The effects of oestrogen treatment on certain plasma constituents associated with vitellogenesis in the elasmobranch Scyliorhinus canicula L

Progesterone initiates tendril formation in the oviducal gland during egg encapsulation in cloudy catshark (Scyliorhinus torazame)

The diverse reproductive strategies of elasmobranchs (sharks, rays, and skates) have attracted research attention, but the endocrine control of reproduction is still incompletely known in elasmobranchs. By long-term monitoring of the egg-laying cycle in cloudy catsharks (Scyliorhinus torazame), we recently demonstrated a transient increase in plasma progesterone (P4) levels just prior to the appearance of the capsulated eggs in the oviducts. In the present study, we examined the in vivo effects of P4 administration in mature female cloudy catsharks. Although no capsulated eggs were observed following the implantation of P4-containing silicone tubing, we did find dark swollen oviducts in the abdominal cavity, in which clumps of long and coiled tendrils were observed. The tendril is an extension of the egg capsule, and the formation of the egg capsule begins with the tendril before main capsule formation. During the period of P4 implantation, the tendrils elongated, and their diameters were significantly increased on day 2 of treatment. Tendril formation was also confirmed on the day of endogenous P4 surge. Tendrils were not formed in catsharks implanted with estradiol-17β or testosterone. Histological analysis of the oviducal gland revealed that P4 implantation induced the secretion of tendril materials from the secretory tubules in the baffle zone, while the tendril materials were stored in the cytoplasm of the secretory cells under low P4 condition. Morphometrically, the area of secreted luminal materials in the secretory tubules was highly correlated to the diameter of tendrils. Our results suggest that the P4 surge during the egg-laying cycle serves as a trigger for egg capsule formation in the oviducal gland of cloudy catshark, but the hormonal signals were incomplete as the main capsule was not formed. Further studies are required to identify the hormones required for ovulation and formation of the main egg capsule.

Changes in sex steroid hormone levels reflect the reproductive status of captive female zebra sharks (Stegostoma fasciatum)

Captive breeding in aquaria is a useful means for ex situ preservation of threatened elasmobranch species. To promote captive breeding, it is important to determine the female reproductive status. However, information regarding reproductive status in female elasmobranchs is limited. Here, we used zebra sharks, Stegostoma fasciatum, as a model for elasmobranch reproduction in captivity. We investigated the relationships among changes in the sex steroid hormone levels, follicle size, and egg-laying period to develop indicators for the female reproductive status. We confirmed that mature female zebra sharks undergo an annual reproductive cycle. Additionally, we showed that the variations in sex steroid hormone levels correlated with reproductive status in mature female zebra sharks. Plasma estradiol-17ß (E2) concentrations increased two months before ovarian follicle development and decreased along with follicle regression. Interestingly, E2 levels were inversely correlated with water temperature (R = -0.901). Moreover, high levels of testosterone (T) correlated well with the laying period. These results strongly suggest that E2 is an indicator for ovarian follicle development, and that T is a useful indicator for both the onset and end of the egg-laying period in captive zebra sharks.

Endocrine control of embryonic diapause in the Australian sharpnose shark Rhizoprionodon taylori

The reproductive cycle of the Australian sharpnose shark, Rhizoprionodon taylori, includes a temporary suspension of development at the commencement of embryogenesis termed embryonic diapause. This study investigated levels of 17β-estradiol (E₂), testosterone (T) and progesterone (P₄) in plasma samples of mature wild female R. taylori captured throughout the reproductive cycle and correlated them with internal morphological changes. Levels of T were elevated through most of the embryonic diapause period, suggesting a role of this hormone in the maintenance of this condition. Increasing plasma T concentrations from late diapause to early active development were associated with a possible role of androgens in the termination of embryonic diapause. As in other elasmobranchs, a concomitant increase of E₂ with ovarian follicle size indicated a direct role of this hormone in regulating vitellogenesis, while a peak in P₄ suggested this hormone is associated with preovulation and ovulation. Additionally, significant correlations between photoperiod or water temperature and maximum follicular diameter and hepatosomatic index suggest that these abiotic factors may also play a role triggering and regulating the synchrony and timing of reproductive events.

