THE RELATIVE SENSITIVITY OF CHICKEN EMBRYOS TO YOLK-INJECTED OR AIR-CELL-INJECTED 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN

We compared the relative sensitivity of chicken embryos exposed to 2,3 ,7,8-tetrachlorodibenzo-p-dioxin (TCDD) injected either into the yolk or into the air cell. The TCDD was injected at the start of incubation (embryonic day 0) and the embryos were sacrificed at multiple times d uring embryonic development. A subset of embryos were allowed to hatch undisturbed. The chick embryo was significantly more sensitive to TCD D when injected into the yolk than when injected into the air cell. Th e resultant median lethal dose (LD50) (122 pg/g egg, determined by pro bit; analysis; 146 pg/g egg determined by interpolation) was 60% lower than the LD50 (297 pg/g egg by probit; 255 pg/g egg determined by int erpolation) for air-cell-injected TCDD. A significant decrease in hatc h weight of embryos exposed to high concentrations of TCDD compared to controls occurred, and this decrease was even more pronounced at a lo wer concentration in the yolk-injected birds. Interestingly, during th e period of embryonic days 11 through 15, the mean weight of the yolk- injected embryos was smaller than the mean weight of the air-cell-inje cted embryos. This difference was not noticably evident just before or just after this developmental period. Embryos exposed to high concent rations of TCDD injected into either the yolk or the air cell tended t o die within the first 2 weeks of incubation. A number of TCDD-exposed embryos survived the entire 21-d incubation period, but only air-cell -injected embryos were able to hatch successfully. Because the injecti on site varies in studies reported by different laboratories, the rela tive sensitivity must be considered when comparing results from differ ent studies.