Abstract
Osteoarthritis (OA) is characterised by the progressive loss of the articular cartilage. This is accompanied by change in phenotype from cells expressing chondrocytic genes to cells, termed ‘degradative’ chondrocytes, that express aggrecanases and collagenases. To understand the cellular events involved, human articular cartilage was obtained from femoral heads after arthroplasty due to OA, fracture of the neck of femur (#NOF) due to osteoporosis, or from a 14 year old male (CDH). Samples were graded according to the new OARSI system (Osteoarthritis and Cartilage, 2006, 14:13–29) and par-affin sections were immunostained for c-Myc (marker of cellular activation), S100 (typically expressed in chon-drocytes), Sox-9 (expressed in early-stage chondrocytes) and nucleostemin (a stem-cell marker). In addition, some specimen were incubated with fluorescent probes to identify metabolically activated cells (CellTracker green).
All chondrocytes, irrespective of OA grade, were immunopositive for S100, but there were differences in the other parameters. Cartilage from the 14-year old (OARSI grade =0) was characterized by no loss of proteoglycans (safranin-O) in the superficial zone and absence of c-Myc, Sox-9 and nucleostemin in all articular chondrocytes. In #NOF cartilage, proteoglycan loss was evident in the very superficial zone. Many chondro-cytes in that zone showed bright green fluorescence with CellTracker-green and were c-Myc positive, consistent with cellular activation. Sox-9 and nucleostemin were absent. Mid-zone and deep zone chondrocytes showed no change. In low-grade OA samples (OARSI = 1-2), the zone of proteoglycan loss had increased, the Cell-Tracker-green/c-Myc positive chondrocytes in that zone had divided to form clusters of 4-8 cells. Occasional cells were positive for nucleostemin. Mid-zone and deep zone chondrocytes still showed no change. In high-grade fib-rillated OA cartilage (OARSI = 3-4) the superficial and mid zones had been eroded, leaving the deep zone at the surface. Chondrocytes were typically found in large clones, which were all immunopositive for c-Myc as well as for nucleostemin and Sox-9.
The results show that cellular activation starts near the surface and progresses to the deep zone. The presence of nucleostemin and Sox-9 suggests that de-differentiation may be involved in the phenotypic change from the chondrocytic to the degradative phenotype.
Correspondence should be addressed to Mr Carlos Wigderowitz, Senior Lecturer, University Department of Orthopaedic and Trauma Surgery, Ninewells Hospital and Medical School, Dundee DD1 9SY.
