The Questions of Developmental Biology

Some scientists, however, say that these claims are exaggerated. Tests on mice had
shown that litter size, sperm concentration, and development were not affected by environmental
concentrations of environmental estrogens. However, recent work by Spearow and colleagues
(1999) has shown a remarkable genetic difference in the sensitivity to estrogen among different
strains of mice. The strain that had been used for testing environmental estrogens, the CD-1
strain, is at least 16 times more resistant to endocrine disruption than the most sensitive strains
such as B6. When estrogen-containing pellets were implanted beneath the skin of young male
CD-1 mice, very little happened. However, when the same pellets were placed beneath the skin of
B6 mice, their testes shrunk, and the number of sperm seen in the seminiferous tubules dropped
dramatically (Figure 21.24). This widespread range of sensitivities has important consequences
for determining safety limits for humans.
Environmental thyroid hormone disruptors
The structure of some PCBs resembles that of thyroid hormones (Figure 21.25), and
exposure to them alters serum thyroid hormone levels in humans. Hydroxylated PCB were found
to have high affinities for the thyroid hormone serum transport protein transthyretin, and can
block thyroxine from binding to this protein. This leads to the elevated excretion of the thyroid
hormones. Thyroid hormones are critical for the growth of the cochlea of the inner ear, and rats
whose mothers were exposed to PCBs had poorly developed cochleas and hearing defects
(Goldey and Crofton in Stone 1995; Cheek et al. 1999).