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We aim to demonstrate the role of Alk receptors in the response of hydrogel expansion. Chondrocytes from rat knees were cultured onto plastic and hydrogel surfaces. Alk-1 and Alk-5 were overexpressed or silenced and the effects on cells during expansion were tested and confirmed using peptide inhibitors for TGFβ. Overexpression of Alk-5 and silencing of Alk-1 led to a loss of the chondrocyte phenotype, proving that they are key regulators of chondrocyte mechanosensing. An analysis of the gene expression profile during the expansion of these modified cartilage cells in plastic showed a better maintenance of the chondrocyte phenotype, at least during the first passages. These passages were also assayed in a mouse model of intramuscular chondrogenesis. Our findings indicate that these two receptors are important mediators in the response of chondrocytes to changes in the mechanical environment, making them suitable targets for modulating chondrogenesis. Inhibition of TGFβ could also be effective in improving chondrocyte activity in aged or expanded cells that overexpress Alk-1.
Low back pain is one of the most common medical conditions in the Western world. Disc degeneration, an inevitable process of ageing, is one of the major causes of low back pain. Autologous chondrocyte transplantation (ACT) is an increasingly popular method of addressing pathological disorders of cartilage. The purpose of our study was to determine whether autologous chondrocytes from elastic cartilage could survive and synthesise a cartilage specific matrix in the intervertebral disc of rabbits. Sixteen lumbar intervertebral discs (IVD) of New Zealand White rabbits were analysed. In 6 IVD, the nucleus pulposus was evacuated and replaced with tissue engineered autologous chondrocytes from auricular cartilage. In the second group, only the nucleus pulposus was evacuated from 6 IVD, with no chondrocytes implantation. Four non-operated IVD were used as a control. Six months after the operation, the animals were euthanized and the IVD were analysed histologically. Autologous cartilage implants were well tolerated by the host for up to six months in vivo. There was only hyaline-like cartilage in the place of the nucleus pulposus. We could not detect any elastic fibres in the new cartilage matrix. In IVD from which only the nucleus pulposus was evacuated and no chondrocytes were implanted, just fibrous tissue was found instead of nucleus pulposus. The overall histological analysis of new cartilage produced after implantation in our study confirmed the hypothesis that ACT from auricular cartilage can be implanted into the IVD instead of the nucleus pulposus and that a significant percentage of implanted chondrocytes survive and produce hyaline-like cartilage.
Chondrocytes differentiate from mesenchymal progenitors and produce templates (anlagen) for the developing bones. Chondrocyte differentiation is controlled by Sox transcription factors. Templates for the neighbour bones are subsequently separated by conversion of differentiated chondrocytes into non-chondrogenic cells and emergence of interzone in which joints cavitation occurs. A central role in initiating synovial joint formation plays Wnt-14/beta-catenin signalling pathway. Moreover, bone morphogenetic proteins and growth and differentiation factors are expressed at the site of joint formation. Joint cavitation is associated with increased hyaluronic acid synthesis. Hyaluronic acid facilitates tissue separation and creation of a functional joint cavity. According to the traditional view articular cartilage represents part of cartilage anlage that is not replaced by bone through endochondral ossification. Recent studies indicate, however, that peri-joint mesenchymal cells take part in interzone formation and that these interzone cells subsequently differentiate into articular chondrocytes and synovial cells. Thus, anlage chondrocytes have a transient character and disappear after cessation of growth plate function while articular chondrocytes have stable and permanent phenotype and function throughout life. (Folia Morphol 2013; 72, 3: 181–187)
Recent results of the clinical outcome of autologous chondrocyte transplantation (ACT) treatment in a group of 28 patients with focal femoral condyle cartilage lesions revealed a correlation trend with the quality of the in vitro cell culture matrix-protein synthesis. No impact of the patients' age and chondrocyte cryopreservation prior to implantation was observed. Further studies are needed to confirm the preliminary results.
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