Effects of exogenous FSH on follicular recruitment in a viviparous lizard Niveoscincus metallicus (Scincidae)

In the viviparous skink Niveoscincus metallicus clutch size appears to be determined before vitellogenesis, and is not altered later by follicular atresia or embryonic loss. This suggests that the number of follicles recruited is determined by the endocrine environment early in the vitellogenic period. Through a series of experiments in which we manipulated gonadotropin concentrations by administering exogenous FSH, we aimed to investigate this hypothesis. Pre-vitellogenic females showed no response to exogenous ovine FSH. In early vitellogenic females, FSH induced follicular recruitment: follicles were enlarged and clutch size increased by recruitment of a second cohort of follicles; some females also ovulated. Females treated with FSH in mid-vitellogenesis had elevated mean plasma estradiol concentrations compared to controls; no follicular recruitment was observed, but most of these animals ovulated. Females treated with a range of doses of FSH in late vitellogenesis ovulated at least one month before natural ovulation, again without recruitment of extra follicles. It appears therefore that in Niveoscincus metallicus exogenous FSH can induce recruitment of additional follicles only if administered during early vitellogenesis. We conclude that in this species clutch size is determined by proximate environmental factors influencing gonadotropin levels early in follicular recruitment, and cannot be increased even if conditions become more favourable once vitellogenesis is established.     

Vitellogenin uptake and vitellin localization in insect follicles examined using monoclonal antibodies and confocal scanning microscopy

Summary

Confocal scanning immunofluorescent microscopy and monoclonal antibodies were used to examine the route of uptake of vitellogenin (VG) by vitellogenic follicles and the ooplasmic localization of vitellin (VN) in the cricket, Acheta domesticus, and the stick insect, Carausius morosus. Uptake and cytoplasmic regionalization of a non-vitellogenic sulfated protein, sp 157/85, by C. morosus oocytes were also examined. By indirect immunofluorescence VG in both species and sp 157/85 were visualized in spaces between follicle cells and in peripheral yolk spheres. One cricket VG polypeptide had a regionalized distribution in the folliclular epithelium, and VN polypeptides in both species and sp 157/85 in C. morosus had regionalized distributions within the ooplasm. Localization of sp 157/85 to the anterior pole of the oocyte appeared to be stage-specific.     

Immuno-localization of ferritin polypeptides in oocytes and somatic tissue of the freshwater snails Lymnaea stagnalis L. and Planorbarius corneus L.

Summary Antibodies were raised in rabbits against the 19000 Mr and 24000 Mr polypeptides of snail ferritin from Lymnaea stagnalis L. Anti-24000 Mr polypeptide antibodies were purified by an affinity-purification step and were made monospecific for their antigen by preabsorption with the 19000 Mr antigen. These purified antibodies were then used for in situ detection of their respective antigens by the indirect immunofluorescence method. The 19000 Mr polypeptide was found widely distributed in tissues of both pulmonate snails investigated (Lymnaea stagnalis L. and Planorbarius corneus L.) with the most intense antigen-directed fluorescence in certain connective tissue cells, secretory cells of the midgut gland and Sertoli cells and epithelia of the gonadal acini. In contrast, the 24000 Mr polypeptide could be detected only in yolk platelets of vitellogenic oocytes. The results indicate that yolk and somatic cell ferritins differ in immunoreactivity and structure and, accordingly may differ in function.This investigation was supported by the Deutsche Forschungsgemeinschaft. I greatly appreciate the advice given to me by Drs. U. Mays and V. Riedel, Münster.     

Ultrastructure and lipid content of the liver of the zebrafish,Brachydanio rerio,related to vitellogenin synthesis

The female zebrafish is capable of producing mature eggs on the fifth day of each reproductive cycle. During this five-day period the ultrastructure of hepatocytes undergoes several changes. The number of nuclear pores increases rapidly during spawning, followed by a proliferation of RER within 24 h. Two days after spawning, glycogen has disappeared and the liver contains large amounts of lipids. The lipid droplets are closely surrounded by elongated mitochondria. Golgi complexes are abundant, secreting dense bodies. Four days after spawning the hepatocytes tend to regain their pre-spawning appearance. It is suggested that the changes in the hepatocytes, which coincide with special phases of ovarian activity, are related to vitellogenin synthesis. Steroids, especially estradiol-17beta, may trigger this process in the liver.     

Probable evolution of the coelacanth's reproductive style: lecithotrophy and orally feeding embryos in cichlid fishes and in Latimeria chalumnae

Synopsis The living coelacanth is a livebearer. Yolk seems to be the main source of nutrients and of oxygen to the embryo (fetus). Long before birth, young may also possibly feed orally on histotrophe secretion and egg debris. This type of reproduction evolved, as in most other fishes, from oviparity. The Carboniferous coelacanth Rhabdoderma exiguum had eggs of much lesser yolk volume and may represent an earlier form of oviparity with hiding, guarding or brooding type of parental care. The Jurassic coelacanth Holophagus (Undina) and the Cretaceous Axelrodichthys appear to have already evolved the internal-bearing style. Much of this evolutionary sequence is similar to that in cichlids. Ancestral cichlids are substrate tenders and nesters, with small eggs, little yolk and a feeding larva with indirect development. Mouthbrooding cichlids evolved a few, large eggs with denser yolk, direct development and, ultimately, orally feeding embryos while yolk is still in ample supply. Mixed feeding from yolk and orally ingested food in cichlids and in coelacanths is shown to be an enhanced mode of food delivery to the embryos over that from each source separately, in order to produce directly a better developed or larger young at the time of release, i.e. independence. Increase in egg size is regarded as an environmentally induced, altered pattern of yolk synthesis and an initial component of the epigenetic mechanism leading towards greater specialization. Carotenoids are incorporated within the yolk to assist the oxidative metabolism of the developing embryo.