In recent years, a hierarchy of techniques has become available for detecting chemicals which may cause endocrine disruption in the aquatic environment. The molecular structure of a chemical provides a first indication about estrogenic activity, i.e. their likelihood of interfering with the female hormone receptor. In vitro competitive binding assays for this receptor and specific cell cultures are also used to demonstrate an estrogenic response, but this does not adequately indicate whether the substance will cause adverse reproductive effects in an entire organism. An elevated level of vitellogenin, a typical female lipoprotein in the plasma of male fish is an in vivo estrogen-mediated response. However, its direct relationship to reproductive developmental effects is as yet unclear. The present study aims at investigating this relationship for assessing endocrine disruption in fish exposed to an estrogenic substance during relevant life stages. A monosex population of male carp, Cyprinus carpio, was exposed to 4-tert-pentylphenol (TPP) and to 17 beta-estradiol as a positive control during the period of sexual differentiation, starting at 50 days post hatch. The fish were sampled every 10 days for the histological examination of the development of the testes, i.e. the formation of the reproductive tract, the multiplication and subsequent meiosis of the primordial perm cells, and gametogenesis in the early gonad. At the end of the experiment, blood was extracted for the quantification of vitellogenin by radioimmunoassays in the plasma. The average number of primordial germ cells (PGCs) per gonadal section was significantly reduced (P < 0.001) at all sampling periods in the testes of carp exposed to 17 beta-estradiol (positive control). The PGCs had commenced oogenesis after 50 days exposure and developed into oocytes as in a 'normal' ovary. The number of PGCs in the testes of the carp exposed to TPP was reduced in a dose dependent manner. Spermatogenesis was severely inhibited in the testes of the TPP-exposed carp, where, few oocytes were ever observed. 20 days exposure to 17 beta-estradiol were sufficient to induce the formation of an oviduct instead of a male vas deferens, whereas this occurred after 30 days in TPP-exposed carp. A dose effect relationship with TPP was established and the EC(50) calculated (63 mu g TPP l(-1)). Vitellogenin levels were significantly elevated in the plasma of the carp exposed to both 17 beta-estradiol concentrations, but only at the highest 4-tert-pentylphenol concentration (256 mu g TPP l(-1), P < 0.05). At the end of the exposure, the no observed effect concentration for the oviduct formation was < 36 mu g TPP l(-1) and that for increased levels of vitellogenin was between 90 and 256 mu g TPP l(-1). (C) 1998 Elsevier Science B.V. All rights reserved.
