Gravitational changes affect tibial growth plates according to Hert's curve

Background: Microgravity significantly affects chondrocyte differentiation within the tibial epiphyseal growth plate of space flown rats. The changes produced in height and number of cells in different zones of the plate are associated with ultrastructural changes in the extracellular matrix. Given the importance of the growth plate in endochondral ossification, we began t o assess the response of the plate to hypergravity, and the countermeasure value of excess G. Methods: Rats of the strain used in Cosmos biosatellite missions were housed under conditions similar to Cosmos flights and subject ed to continuous hypergravity (2 G) for 14 d, in a 12-ft radius centrifuge. Results: Histomorphometrical analyses of tibial growth plates from these r ats found the hypertrophic/calcification zone to be significantly reduced i n both height and cell number, and the proliferation zone in cell number. C onclusions: These results, along with those of spaceflight and of studies u sing suspension-centrifugation, indicate that rat growth plate responds to gravitational changes according to Hert's curve: i.e., a) an increased base line (minimal) loading reduces cartilage differentiation; and b) a reduced baseline loading may lead to increased cartilage differentiation but only w ithin a range, beyond which lack of differentiation results. The plasticity of the plate, i.e., its ability to increase or decrease its activity in re sponse to changes in gravity suggests the possibility of a range of G that will produce the load necessary to maintain normal growth of the plate, i.e ., possible countermeasures to the effects of either hypo- or hyper-gravity .