Effects of TCDD and its congeners 3,3',4,4'-tetrachloroazoxybenzene and 3,3',4,4'-tetrachlorobiphenyl on lymphoid development in the thymus of avian embryos.

Thymus anlagen from 11-day-old chick and 14-day-old turkey and duck embryos were cultured in media containing 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) for 5 days. The maximal TCDD-induced decrease in lymphoid cell number of chick embryo thymus (to about 60% of the control number) occurred at concentrations of 10(-10) M and above. To produce the same effect on lymphoid cell number in the cultures of thymus anlagen from turkey and duck embryos, about a 100-fold higher concentration of TCDD was needed. The toxicity of the TCDD congeners 3,3'4,4'-tetrachloroazoxybenzene (TCAOB) and 3,3',4,4'-tetrachlorobiphenyl (TCB) to embryonic chicken thymus was tested in vitro and in ovo. In chick embryo thymus cultures, TCAOB and TCB were about two orders of magnitude less toxic than TCDD. Injection of TCAOB and TCB into chicken eggs preincubated for 11 days resulted in a dose-dependent decrease in thymic lymphoid cell number 5 days later, declining to about 14% of the controls at 10 micrograms TCAOB/kg egg. The ED50 value was estimated to be 3.6 and 60 micrograms/kg egg for TCAOB and TCB, respectively.