Factors affecting egg quality are determined by the intrinsic properties of the egg itself and the environment in which the egg is fertilized and subsequently incubated. Egg quality in fish is very variable. Some of the factors affecting egg quality in fish are known, but many (probably most) are unknown. Components that do affect egg quality include the endocrine status of the female during the growth of the oocyte in the ovary, the diet of the broodfish, the complement of nutrients deposited into the oocyte, and the physiochemical conditions of the water in which the eggs are subsequently incubated. In captive broodfish, the husbandry practices to which fish are subjected are probably a major contributory factor affecting egg quality. Our knowledge of the genetic influences on egg quality is very limited indeed. We know that parental genes strongly influence both fecundity and egg quality, but almost nothing is known about gene expression and/or mRNA translation in fish oocytes/embryos. This is surprising because the products synthesized in ovoand the mechanisms controlling their expression are likely to play a central role in determining egg quality. The genetic mechanisms underpinning oocyte and embryo growth and development are a priority for research

Egg quality in fish: what makes a good egg?

Oocyte growth and development is an important issue in fish and fisheries biology. This paper reviews the information available on oocyte growth patterns and the rates and dynamics of oocyte growth in teleosts. In synchronous spawners, the weight of the gonad may represent as much as 40% of the overall body weight of the fish. In asynchronous spawners, the weight of the mature ovary is considerably less than in synchronous ovulators, but the ovary shows a more regular periodicity and may grow repeatedly many times during the breeding season. There is a huge variability in egg size in teleosts, with the largest known measuring up to 8 cm in diameter. Within the limits of variance set by genetic constraints, egg size may vary between populations of the same species. Oocytes in all teleosts undergo the same basic pattern of growth: oogenesis, primary oocyte growth, cortical alveolus stage, vitellogenesis, maturation and ovulation. The mechanisms that control oocyte growth are addressed in this review, albeit that the available information, as in all other vertebrates, is very limited. The main hormones that have been shown to affect ovarian growth are gonadotrophin, thyroid hormones, growth hormone, insulin and insulin-like growth factors. An overview of the determinants of fecundity, with particular reference to oocyte recruitment and atresia, is the focus of the second part of the paper. Genetics and nutrition have major effects on fecundity, and studies so far suggest that the determinants of fecundity usually operate during the early part of gametogenesis. The role of atresia in determining fecundity is less clear. The final part of this review highlights some areas of study that are priorities for research on ovarian development in fish.

Oocyte growth and development in teleosts

This report is an overview of the current state of the science relative to environmental endocrine disruption in humans, laboratory testing, and wildlife species. Background information is presented on the field of endocrinology, the nature of hormones, and potential sites for endocrine disruption, with specific examples of chemicals affecting these sites. An attempt is made to present objectively the issue of endocrine disruption, consider working hypotheses, offer opposing viewpoints, analyze the available information, and provide a reasonable assessment of the problem. Emphasis is placed on disruption of central nervous system--pituitary integration of hormonal and sexual behavioral activity, female and male reproductive system development and function, and thyroid function. In addition, the potential role of environmental endocrine disruption in the induction of breast, testicular, and prostate cancers, as well as endometriosis, is evaluated. The interrelationship of the endocrine and immune system is documented. With respect to endocrine-related ecological effects, specific case examples from the peer-reviewed literature of marine invertebrates and representatives of the five classes of vertebrates are presented and discussed. The report identifies some data gaps in our understanding of the environmental endocrine disruption issue and recommends a few research needs. Finally, the report states the U.S. Environmental Protection Agency Science Policy Council's interim position on endocrine disruption and lists some of the ongoing activities to deal with this matter.

Environmental endocrine disruption: an effects assessment and analysis.

