Distinct horizontal patterns in the spatial organization of superficial zone chondrocytes of human joints

Abstract

A better understanding of the unique cellular and functional properties of the superficial zone of articular cartilage may aid current strategies in tissue engineering which attempts a layered design for the repair of cartilage lesions to avert or postpone the onset of osteoarthritis. However, data pertaining to the cellular organization of non-degenerated superficial zone of articular cartilage is not available for most human joints. The present study analyzed the arrangement of chondrocytes of non-degenerated human joints (shoulder, elbow, knee, and ankle) by using fluorescence microscopy of the superficial zone in a top-down view. The resulting horizontal chondrocyte arrangements were tested for randomness, homogeneity or a significant grouping via point pattern analysis and were correlated with the joint type in which they occurred. The present study demonstrated that human superficial chondrocytes occurred in four distinct patterns of strings, clusters, pairs or single chondrocytes. Those patterns represented a significant grouping (p < 0.0001) with horizontal alignment. Each articular joint surface was dominated by only one of these four patterns (p < 0.001). Specific patterns correlated with specific diarthrodial joint types (p < 0.001). Further studies need to establish whether these organizational patterns are a consequence of their surrounding environment or whether they are linked to a functional purpose.

Figure 1. Different focal planes of patellofemoral groove clusters

Representative images of the most superficial (A, approximately a few micrometers beneath the tissue surface) and increased tissue depths (B–D, approximately 30 micrometer steps deeper into the tissue) within the superficial zone. The square, circle and triangle represent different groups of cells situated in different focal planes. Scale bar, 100 µm.

Figure 2. Chondrocyte cell patterns of the superficial zone

Fluorescent microscopic representative images showing cells stained with propidium iodide of strings (A–B), clusters (C–D) and pairs (E–F). Single cells were dispersed among the other patterns. Scale bar, 100 µm.

Figure 3. Percentage of lower and upper extremity 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.

Figure 4. 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 5. Cell densities of the superficial and the deeper zones of the upper and lower extremities

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