic and chondrogenic differentiation. This cell source has potential use in tissue engineering applications.

Introduction: Articular cartilage is frequently damaged but only shows a limited capacity for repair. Autologous chondrocytes are being used for the repair of focal articular cartilage defects in the ankle but their use has limitations. The use of undifferentiated progenitor cells from other sources is limited by the fact that these cells loose there stem cell characterisation with passage in culture. The fat pad derived stem cells are a possible alternative that maintain multipotentiality at higher passages. We explore the hypothesis that their cell surface characterisation will resemble that of mesenchymal stem cells and will not alter with passage.

Materials and Methods: Cells were isolated from the human fat pad and expanded in monolayer culture. On confluence, they were harvested by digestion and replated at a ratio of 1:3. Cells from passage 2, 4, 6, 8 and 10 were stained and analysed using flow cytometry for a panel of stem cell surface antibodies.

Results: Fat pad derived cells stained strongly for CD13, 29, 44 and 90 (markers of mesenchymal stem cells). The cells stained poorly for 3G5 (pericyte marker), CD34 and CD56 (marker for haematopoetic lineage), and LNG FR and STRO 1 (markers of bone marrow stem cells). These results suggest that the fat pad cell population has surface expression characteristics of mesenchymal stem cells, but differ from bone marrow derived stem cells. It is also important to note that the expression of these cell-surface markers was maintained up to passage 10.

Conclusion: The consistent pattern of cell surface expression, with little change with passage, shows that the proliferation and expansion of the fat pad stem cell population does not lead to major changes in phenotype of these cells. This can potentially allow a significant increase in number sufficient for clinical applications without loosing their multipotentiality.