Impairments of Endocrine Functions: Case Studies - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 238

TABLE 9.1
Terminology for Gonadal Abnor malities in Amphibians

Term Description

Ambisexuality Immature state of gonads during larval development. The gonad has

potential for development into ovarian or testicular tissue

Testicular oocytes Oocytes present in the testis regardless of state of maturity

Rudimentary testicular oocytes Oocytes present as a normal part of early development, and which

disappear with maturation

Testicular oogenesis Genesis of oocytes in testis tissue

Testicular dysgenesis Abnormal or testicular development or degeneration of testicular tissue

Segmented gonads Gonads segmented as discrete subunits and separated by connective tissue

Intersex Phenotypic sex different from genotypic sex

Mixed gonadal tissue Testicular and ovarian tissue occurs within the same individual, either

unilaterally appearing as distinct ovaries and testes

Ovotestis Occurrence of ovarian and testicular tissue in the same gonad, typically

with >30% of one or other tissue type. Not necessarily indicative of
functional hermaphroditism

Functional hermaphroditism Both male and female reproductive structures, and fully functional as a

male or female

Developmental hermaphroditism Ovotestis present during early sexual development. Further

development of males concomitant with degeneration of oocytes.
Residual oocytes are not functionally relevant

Rudimentary hermaphroditism Nonfunctional form of hermaphroditism in which a few germ cells of

opposite sex are present in gonad

Source: Modified from Hecker, M. et al., Rev. Environ. Contam. Toxicol. , 187, 103-131, 2006.

articles (Tavera-Mendoza et al. 2002a, b). The authors presented histological evidence of
gonadal abnormalities (see Table 9.1 for descriptions) in African clawed frogs ( Xenopus
laevis ) after exposure to 21 μg atrazine L -1 during sexual differentiation of the gonads.
Specifically, they described degeneration of testicular tissue, manifested as testicular
resorption and a decline in spermatogonial cell nests (Tavera-Mendoza et al. 2002a), and
resorption of oogonia in ovaries (Tavera-Mendoza et al. 2002b). In the same year, Hayes et
al. (2002a) also described gonadal abnormalities following exposure of X. laevis tadpoles
throughout their larval development to atrazine at 25 μg L -1 , but also at the much lower
concentration of 0.1 μg L -1 . Those abnormalities were predominantly gross morphological
abnormalities of the gonads, including multiple gonads or a mixture of testes and gonads
(hermaphroditism). The identity of tissue types (i.e., testis or ovary) was validated with
histology. Both groups of authors implicated increased aromatase activity as the mecha-
nism for the effect of atrazine (see below); however, apart from involving the gonads, the
actual morphologies described by Tavera-Mendoza et al. (2002a) and Hayes et al. (2002a)
were quite different. Tavera-Mendoza et al. (2002a) described gonadal dysgenesis, whereas
Hayes et al. were describing gonadal ambiguities and emasculation of male frogs.
Since these two studies were published, the use of histopathological markers as bio-
markers of exposure and effects of atrazine have been central to amphibian studies focus-
ing on endocrine disruption. Following up on their studies in X. laevis , Hayes et al. (2002b,
2003) published data describing testicular dysgenesis among American native frogs ( Rana
pipiens ) and the occurrence of mixed sex gonads characterized by the presence of testicu-
lar oocytes (TOs; see Figure 9.12), again at 25 μg L -1 and the much lower concentration of
0.1 μg L -1 . Exposed males developed TOs (29% and 8% at 0.1 and 25 ppb, respectively) or