Effect of 17beta-estradiol and progesterone on vitellogenesis in the spotted ray Torpedo marmorata Risso 1810 (Elasmobranchii: Torpediniformes): studies on females and on estrogen-treated males

The influence of 17beta-estradiol (E(2)) on vertebrate vitellogenesis is well ascertained. The aim of the present paper is to study the involvement of E(2) and progesterone (P) in the induction and regulation of vitellogenesis in females and experimental E(2)-treated males of Torpedo marmorata. We analyzed females in various stages of the reproductive cycle and E(2) experimentally treated males. The presence of vitellogenin was investigated in the plasma and in the liver by western blot and immunohistochemistry; its site of synthesis was investigated by in situ hybridization. The steroid levels in the plasma were measured by Enzyme Immunoassay. In treated males, E(2) induces in the liver the synthesis of VTG which is then secreted into the bloodstream as a 205-kDa polypeptide, the same that is found in the plasma of non-pregnant vitellogenic females. In females, E(2) is naturally present in the plasma and its level is correlated with VTG synthesis in the liver and with the female reproductive cycle. Indeed, large amounts of E(2) are only found in mature vitellogenic females, whose liver is involved in VTG synthesis and secretion. By contrast, small amounts of E(2) are evident in juveniles whose ovaries are lacking in vitellogenic follicles and in females preparing for ovulation. Low titers are also found in gravid females, whose liver is not engaged in VTG synthesis. We show that P, which is absent in untreated males and juvenile females, is evident in the blood serum of E(2)-treated males and sexually mature females. Interestingly, in treated males P appears in the plasma just 24h after the first injection of E(2) and its titer increases; a week after the last injections, the P level is similar to that recorded in non-gravid vitellogenic females. Finally, it is noteworthy that the highest titer of P was recorded in pregnant females. We demonstrate that in Torpedo vitellogenin synthesis, as in other vertebrates, is under the control of E(2) but also that this synthesis is probably under the control of progesterone.

Electrochemical Immunoassay for Vitellogenin Based on Sequential Injection Using Antigen-immobilized Magnetic Microbeads

A rapid and sensitive immunoassay for the determination of vitellogenin (Vg) is described. The method involves a sequential injection analysis (SIA) system equipped with an amperometric detector and a neodymium magnet. Magnetic beads, onto which an antigen (Vg) was immobilized, were used as a solid support in an immunoassay. The introduction, trapping and release of magnetic beads in an immunoreaction cell were controlled by means of the neodymium magnet and by adjusting the flow of the carrier solution. The immunoassay was based on an indirect competitive immunoreaction of an alkaline phosphatase (ALP) labeled anti-Vg monoclonal antibody between the fraction of Vg immobilized on the magnetic beads and Vg in the sample solution. The immobilization of Vg on the beads involved coupling an amino group moiety of Vg with the magnetic beads after activation of a carboxylate moiety on the surface of magnetic beads that had been coated with a polylactate film. The Vg-immobilized magnetic beads were introduced and trapped in the immunoreaction cell equipped with the neodymium magnet; a Vg sample solution containing an ALP labeled anti-Vg antibody at a constant concentration and a p-aminophenyl phosphate (PAPP) solution were sequentially introduced into the immunoreaction cell. The product of the enzyme reaction of PAPP with ALP on the antibody, paminophenol, was transported to an amperometric detector, the applied voltage of which was set at +0.2 V vs. an Ag/AgCl reference electrode. A sigmoid calibration curve was obtained when the logarithm of the concentration of Vg was plotted against the peak current of the amperometric detector using various concentrations of standard Vg sample solutions (0-500 ppb). The time required for the analysis is less than 15 min.