A period of oocyte growth is followed by a process called oocyte maturation (the resumption of meiosis) which occurs prior to ovulation and is a prerequisite for successful fertilization. Our studies using fish models have revealed that oocyte maturation is a three-step induction process involving gonadotropin (LH), maturation-inducing hormone (MIH), and maturation-promoting factor (MPF). LH acts on the ovarian follicle layer to produce MIH (17α, 20β-dihydroxy-4-pregnen-3-one, 17α, 20β-DP, in most fishes). The interaction of ovarian thecal and granulosa cell layers (two-cell type model), is required for the synthesis of 17α,20β-DP. The dramatic increase in the capacity of postvitellogenic follicles to produce 17α,20β-DP in response to LH is correlated with decreases in P450c17 (P450c17-I) and P450 aromatase (oP450arom) mRNA and increases in the novel form of P450c17 (P450c17-II) and 20β-hydroxysteroid dehydrogenase (20β-HSD) mRNA. Transcription factors such as Ad4BP/SF-1, Foxl2, and CREB may be involved in the regulation of expression of these steroidogenic enzymes. A distinct family of G-protein-coupled membrane-bound MIH receptors has been shown to mediate non-genomic actions of 17α, 20β-DP. The MIH signal induces the de novo synthesis of cyclin B from the stored mRNA, which activates a preexisting 35 kDa cdc2 kinase via phosphorylation of its threonine 161 by cyclin-dependent kinase activating kinase, thus producing the 34 kDa active cdc2 (active MPF). Upon egg activation, MPF is inactivated by degradation of cyclin B. This process is initiated by the 26S proteasome through the first cut in its NH2 terminus at lysine 57.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1440-169X.2008.01019.x

Regulation of oocyte maturation in fish.

Alkylphenol ethoxylates (APEs) and related compounds recently have been reported to be estrogenic because it has been demonstrated in laboratory studies that they mimic the effects of estradiol bothin vitro andin vivo. Chemicals referred to as “environmental estrogens” are suspected of causing health effects in both humans and wildlife through disruption of the endocrine system. In this review, the occurrence, environmental fate, and biological effects of APEs are presented. To provide understanding of the potential for endocrine disruption due to environmental estrogens, the physiology of estrogens in mammals and fish is also reviewed. The estrogenic potency of other environmental estrogens is compared to the potency of APE degradation products. The reproductive effects of estrogenic compounds are considered when evaluating the potential health effects of APEs. Given the reported environmental concentrations and bioconcentration factors of APE products, the potential for these compounds to produc...

Environmental estrogenic effects of alkylphenol ethoxylates

Publisher Summary This chapter discusses the yolk formation of teleost fishes The process of vitellogenesis in teleosts has been shown to be similar to that operating in other oviparous vertebrates The contribution of autosynthetic processes (endogenous vitellogenesis) to the yolk mass in the teleost ovary, relative to exogenous yolk acquired by incorporation of vitellogenin, has not been estimated Exogenous vitellogenesis can be considered to consist of two phases The first phase involves the induction of hepatic vitellogenin production under stimulation of ovarian estrogen During the second phase vitellogenin is taken up from the blood stream and incorporated into ovarian yolk proteins In salmonids maturational gonadotropin occurs at high levels in plasma around spawning time but is near the lower limit of the radioimmunoassay during the phase of active incorporation of vitellogenin There appears to be a small increase in maturational gonadotropin coincident with an increase in estradiol in trout plasma early in vitellogenesis, and antibody to maturational gonadotropin inhibits ovarian growth immediately prior to the rapid and massive increase in the ovarian weight Therefore, there appears to be a low level of maturational hormone present when the fish resumes ovarian development after spawning, but it is sufficient to establish vitellogenin production by the liver

8 Yolk Formation and Differentiation in Teleost Fishes

Studies on two types of gonadotropins from both salmon and carp pituitaries

"Big" and "little" forms of plaice vitellogenic and maturational hormones.

Publisher Summary This chapter discusses the isolation, biochemistry, and function of teleost gonadotropins Methods of purification include solvent fractionation, affinity chromatography, gel filtration, and ion-exchange chromatography Reports on purification of a single gonadotropin from various teleost species appear to be in contrast to the discovery of vitellogenic hormone and maturational hormone The difference stems from the use of or lack of affinity chromatography (Con A-Sepharose) as one of the purification procedures and the fact that most workers did not monitor vitellogenic activity during the course of purification The gonadotropins that were isolated, as judged from the spectrum of biological activities, correspond to maturational hormones Maturational gonadotropin levels in plasma do not appear to correlate well with the reproductive processes Substantial evidence has been collected for the existence of two gonadotropins, vitellogenic hormone, and maturational hormone in teleost The two hormones manifest distinctive chromatographic, compositional, immunological, and biological characteristics Antiserum to each of these two hormones produces inhibitory effects on those aspects of fish reproduction, which are consistent with the biological activities of the hormone

T. Bun Ng

Papers

8 Yolk Formation and Differentiation in Teleost Fishes

Studies on two types of gonadotropins from both salmon and carp pituitaries

"Big" and "little" forms of plaice vitellogenic and maturational hormones.

5 Teleost Gonadotropins: Isolation, Biochemistry, and Function

Studies on two types of gonadotropins from both American plaice and winter flounder pituitaries.