Reproduction of Striped Bass Morone saxatilis; a structural, biochemical and functional characterization of atresia
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2002-06-26
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Abstract
Ovarian atresia was examined in striped bass in order to thoroughly characterize this physiological process. If the proper hormonal and temperature cues are not received, gravid female striped bass will initiate ovarian atresia. Although domestic broodfish used in aquaculture routinely complete ovarian growth, injections of gonadotropin or gonadotropin-releasing hormone are required to induce final oocyte maturation. Those fish not treated with hormones in a timely fashion will, instead, initiate ovarian atresia, thereby failing to achieve their reproductive potential for the year.
In progressively atretic striped bass, hormonal profiles showed a steady decline in circulating estradiol-17β and testosterone levels where as levels of the final maturation-inducing steroids, 17α, 20β-dihydroxy-4-pregnen-3-one (17α, 20β-P) and 17α, 20β-21-trihydroxy-4-pregnen-3-one (20β-S) remained basal. Testosterone levels declined significantly across the last three stages of atresia, suggesting that they maybe a useful tool in detecting the onset of atresia. In addition, in vitro investigations showed a general decline in steroidogenesis and follicle responsiveness to gonadotropin during atresia. A steady decline in circulating vitellogenin (Vg) levels was found as the follicles proceeded from post-vitellogenic to late atretic stages. There was also evidence of a bimodal distribution of Vg concentrations at atresia Stage 2 (mid-atresia), which suggested that absolute values of Vg would not be a useful indicator of atresia status or spawning eligibility. Contrary to prior findings in fishes, the Vg-derived yolk proteins, lipovitellin (Lv) and β’-component were not found intact or partially intact in the blood plasma of atretic animals. Results of SDS-PAGE and Western blotting of plasma and ovarian extracts from atretic and non-atretic fish indicated that these yolk proteins are completely hydrolyzed before export from the atretic follicle. This correlated with light microscopy evaluations of atretic follicles that portrayed a general decline in the intensity of staining of the yolk granules with metanil yellow. Vg was detected via SDS-PAGE, Western blotting as well as ELISA in plasma samples from vitellogenic, post-vitellogenic and atretic animals; however, it was only detected in ovarian extracts from vitellogenic fish. Thus, absence of newly endocytosed Vg detected by Western blotting may be a useful marker for post-vitellogenesis in striped bass.
Using transmission electron microscopy, it was found that edema in the granulosa cell layer is an unambiguous characteristic of early atresia. Similar to what has been found in mammalian systems, the granulosa cells of atretic follicles appeared to lose their steroidogenic capabilities as they went through drastic morphological changes. During this time, the basement membrane, which separates the theca and granulosa cell layers of the follicle, appeared to remain intact. Therefore, evidence of phagosomes within the oocyte suggested that phagocytes of granulosa cell origin invaded the oocyte through a disintegrated zona radiata to engulf and recycle lipids, proteins and other components of the ooplasm.
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Keywords
yolk protein, vitellogenin, striped bass, atresia, follicle
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Degree
MS
Discipline
Zoology