Ovarian follicle cells in torpedo marmorata synthesize vitellogenin

The pattern of vitellogenesis is similar in all non-mammalian vertebrates: the liver, under oestrogenic stimulus, synthesizes vitellogenin (VTG) that, via the maternal circulation, is delivered to the oocyte and here internalized by receptor-mediated endocytosis (Wallace, 1985: Development Biology. A comprehensive synthesis. Vol. 1 Oogenesis:127-177; Schneider, 1996: Int Rev Cytol 166:103-134; LaFleur, 1999: Encyclopedia of Reproduction Vol. 4:985-992). The contribution to vitellogenesis of different components of the ovarian follicle has also been reported in amphibians (Wallace, 1985), squamate reptiles (Ghiara and Limatola, 1980: Acta Embryol Morphol Exper 1:5-6; Andreuccetti, 1992: J Morphol 212:1-11), and recently, supporting previous reports (Chieffi and Pierantoni, 1987: Hormones and Reproduction in Fishes, Amphibians and Reptiles Single vol.:117-144), in Torpedo marmorata (Prisco et al., 2001: Perspective in comparative endocrinology: Unity and diversity Single vol.:1197-1201; Prisco et al., 2002b: Gen Comp Endocrinol 128:171-179). The present investigation, performed with immunoblotting, immunohistochemical, and in situ hybridization techniques during different stages of follicular growth in T. marmorata, shows that, as previously supposed (Prisco et al., 2002b), granulosa cells in both previtellogenic and vitellogenic phases actively synthesize VTG. This is the first time among vertebrates that the synthesis of this protein has been found to occur also within the ovarian follicle. The present data also demonstrate that the contribution of granulosa cells becomes particularly evident during vitellogenesis. Indeed, in vitellogenic follicles, small, intermediate, and pyriform-like cells cross-react with an anti-VTG antibody and are positive to a hybridization signal with a VTG mRNA probe. By contrast, in previtellogenesis only the enlarged cells, i.e., intermediate and pyriform-like cells, are involved in VTG synthesis.

Multihormonal control of vitellogenesis in lower vertebrates

The comparative approach on how and when vitellogenesis occurs in the diverse reproductive strategies displayed by aquatic and terrestrial lower vertebrates is presented in this chapter; moreover, attention has been paid to the multihormonal control of hepatic vitellogenin synthesis as it is related to seasonal changes and to vitellogenin use by growing oocytes. The hormonal mechanisms regulating vitellogenin synthesis are also considered, and the effects of environmental estrogens on the feminization process in wildlife and humans have been reported. It is then considered how fundamental nonmammalian models appear to be, for vitellogenesis research, addressed to clarifying the yolkless egg and the evolution of eutherian viviparity.

An ultrastructural study on the vitellogenesis in the spotted ray Torpedo marmorata

The present investigation strongly suggests that in Torpedo the oocyte growth is not only due to the uptake of exogenous molecules, but also by the oocyte itself and the granulosa cells. The oocyte, starting from the early previtellogenic follicles (see also Mol. Reprod. Dev. 61 (2002) 78), synthesizes large amounts of glycogen. Later, as the oocyte growth goes on, the cytoplasm of granulosa cells progressively bears numerous islets of glycogen, which are also evident inside the intercellular bridges and in the oocyte cortex, suggesting that they may flow from granulosa cells to the oocyte. The contribution of granulosa cells seems to become most relevant during the vitellogenesis. In vitellogenic follicles, both small, intermediate, and pyriform-like cells bear numerous vacuoles containing vitellogenin-like material, suggesting strongly that in Torpedo, differently from other vertebrate species, granulosa cells could be engaged in vitellogenesis. The present investigation does not allow us to know if such a material is due to a transcytosis process and/or is synthesized inside them. The organization of granulosa seems to exclude the possibility that it is transferred to granulosa via transcytosis. On the contrary, granulosa cells, especially in vitellogenic follicles, display the morphological organization of metabolically active cells, so they could be engaged in vitellogenin synthesis. This interpretation is consistent with the observation that granulosa cells are positively stained by OZI (osmium tetroxide-zinc iodide) and that the same positivity is evident on intercellular spaces, containing vitellogenin-like material, and on nascent yolk globules.

Annual cycles of steroid hormone production, gonad development, and reproductive behavior in the Atlantic stingray

The mating season of the Atlantic stingray (Dasyatis sabina), which begins in August and continues through April, is the longest documented for any elasmobranch fish. Despite this protracted mating period, female stingrays ovulate synchronously at the end of the mating season and there is no evidence for sperm storage by females. Thus, the proximate causal factors and ultimate function of this extended preovulatory mating are unknown. Annual cycles of the gonadal steroids testosterone (T), dihydrotestosterone (DHT), 17beta-estradiol (E2), and progesterone (P4) were measured for 26 months in a wild estuarine population of Atlantic stingrays to test for associations with their reproductive biology, gametogenesis, and sexual behavior. Serum androgen levels in males showed four phases within an annual cycle: (1) androgen suppression between reproductive seasons (April-July), (2) primary androgen increase during the onset of spermatocyte development (August-October), (3) androgen decrease following maximum testis growth and spermatocyte development (November-December), and (4) secondary androgen increase during the peak of sperm maturation (January-March). Increases in male E2 and P4 were correlated with spermatocyte/spermatocyst formation, maximum testis weight, and the primary (but not secondary) androgen surge. We propose that the production of male androgens across the full seven-month preovulatory mating period promotes their aggressive reproductive behavior and drives the protracted mating season of this species. In females, serum T and DHT showed relatively brief increases near ovulation, whereas E2 and P4 showed brief increases near both ovulation and parturition. The increase in female androgens near ovulation may increase female aggression when they are impregnable by courting males and enhance their choice of mates. This estuary sample population shows higher absolute steroid levels and distinct differences in temporal cycles compared to another Florida fresh water lake population, but the cause and significance of these differences are unknown. Experiments are needed to confirm that the aggressive and protracted mating behavior is the result of prolonged male androgen production and to determine whether the sustained preovulatory mating serves some function related to female reproduction.

