Validation of an Amphibian Lifecycle Test Method for Measuring Endocrine Disruption

The Phase I study resulted in the development of an assay that tests substances

that have the capacity to disturb reproductive and developmental processes

during the lifecycle of a vertebrate animal by interfering with the endocrine

system. The primary goal of this Phase II research project is to standardize,

validate, and commercialize an amphibian lifecycle model using Xenopus as a

system for the evaluation of endocrine-disrupting chemicals (EDCs) found in

the workplace or in the environment. Specifically, Fort Environmental Laboratories,

Inc., standardized and evaluated the use of X. tropicalis as a model system

to evaluate the effect of EDCs on various aspects of the amphibian lifecycle

by conducting studies with a series of known mammalian EDCs and chemicals with

unknown activity. 17ß-estradiol (E2), androstene dione, and methoxychlor

were used to develop and standardize the model. Results from these studies

were used to generate a final study protocol from which a Guidance Document

ultimately will be prepared following Phase II interlaboratory validation studies.

Results from Phase I strongly indicated that chronic exposure to E2 slightly

delayed development, had a marginal affect on the normalcy of early larval

development, dramatically skewed sex ratios toward the female gender, induced

gonadal malformations that included some intersexual development at low test

concentrations, and induced liver abnormalities. Furthermore, chronic E2 exposure

impaired both female and male reproductive fitness. Chronic exposure to E2

decreased breeding success in both sexes, decreased fertilization in exposed

females, and decreased embryo-larval viability in the F1 progeny.

Chronic exposure to androstene dione dramatically skewed sex ratios toward

the male gender, induced gonadal malformations that included some intersexual

development at low test concentrations, and caused liver abnormalities. Androstene

dione reduced reproductive fitness in female specimens, but not male specimens.

Chronic exposure to androstene dione did not markedly alter breeding success

in both sexes or decrease embryo-larval viability in F1 progeny, in contrast

to the effects of E2.

Finally, chronic exposure to methoxychlor delayed development, including

slowing the rate of metamorphosis; had a slight affect on the normalcy of larval

development; caused skewed sex ratios toward the female gender; and induced

gonad, liver, and thyroid abnormalities in juvenile specimens. Furthermore,

chronic methoxychlor exposure impaired both female and male reproduction fitness.

Increasing concerns over the widespread finding of EDCs in the environment

have dramatically increased the need for standardized assays, such as the XLCA,

because no other assay of this type is available today.