|
Media Resources :: Newsroom
Environmental Estrogens Do Not
Always Result in Irreversible Sex Changes
19 April 2005
Zebrafish differ from other species used in tests with
endocrine-disrupting compounds in that they do not always
undergo irreversible sex changes with life-cycle exposure
to estrogens, said researchers in a new study. Previous
studies of fish, such as medaka and fathead minnows,
indicated that life-cycle exposure to estrogens induced
full phenotypic sex changes.
According to the study published in the latest issue
of Environmental Toxicology
and Chemistry, continuous exposure to estrogen
halts gonadal development in genetically male zebrafish,
causing feminization in 100% of those exposed and thus
inhibiting reproduction. Males, however, are able to
resume testicular differentiation when exposure ceases.
Zebrafish were exposed in a flow-through system to
an environmentally relevant concentration of the synthetic
estrogen 17a-ethinylestradiol (EE2). EE2 is one of the
most commonly used oral contraceptives and has been
detected in sewage treatment and in surface waters in
Europe and the United States.
The team of four researchers compared partial to full
life-cycle exposure, given that long-term assessment
is required to fully understand the potency and the
extent to which environmental estrogens disrupt the
development and reproductive function of fish.
The zebrafish life-cycle, from fertilization to the
reproductive stage, is only three to four months. A
distinguishing characteristic of zebrafish is its development
as a "juvenile hermaphrodite." This means
that zebrafish pass through a non-functional female
phase, regardless of genetic sex, when the gonads consist
solely of primary growth stage oocytes, or eggs before
maturation.
The researchers found that early life-stage exposure
led to a lasting induction in adult females of plasma
vitellogenin (VTG), a protein widely used as an indicator
of environmental exposure to water-borne estrogenic
compounds, but did not alter the sex ratio or the reproductive
capabilities. Full life-cycle exposure resulted in elevated
VTG concentrations and caused gonadal feminization in
100% of exposed fish.
Two types of ovaries were observed in the study's continuously
exposed adult fish: immature ovaries with primary growth
stage oocytes and mature ovaries containing the full
range of all oocyte stages. Fish with immature ovaries
had plasma VTG levels like control males, while fish
with mature ovaries had female-like VTG levels. The
effects of full life-cycle exposure were at least partly
reversible, and 26% of fish of the previous all-female
group developed fully differentiated testes. This finding
may be due to dimorphic feminization, where some feminized
male fish develop mature female ovaries and others do
not.
Results found in this study have implications not only
for tests of endocrine-disrupting chemicals, the researchers
said, but also for risk assessments and interpretation
of experimental results.
To read the entire study, click here: http://www.allenpress.com/pdf/entc_24_416_1088_1098_(2).pdf
Environmental Toxicology
and Chemistry is a monthly journal of the Society
of Environmental Toxicology and Chemistry (SETAC). For
more information about the Society, visit www.setac.org.
For more information about Environmental
Toxicology and Chemistry, visit http://setac.allenpress.com.
[An Environmentally Relevant Concentration
of Estrogen Induces Arrest of Male Gonad Development
in Zebrafish, Danio rerio, Environmental
Toxicology and Chemistry], 2005; Vol 24 (5):1088-1098
Contact:
April M. Phillips
T 850 469 1500 x 28
aprilp@setac.org
|
