No abstract

The chondrogenic differentiation of bone marrow-derived human mesenchymal stem cells (MSCs) in a collagen type I hydrogel, which is in clinical use for matrix-based autologous chondrocyte transplantation (ACT), was investigated. Collagen hydrogels with 2.5 x 10(5) MSCs/mL were fabricated and cultured for 3 weeks in a serum-free, defined, chondrogenic differentiation medium containing 10 ng/mL TGF-beta1 or 100 ng/mL BMP-2. Histochemistry revealed morphologically distinct, chondrocyte-like cells, surrounded by a sulfated proteoglycan-rich extracellular matrix in the TGF-beta1 and BMP-2 treated group, with more elongated cells seen in the BMP-2 treated group. Immunohistochemistry detected collagen type II (Col II) in the TGF-beta1 and BMP-2 treated group. Collagen type X (Col X) staining was positive in the TGF-beta1 but only very weak in the BMP-2 treated group. RT-PCR analyses revealed a specific chondrogenic differentiation with the expression of the cartilage specific marker genes Col II, Col X, and aggrecan (AGN) in the TGF-beta1 and the BMP-2 treated group, with earlier expression of these marker genes in the TGF-beta1 treated group. Interestingly, MSC-gels cultured in DMEM with 10% FBS (control) indicated few isolated chondrocyte-like cells but no expression of Col II or Col X could be detected. The results show, that MSCs cultured in a collagen type I hydrogel are able to undergo a distinct chondrogenic differentiation pathway, similar to that described for MSCs cultured in high-density pellet cultures. These findings are valuable in terms of ex vivo predifferentiation or in situ differentiation of MSCs in collagen hydrogels for articular cartilage repair.

Blood vessel formation is a prerequisite for bone healing. In this study, we tested the hypothesis that a delay in bone healing is associated with an altered regulation of blood vessel formation. A tibial osteotomy was performed in two groups of sheep and stabilized with either a rigid external fixator leading to standard healing or with a highly rotationally unstable one leading to delayed healing. At days 4, 7, 9, 11, 14, 21, and 42 after surgery, total RNA was extracted from the callus. Gene expressions of vWF, an endothelial cell marker, and of several molecules related to blood vessel formation were studied by qPCR. Furthermore, histology was performed on fracture hematoma and callus sections. Histologically, the first blood vessels were detected at day 7 in both groups. mRNA expression levels of vWF, Ang1, Ang2, VEGF, CYR61, FGF2, MMP2, and TIMP1 were distinctly lower in the delayed compared to the standard healing group at several time points. Based on differential expression patterns, days 7 and 21 postoperatively were revealed to be essential time points for vascularization of the ovine fracture callus. This work demonstrates for the first time a differential regulation of blood vessel formation between standard and mechanically induced delayed healing in a sheep osteotomy model. ß

Core binding factor alpha 1 (Cbfa1) is an osteoblast-specific transcription factor essential to develop a mature osteoblast phenotype. However, its exact role in the signaling of various osteotropic-differentiating agents is still unclear. In this study, we assessed the effects of 1,25-(OH)(2)-D3 (D3), ascorbic acid, bone morphogenetic protein-2 (BMP-2), dexamethasone (Dex), and transforming growth factor-beta (TGF-beta) on Cbfa1 and osteocalcin (OCN) mRNA steady state levels (by semiquantitative RT-PCR) in an in vitro model of osteoblast differentiation. TGF-beta increased Cbfa1 mRNA levels in normal primary human osteoblasts (pHOB) by 2.6-fold in a time-dependent fashion with maximum effect on day 28 (P < 0.001). Similarly, the glucocorticoid Dex enhanced Cbfa1 gene expression by pHOB in a time-dependent fashion by up to 4.6-fold (P < 0.001). In contrast, Dex inhibited OCN gene expression levels by 68% (P < 0.01). Treatment with BMP-2 resulted in an earlier enhancement of Cbfa1 and led to a 4.2-fold increase with a maximum on day 21 (P < 0.001). Ascorbic acid did not modulate Cbfa1 and OCN gene expression. The effect of vitamin D (D3) on Cbfa1 mRNA expression was influenced by the duration of treatment, being inhibitory after 1 h and having a stimulatory effect after 48 h. Time course experiments indicated a stimulatory effect of D3 on Cbfa1 mRNA levels (by 2.5-fold after 48 h; P < 0.01). Analysis of the late cellular differentiation marker osteocalcin revealed that D3 increased OCN gene expression by 14-fold (P < 0.001). In conclusion, in normal primary human osteoblasts, the rapid and pronounced increase of OCN after treatment with D3 seems not to be mediated by Cbfa1. These data imply that Cbfa1 gene expression is differentially regulated by various osteoblastic differentiating agents and is dependent on the stage of maturation.

No abstract