119 HW mic jayes (Figure 94). Within the first week after 5000 middle segment of the femoral cartilage plate. Ampu- srolonged jage projected into the metaphysis and becamepar‘cond, the rally ossified (Figure 94). That this change was one sitting, and it usually involves separation of trabeculae from the growth apparatus.” Marked strain differences were observed in the marrow. Two days after 5000 Rads the marrowcells were noticeably depopulated and increasing numbersof fat cells apepared toward the end of the first week in all strains. Marrow regeneration occurred during thesecond week, but it was slowest in strain HB mice. The _and HRB Rads, resorption of cartilage had proceeded so irregud to nar. larly that numerous tongues and/or nodules of carti- Ost twice Javed until the 4th week after irradiation with 7200 igure 91)E ads (and only in strain H) again suggests that the mpted tok sighcr dose level prolonged the postirradiation recovthis wasf ery period, The differences were undoubtedly related issible tok to the ability of the metaphyseal capillaries to invade basis of the cartilage, and this process is known to be depend[W mice ent upon the state of mineralization of the cartilage normal-f matrix.@3) Capillaries were found running parallel to, bility of ruther than perpendicular to, the face of the cartilage dliferate curing the first week after both 5000 Rads and 7200 te thick-F liuds, but while a few days were required in the 4000ive beenk Rad groups before normal invasion of the cartilage plate was reestablished, several weeks were required ed as af following 7200 Rads. In this respect, following 7200 and this Rads the bones from strain H mice had nodules of parr matrix€ tially ossified cellular cartilage in the metaphysis as bly hy-§ long as 120 days after irradiation, Remnants of celluh carti-F ur curtilage were never found after the second week i the metaphyses of mice irradiated with 5000 Rads. ‘sification processes, while irregular and slowfollowinv 3000 Rads, always resulted in the formation of true cidochondral bone. However, 7200 Rads appeared so disruptive to the normal processes of chondrocyte and tissue maturation in the growth plates that only simple lamellar bone was deposited on the subchondral] surfuces (areas which were probably of normal mineral content) during the first two postirradiation weeks. Ikesumption of the normal cartilage metabolism and it formation of endochondral trabeculae were delayed until the third week. In view of the continuing formaion of trabeeular bone in all the mice, the population and funetion of osteoblasts seemed little affected byir- radiation, A similar conclusion was reached by Kem- ber.It is of interest that the trabeculae in mice of the HB strain were shorter at all postirradiation penods, but the significane of this is unknown since no uurradiated control animals of this strain and of the “Ime age were available for comparison. However, it “il be reealled that in this study HB strain mice also retuned more medullary fat and sustained the more -tvere damage to their cartilages. hb growth ; whole- F shows § ands of E 1 proc: E eosin. Because the mice were irradiated at an age when they normally begin to develop islands of degenerate cartilage in their growth plates, it was difficult to relute this change to the effects of irradiation. However, eecnerate changes in the matrix became more severe ‘h time at both dose levels. Small rents developed the tissues following 7200 Rads. but onlyin one in- “ance (strain H) did we observe amputation of a tations are more frequently observed in mice after high exposures of radiation (2000 Rads) delivered at time course for the development of fatty marrow was earlier after an exposure of 7200 Rads—at 2 days in strain HB mice and at 3 days in strain HW mice. Extravasation of red cells occurred at one day in strain Hand at two days in strains HW and HB. Less fat was noted during the second week, and healing was com- pleted by the third week—-a delay of one week compared to the 5000-Rad groups. SUMMARY Cas7 mice of three strains received °Sr-9°Y partial body irradiation with a. 5000- and 7200-Rad surface dose for a limit of 16 hr. In this preliminary study, the cartilages of strains H (hairless) and HB (haired black) mice responded similarly to the two doses. The cartilages were thinnest 5 days after irradiation owing to suppression of proliferative activity, but partial recovery was achieved earlier on the average in strain H. The time requiredto elicit these changes in strain HW (haired analogues) was extended. With the histologic observations, data suggested also that the recovery period in HWstrain mice following 7200 Rads was somewhat longer than in the other twostrains. REFERENCES 1. Segale, G. C. Sull’azione biologica dei raggi Réntgen e del radium sulle cartilagini epiphysarie. Rad. Med. 7, 234247 (1920). 2. Hinkel, C. L. Theeffect of roentgen rays upon the growing long bones of albino rats. I. Quantitative studies on the growth limitation following irradiation. Am. J. Roentgenol. 47, 439-457 (1942). 3. Hinkel. C. L. The effeet of roentgen rays upon the growing long bones of albino rats. II. Histopathological changes involving endochondral growth centers. Am. J. Roentgenol, 49, 321-347 (1943). 4. Sams, A. The effeet of 2000 R of X-rays on the internal structure of the mouse tibia. Int. J. Radiat. Biel. 11, 51-68 (1966). 5. Gall, E. A., Lingley, J. R., and Hileken, J. A. Comparative experimental studies of 200 kilovolt and 1000 kilovolt roentgen rays. I. The biological effects on the epiphysis of the albino rat, Am. J, Path, 16, 605-618 (1940). 6. Dahl, B. Effets des rayons x sur les os longs en développement étude radiographique et anatomique. J. Radiol. ef d'Electrof. 18, 131-140 (1934).