Abstract. In bone forming cartilage in vivo, cells undergo terminal differentiation, whereas most of the cells in normal articular cartilage do not . Chondrocyte hypertrophy can be induced also in vitro by diffusible signals. We have identified growth factors or hormones acting individually on 17-d chick embryo sternal chondrocytes cultured in agarose gels under strictly serumfree conditions. Insulin-like growth factor I or insulin triggered the first steps of chondrocyte maturation, i.e., cell proliferation and increased matrix deposition while the chondrocytic phenotype was maintained. However, cells did not progress to the hypertrophic stage. Proliferation and stimulated collagen production was preceded by a lag period, indicating that synthesis C ARTILAGE cells in situ modulate their phenotype in response to environmental stimuli . Pluripotent mesenchYmal cells differentiate into resting chondrocYtes characterized by a moderate rate of synthesis of aggregating proteoglycans and collagens 11, IX, and XI. Most of the cells maintain this phenotype in joints where cartilage is permanent . During endochondral ossification in development, growth, and repair of bones, however, resting chondrocytes differentiate further. Initially, they proliferate and, later, they become hypertrophic in that they drastically increase both their volume and metabolic activity. Hypertrphic cells deposit large quantities ofextracellular matrix containing cartilage-specific proteoglycans and collagens including collagen X, a marker for this stage ofdifferentiation (Schmid and Linsenmayer, 1985). In contrast to resting and proliferative cartilage cells, hypertrophic cells also produce alkaline phosphatase .The phenotypic flexibility ofchondrocytes is also apparent in vitro . In monolayer culture, the cells can gradually lose the cartilage phenotype and begin to resemble mesenchymal cells (Holtzer et al ., 1960) and they produce macromolecules not normally found in cartilage (Mayne et al ., 1976;von der Mark et al ., 1977) . By contrast, in suspension culture stabilized by semisolid gels (Horwitz and Dorfman, 1970), chondrocytes remain differentiated and cells modulated in monolayer culture reexpress the cartilage phenotype C The Rockefeller University Press,
Abstract. Chondrocytes in dense suspension culture in agarose survive in serum-free DME because they secrete low molecular mass compounds supporting their own viability. This activity can be replaced by pyruvate, or sulfhydryl compounds, e.g., cysteine or dithioerythritol. Catalase, an enzyme decomposing H202, also protects the cells, whereas superoxide dismutase has no effect. Therefore, chondrocytes in culture are sensitive to toxic compounds derived from molecular oxygen, i.e., hydroxyl radicals or hydrogen peroxide spontaneously generated in DME containing ascorbate and ferrous ions. Poly-ADP-ribosylation is an important step in the cascade of events triggered by these compounds.To survive, chondrocytes do not require stimulation by growth factors. They remain resting cells in fully defined, serum-free culture also at low density. Proliferation and hypertrophy can be induced by serum, but not by low cell density alone.URING respiration, cells reduce large amounts of molecular oxygen to water, but minor quantities of H202 and 02-" or OH" radicals are also formed. Aerobic cells protect themselves against these toxic oxygen species by decomposing them with superoxide dismutases, catalase, and/or peroxidases or with antioxidants, such as ot-tocopherol, carotinoids, bilirubin, pyruvate, glutathione, or cysteine (for review, see Cadenas, 1989).Hyaline cartilage is an avascular tissue with relatively few cells embedded in abundant extracellular matrix. Therefore, chondrocyte metabolism and differentiation depends on diffusion of nutrients and regulatory factors, either from surrounding tissues or from one cartilage cell to another. Cartilage matrix is penetrated even by large molecules because it consists of a loose network of collagen fibrils entrapping the highly charged, polyanionic proteoglycans which bind large amounts of water (Maroudas and Bannon, 1981). When compared with cells of vascularized tissues, chondrocytes in situ are exposed to very low oxygen tensions (Maroudas, 1973;Stockwell, 1983). It is conceivable, therefore, that they are exceptionally sensitive to oxygen, particularly at pressures normally encountered in tissue culture. In fact, development of the chondrocytic phenotype by mesenchymal cells in culture is favored by reduced oxygen pressures (Pawelek, 1969).Chick embryo sternal chondrocytes in suspension culture survive even in the absence of serum when grown at high density. The conditioned media support the viability of similar cells at low density suggesting that chondrocytes produce factors required for their own survival (Bruckner et al., 1989). Here, we identify this activity as a low molecular weight antio×idant and as the only prerequisite for cell viability. Materials and Methods Agarose CulturesChick embryo sternal chondrocytes were cultured in agarose at densities between 0.08 and 1.0 x 106 cells/ml (Benya and Shaffer, 1982;Bruckner et al., 1989). Briefly, 60-ram Petri dishes were coated with agaros¢ and cell suspensions in low melting agarose were overlayered. To facilitate their...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2025 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.