Distribution of genes for parathyroid hormone (PTH)-related peptide, Indian hedgehog, PTH receptor and patched in the process of experimental spondylosis in mice

Functional characterization of hypertrophy in chondrogenesis of human mesenchymal stem cells

OBJECTIVE

Mesenchymal stem cells (MSCs) are promising candidate cells for cartilage tissue engineering. Expression of cartilage hypertrophy markers (e.g., type X collagen) by MSCs undergoing chondrogenesis raises concern for a tissue engineering application for MSCs, because hypertrophy would result in apoptosis and ossification. To analyze the biologic basis of MSC hypertrophy, we examined the response of chondrifying MSCs to culture conditions known to influence chondrocyte hypertrophy, using an array of hypertrophy-associated markers.

METHODS

Human MSC pellet cultures were predifferentiated for 2 weeks in a chondrogenic medium, and hypertrophy was induced by withdrawing transforming growth factor beta (TGFbeta), reducing the concentration of dexamethasone, and adding thyroid hormone (T3). Cultures were characterized by histologic, immunohistochemical, and biochemical methods, and gene expression was assessed using quantitative reverse transcription-polymerase chain reaction.

RESULTS

The combination of TGFbeta withdrawal, a reduction in the level of dexamethasone, and the addition of T3 was essential for hypertrophy induction. Cytomorphologic changes were accompanied by increased alkaline phosphatase activity, matrix mineralization, and changes in various markers of hypertrophy, including type X collagen, fibroblast growth factor receptors 1-3, parathyroid hormone-related protein receptor, retinoic acid receptor gamma, matrix metalloproteinase 13, Indian hedgehog, osteocalcin, and the proapoptotic gene p53. However, hypertrophy was not induced uniformly throughout the pellet culture, and distinct regions of dedifferentiation were observed.

CONCLUSION

Chondrogenically differentiating MSCs behave in a manner functionally similar to that of growth plate chondrocytes, expressing a very similar hypertrophic phenotype. Under the in vitro culture conditions used here, MSC-derived chondrocytes underwent a differentiation program analogous to that observed during endochondral embryonic skeletal development, with the potential for terminal differentiation. This culture system is applicable for the screening of hypertrophy-inhibitory conditions and agents that may be useful to enhance MSC performance in cartilage tissue engineering.

PTHrP, PTHr, and FGFR3 are involved in the process of endochondral ossification in human osteophytes

To elucidate the process of endochondral ossification in human osteophytes we have studied the expression of parathyroid hormone-related protein (PTHrP), its receptor (PTHr), and fibroblast growth factor receptor 3 (FGFR3). Osteophytes from patients undergoing total knee replacement ( n=13), and fetal growth plate cartilages ( n=4) were processed for safranin O staining and immunohistochemistry. Chondrocytes and their matrix were preferentially stained for PTHrP in the middle and deep zones of the osteophytes examined. Ossified areas did not show a positive staining. In fetal joints the cartilaginous surface and the perichondrium as well as the osteoblasts in the trabecular bone were positive. PTHr was expressed at large in chondrocytes and osteoblasts of all osteophytes and fetal joints. Cells of the perichondrium were also positive. The FGFR3 antibody stained only single chondrocytes in some osteophytes, and groups of cells in others. In fetal samples, chondrocytes of the proliferating and the hypertrophic zone showed staining for FGFR3. This is the first report on the expression of PTHrP, PTHr, and FGFR3 in human osteophytes. As in fetal joints these mediators might regulate proliferation and differentiation of chondrocytes playing an important role in osteo(chondro)phyte growth.