SEQUENTIAL HISTOMORPHOMETRIC CHANGES IN SPRAGUE-DAWLEY RAT GROWTH PLATE FOLLOWING IRRADIATION

Spadaro J, Damron TA, Farnum CN*, SUNY-HSC Orthopedic Laboratory, *Department of Anatomy, Cornell University College of Veterinary Medicine

PURPOSE: To examine the sequential histological and morphometric changes in growth plate following irradiation in an animal model in order to understand the mechanism of those changes and to provide a basis for comparison to radioprotectant strategies.

METHODS: Thirty Spague-Dawley 3 to 4 week old weanling rats were irradiated with 17.5 Gy to the right knee. The left limb was not irradiated. Six rats each were randomly assigned to harvest at five time periods: 0.5, 1,2,3, and 4 weeks. Animals were injected with oxytetracycline (OTC, 50 mg/kg I.P.) 48 hours prior to euthanasia and with bromodeoxyuridine (BrdU, 25 mg/kg I.P.) 1 hour prior to euthanasia. Following euthanasia, histologic sections of bilateral proximal tibial growth plates were harvested and fixed in RHT-glutaraldehydeparaformaldehyde fixative. Growth rate was calculated based upon the OTC fluorescent band. BrdU staining was utilized to assess active proliferation.

RESULTS: At 0.5 weeks following radiation there was a diminished growth rate (63% normal). Normal growth plate height (reserve through hypertrophic zone) was maintained by abnormal expansion of the proliferative zone at the expense of a shortened hypertrophic zone. At 1 week, growth rate dropped to its lowest level (11% normal). Growth plate height fell to 66% normal with persistence of the expanced proliferative and shortened hypertrophic zones. Active proliferation , albeit notably decreased, was evident by BrdU staining even during the 0.5 and 1 week time periods. Diminished cellularity and increased disorganization was noted within the proliferative zone. By 2 weeks, both growth rate and growth plate width had returned to normal. However, disorganization and diminished cellularity persisted through 4 weeks.

CONCLUSIONS: The profound effects of irradiation on the growth plate in this animal model are most pronounced histologically beginning at 1 week post-irradiation and persisting through 4 weeks. From a functional standpoint, the growth rate effects are already significant at 0.5 weeks following irradiation and reach peak detriment at 1 week before beginning recovery towards normal. These findings provide a basis for further study of the mechanisms of these effects and their potential amelioration with radioprotectant strategies.