Acute radiation exposure in the clearnose skate, Raja eglanteria: A histological study of the immune system

Clearnose skates (25g) were exposed to 0–75Gy of x-radiation (n = 122). The skates were sacrificed 10, 12, 20, 30 and 40 days post-irradiation. Thymus, spleen, Leydig's organ and epigonal tissues were fixed in 10% neutral buffered formalin and processed for histology. Peripheral blood smears were prepared for differential cell enumeration. Five histological cross-sections of thymus at 500μm intervals were digitized for area analysis. Spleens and epigonals were weighed to the nearest 0.01mg. Animal morphometrics, thymus area, blood differentials, spleen and epigonal weights were compared by ANOVA. There were no significant differences in animal weights, disc widths or total lengths. Beginning at 1.5Gy there was a significant logarithmic decline in thymus area as a function of radiation dose that became asymptotic by 24Gy. Histologically, the medulla was infiltrated with large cysts containing apoptotic thymocytes and eosinophils. Repopulation of the thymus began by day 30 after 9Gy but was not yet complete after 40 days. The spleen significantly dropped in weight after 1.5Gy and had a logarithmic decline that became asymptotic by 24Gy. Histologically the red and white pulps were destroyed after 9–15Gy. There was recovery of the red pulp 30 days after a 9Gy exposure but it was incomplete by 40 days. White pulp did not recover during the experimental period. The epigonal had a logarithmic decline in weight that became significant after exposure to 9Gy and asymptotic by 24Gy. Stem cells were observed in animals exposed to 0–15Gy but were absent in higher treatments. When skates were exposed to greater than 15Gy granulocytes were observed only occasionally in the connective tissue stroma. Stem cells were present 20 days after exposure to 9Gy but recovery was not complete by 40 days. Beginning at 1.5Gy there was a significant logarithmic decline in peripheral white blood cell number as a function of radiation dose that became asymptotic by 24Gy. There was incomplete recovery in white blood cell numbers after 40 days in skates exposed to 9Gy. Only the thymus showed signs of recovery during the experimental period after exposure to 13.5 or 18Gy. An LD50/30 was calculated to be 9–18Gy.