Identification of vitellogenin in the little skate (Raja erinacea)

1. Vitellogenin was isolated from mature female skates by selective precipitation with MgCl2/EDTA followed by chromatography on DEAE-cellulose columns. 2. A single monomer of approximately 205 kDa was identified on 6.0% SDS-PAGE gels. 3. In addition, isolation of yolk proteins with ammonium sulfate yielded proteins of 94 and 38 kDa (putative phosvitins) and putative lipovitellins of ca 105, 91 and 67 kDa. 4. In vivo phosphate incorporation in female and male skates implanted with estradiol indicated that vitellogenin was phosphorylated. 5. Total protein phosphate incorporation was significantly higher in females than male skates. 6. In male skates treated with estradiol, phosphate incorporation increased from 2 days after implantation to a maximum at approximately 11 days after implantation. 7. Determination of the rate of disappearance of 32P-labeled protein suggests a half-life of ca 200 hr in normal female skate plasma.

The purification and partial characterization of carp, Cyprinus carpio, vitellogenin

A procedure is described for the isolation of intact vitellogenin (c-VTG) from the carp, Cyprinus carpio. VTG was induced in juvenile females using oestradiol-17β and purified from the plasma using a combination of gel-filtration chromatography on Sepharose 6B and ion exchange chromatography on DEAE-cellulose. Purification procedures were conducted at low temperatures (below 9°C) in the presence of the proteolytic enzyme inhibitor aprotinin to prevent degradation. Intact c-VTG had an apparent molecular mass of 390,000 Daltons, but when extracted from plasma in the absence of aprotinin it underwent proteolysis into at least 2 protein fragments (apparent molecular masses of 230,000 and 96,000 Daltons), showing an instability of the native dimer. An amino acid analysis of c-VTG showed that its composition was almost identical to goldfish VTG, a species closely allied to the true carps and also similar to other oviparous vertebrate VTGs. Collectively, these data indicate that using these purification procedures VTG from carp, and probably other teleost species, can be isolated in an intact, highly purified form.

Concanavalin A reactivity of vitellogenin and yolk proteins of the threespined stickleback Gasterosteus aculeatus (Teleostei)

1. Concanavalin A (con A) reactive proteins have been detected in the plasma and ovaries of the oestradiol treated Gasterosteus aculeatus. 2. Concanavalin A-horseradish peroxidase (HRP) technique applied on nitrocellulose membranes reveals that vitellogenin (Vg) is the only mannose and glucose rich glycoprotein present in the plasma of oestradiol treated sticklebacks. Stickleback Vg can be purified by con A-Sepharose chromatography. 3. Con A reactivity in the ovary changes in the course of development of the oocytes. First, the yolk vesicles, which are synthesized by the oocyte itself, become con A positive. Later, the yolk granules, which contain vitellogenin synthesized in the liver and taken up from the plasma, show a clear affinity for con A. Con A staining disappears when mannopyranoside is added. 4. No con A staining is found in the periodic acid/Schiff staining chorion.

A comparative study of some properties of vitellogenin (Vg) and yolk proteins in a number of freshwater and marine teleost fishes

1. Injection of oestradiol results in induction of vitellogenic proteins in the plasma of stickleback, sea bass, sea horse, pipe-fish, rainbow trout and roach. 2. Immunoblotting after SDS-PAA gradient gel electrophoresis of plasma proteins of stickleback, sea bass and rainbow trout reveals that vitellogenins from these species share common antigenic determinants. 3. Yolk proteins are recognized by antivitellogenin antisera which shows that proteolytical cleavage does not destroy all antigenic determinants. 4. Vitellogenic proteins are present only in the yolk granules and near the theca cell layer as is shown by immunocytochemical methods.