Stress relaxation of growth plate tissue under uniform compressive load: Relationship between mechanical response and extracellular matrix bio-composition and structure

The main objective of this study was to characterize the histomorphological characteristics and mechanical behaviour of growth plates at both cellular and tissue levels and to evaluate the biochemical composition and collagen fiber orientation of growth plate tissue in the three functionally distinct zones and to further establish associations between zonal mechanical behavior and biochemical composition of the growth plate.

Five groups of growth plate explants from 4-week old swine were used in this project. Histomorphological analyses of growth plate at tissue and cellular levels revealed the heterogeneous and zone-dependent morphological state of the growth plate. Significant variation in the chondrocytes morphology was observed within different histological zones. Maximum chondrocytes volume and surface area were found in the hypertrophic zone compared to the reserve and proliferative zones. Chondrocyte volume and surface area increased about five- and three-fold respectively as approaching the chondro-osseous junction from the pool of reserve cells. Chondrocytes from the proliferative zone were the most discoidal cells among three different histological zones. Significant differences were also observed in cell/matrix volume ratio between the three zones. Minimum and maximum cell/matrix volume ratios were identified in the reserve and proliferative zone, respectively.

First of all, at the histomorphological level, the marked heterogeneity in cell size through the different histological zones of the growth plate observed in this study are consistent with previous studies on chondrocyte morphology using conventional histology and stereological methods. Chondrocytes undergo spatial shape changes while progressing from the reserve zone to the chondro-osseuse junction. Reserve and hypertrophic chondrocytes were round relative to the flattened proliferative chondrocytes. This was confirmed by the significantly lower sphericity values of proliferative chondrocytes, as compared to reserve and hypertrophic zones. Tissue and cellular morphology may have noteworthy contribution to the growth plate behavior during growth process. Thus, the ability to obtain in situ cell morphometry and monitor the changes in the growth direction could improve our understanding of the mechanisms through which abnormal growth is triggered.

Secondly, chondrocytes and their surrounding extracellular matrix undergo significant zone-dependent morphological changes with compression, most probably due to heterogeneous mechanical properties characterizing the three zones, where the reserve zone was found stiffer along and perpendicular to the compression axis. In our study, hypertrophic chondrocytes showed the greatest deformations among chondrocytes of the three histological zones. Hence, hypertrophic chondrocytes could be more prone to trigger altered biological messages, potentially through cell membrane stretch, which is believed to modulate second messenger activity, and eventually cause growth deceleration under mechanical compression.

The growth plate collagen content and collagen fiber orientation were also non uniform through the growth plate thickness. Random dispersion of chondrocytes in the reserve zone and the columnar arrangement of chondrocytes in proliferative and hypertrophic zones correlate with the observed orientation of collagen fibers in these zones. Moreover, our data corroborates existing data on growth plate bio-composition.

Finally, the zone-dependent biomechanical behavior of the growth plate under compression is related to its collagen content and collagen fiber organization. Reserve zone, which was less susceptible to strains compared to proliferative and hypertophic zones, contained the maximum collagen content with fibers aligned perpendicular to growth direction. Conversely, lower collagen content and longitudinally oriented collagen fibers were detected in the proliferative and hypertrophic zones with high proneness to strains. Overall, the proliferative and hypertrophic zones, where lower collagen levels and longitudinally organized collagen fibers were found, could be more susceptible to compressive strains at both cellular and tissue levels. The more rigid reserve zone could play a more significant role of mechanical support compared to the proliferative and hypertrophic zones, which would be more likely to be involved in the process of growth modulation. These data add to our understanding of the relationship between compressive forces experienced by growth plate chondrocytes and their extracellular environment. (Abstract shortened by UMI.)