Abstract
In teleosts, gonadotropins (Fsh: follicle-stimulating hormone and Lh: luteinizing hormone) secreted from the pituitary are key factors regulating gonadal steroid production and gametogenesis. At present, acquired insights into the involvement of gonadotropins in oogenesis are mainly related to the advanced stages i.e. vitellogenesis and final maturation. In contrast, the number of studies on the role of gonadotropins during the previtellogenic stage is limited although the previtellogenic stage is essential for the success of the onset of puberty and fertilization. Therefore, the main aim of the present study is to elucidate the role of gonadotropic hormones during the early stages of oogenesis using New Zealand shortfinned eels, Anguilla australis as a model. In addition to the main aim, the thesis addressed eel (Anguilla spp) composition and distribution in my home country, Vietnam, to bridge local science and current state-of-the-art techniques with a recognized fish model. The use of A. australis in the present study provided a phylogenetic contrast to published work to date. Wild previtellogenic eels are locally abundant in New Zealand and this is an advantage to obtain insights into gonadotropin biology during early oogenesis.
In order to achieve the main aim, three experimental chapters including field work, in vivo and in vitro studies were conducted. In particular, wild New Zealand shortfinned eels at different stages of ovarian development from yellow to silver eels were sampled to assess: (i) mRNA levels of pituitary gonadotropin subunits and protein levels of Fsh in the pituitary, and mRNA levels of ovarian gonadotropin receptors, and (ii) effects of gonadotropic hormones-associated changes in plasma sex steroid levels and oocyte cytology. Following the field work experiment, in vivo administration of recombinant Fsh (rec-Fsh) and human chorionic gonadotropin (hCG) to previtellogenic eels was pursued to clarify the biological function of gonadotropic hormones in ovarian development during the early stages of oogenesis. Lastly, short-term (24 hours) and long-term (21 days) in vitro culture of previtellogenic ovarian fragments with either rec-Fsh or hCG in the presence or absence of insulin-like growth factor-1 (IGF1) was performed to investigate the potential interaction between effects of gonadotropic hormones and metabolic hormones on oogenesis at the previtellogenic stage. It should be noted that the present study is the first to report on the use of rec-Fsh in previtellogenic eels to evaluate the role of Fsh in ovarian development during previtellogenesis.
This study provides the first definitive evidence on the composition of eel species in Vietnam, reflected in identification of three species, i.e., A. marmorata, A. bicolor and A. japonica (Chapter 2). Data from field work (Chapter 3) suggested that the synthesis of Fsh in the pituitary reached a peak in yellow eels which had ovaries in the oil droplet stage. Meanwhile, the findings from the in vivo and in vitro experiments (Chapter 4 and 5) indicated a clear ability of gonadotropic hormones to stimulate oocyte growth in previtellogenic eels. In particular, the use of rec-Fsh and hCG increased gonadosomatic index (in vivo) and oocyte diameter (in vivo and in vitro). The eels treated with rec-Fsh also showed more developed oocytes (Chapter 4). Therefore, the use of rec-Fsh or hCG could be potential means to induce puberty in captive eels through stimulation of oocyte growth and development. There was no clear evidence for interaction between effects of gonadotropins and metabolic hormone (IGF1) influencing the growth of previtellogenic oocytes of A. australis.
This study provides compelling evidence of involvement of gonadotropic hormones in oocyte growth of previtellogenic eels in in vitro and in vivo studies. This study sheds new light on control of previtellogenic oocyte growth and the role that gonadotropins may play in this.