The development of an aquatic bivalve model for evaluating the toxic effects on gametogenesis following 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure in the eastern oyster (Crassostrea virginica)

The gametogenic protocol developed in this research can serve as a model for evaluating the toxic effects of chemicals on oogenesis and spermatogenesis. The compound 2,3,7,8-tetrachlordibenzo-p-dioxin (TCDD) was selected as a “proof of principle” toxicant to examine developmental toxicity in an invertebrate system. The studies were designed to (1) test the model using TCDD and (2) to use histopathology and biochemical parameters such as ornithine decarboxylase (ODC) activity, polyamine levels (putrescine, spermidine, and spermine), and vitellogenin levels as endpoints during the critical stages of gametogenesis in the eastern oyster (Crassostrea virginica). TCDD at 10 pg/gram caused significant histopathological gonadal lesions by day 14 of gametogenesis in both male and female oysters. These lesions resulted in complete inhibition of gonadogenesis. The gonadal lesions observed in the TCDD-treated oysters were significantly different from the lesions observed in oysters treated with α-difluoromethylornithine (DFMO), a known ODC inhibitor, benzo(a]pyrene, and non-chlorinated dioxin. A total body dose of 2 pg/gram and 10 pg/gram TCDD caused a dose-related response resulting in abnormal gonad development. The lesions included the following: (1) abnormal gonadal development resulting in inhibition of oocyte growth and maturation, (2) improper oocyte division, (3) a significant decrease in the Leydig cell population, (4) a significant delay in spermatogenesis, and (5) unsynchronized sperm development. Biochemically, TCDD (10 pg/gram) resulted in a 1.27 fold decrease in ODC activity, a 4.22 fold decrease in putrescine levels, and an 11.2 fold decrease in circulating vitellogenin levels compared to controls. These biochemical parameters were consistent with the histopathologic lesions observed in the gonad.; The eastern oyster is the most sensitive animal tested to date for reproductive effects. Therefore, the eastern oyster can be used as a sensitive toxicological model for examining the effects of dioxin-like compounds and other xenobiotics on gametogenesis. These studies show that environmentally relevant concentrations of 2,3,7,8-TCDD (2–10 pg/gram TCDD) have a significant adverse effect on oyster gametogenesis. The 2,3,7,8-CDD effects reported in this dissertation may explain the lack of bivalve populations in New Jersey waters.