Proliferative remodeling of the spatial organization of human superficial chondrocytes distant from focal early osteoarthritis

Abstract

Objective: Human superficial chondrocytes show distinct spatial organizations, and they commonly aggregate near osteoarthritic (OA) fissures. The aim of this study was to determine whether remodeling or destruction of the spatial chondrocyte organization might occur at a distance from focal (early) lesions in patients with OA.

Methods: Samples of intact cartilage (condyles, patellofemoral groove, and proximal tibia) lying distant from focal lesions of OA in grade 2 joints were compared with location-matched nondegenerative (grade 0-1) cartilage samples. Chondrocyte nuclei were stained with propidium iodide, examined by fluorescence microscopy, and the findings were recorded in a top-down view. Chondrocyte arrangements were tested for randomness or significant grouping via point pattern analyses (Clark and Evans Aggregation Index) and were correlated with the OA grade and the surface cell densities.

Results: In grade 2 cartilage samples, superficial chondrocytes were situated in horizontal patterns, such as strings, clusters, pairs, and singles, comparable to the patterns in nondegenerative cartilage. In intact cartilage samples from grade 2 joints, the spatial organization included a novel pattern, consisting of chondrocytes that were aligned in 2 parallel lines, building double strings. These double strings correlated significantly with an increased number of chondrocytes per group and an increased corresponding superficial zone cell density. They were observed in all grade 2 condyles and some grade 2 tibiae, but never in grade 0-1 cartilage.

Conclusion: This study is the first to identify a distinct spatial reorganization of human superficial chondrocytes in response to distant early OA lesions, suggesting that proliferation had occurred distant from focal early OA lesions. This spatial reorganization may serve to recruit metabolically active units as an attempt to repair focal damage.

Figure 1. Top-down view onto the articular surface depicting the horizontally orientated organizational patterns of the superficial zone chondrocytes with and without early OA

Fluorescent microscopic representative images showing chondrocyte nuclei stained with propidium iodide of the femoral condyles (A-B, E-F), and the patellofemoral groove (C-D). Shown is the normal, non-osteoarthritic articular surface of grade 0–1 joints (A, C), the remaining, intact articular surface distant to a focal OA lesion of grade 2 joints (B, D), and a focal lesion of the articular surface of grade 2 condyles (E, F). Square: single chondrocyte; triangle: pair; circle: cluster; ellipse: string; rectangle: double string. Scale bars, 100 μm.

Figure 2. Percentage of chondrocytes that were grouped in strings, clusters, pairs or as singles

Values are expressed as the mean ± standard error of the mean (SEM). p values are listed in Table I. Strings, clusters, pairs and singles are shown in Fig. 1.

Figure 3. Percentage of chondrocytes that appear as single cells or in groups of 2, 3, 4, 5, 6 to 8, or 9 to 12 cells

Values are expressed as the mean ± SEM.

Figure 4. Cell densities of the superficial and the deeper zones of the human knee joint

Values are expressed as the mean ± SEM. *, p < 0.05. The cell density of the distal femur consisted of the groove and the condylar regions; separate densities for both regions were not calculated.