Mechanisms of oocyte development in European sea bass (Dicentrarchus labrax L.): investigations via application of unilateral ovariectomy

Abstract

Unilateral ovariectomy (ULO) was performed in European sea bass (Dicentrarchus labrax L.) during late pre-vitellogenesis/early vitellogenesis. Plasma steroid levels and the expression of a suite of potential oogenesis-relevant genes in the ovary, brain, and pituitary were evaluated with the aim of understanding their involvement in the compensatory oocyte development occurring within the remaining ovarian lobe. After 69 days of surgery the remaining ovarian lobe in ULO fish was gravimetrically equivalent to an intactpaired ovary of sham operated, control fish. This compensatory ovarian growth was based on an increased number of early perinucleolar oocytes and mid-late stage vitellogenic follicles without an apparent recruitment of primary oocytes into the secondary growth phase. Plasma steroid levels were similar in ULO and control females at all time points analyzed, suggesting an increased steroid production of the remaining ovarian lobe in hemi-castrated females. Results of the gene expression survey conducted indicate that the signaling pathways mediated by Fsh and Gnrh1 constitute the central axes orchestrating the observed ovarian compensatory growth. In addition, steroid receptors, Star protein, Igfs, and members of the transforming growth factor beta superfamily including anti-Mullerian hormone and bone morphogenetic protein 4 were identified as potentially relevant players within this process, although their specific actions and interactions remain to be established. Our results demonstrate that ULO provides an excellent in vivo model for elucidating the interconnected endocrine and molecular mechanisms controlling oocyte development in European sea bass.

Bone morphogenetic protein 15 and growth differentiation factor 9 expression in the ovary of European sea bass (Dicentrarchus labrax): Cellular localization, developmental profiles, and response to unilateral ovariectomy

“…The mRNAs and/or the proteins of both factors have also been detected in follicular somatic cells in some species, such as zebrafish [7,33], rainbow trout [32], goat [48], and hen [15,29] 17,46]. Interestingly, the expression levels of BMP15 protein (as well as of its mRNA) increase in response to FSH treatment in mouse ovary, but are down-regulated in both FSH receptor null and +/-gonads [54].…”

“…In addition, other common reference genes, like bactin or 28S ribosomal RNA, have already been proved not to be suitable for their use in European sea bass ovarian samples [22]. Thus, we decided to use non-normalized relative transcript amounts for the target genes studied as recently reported [17,18]. This strategy is supported by studies showing opposite trends between normalized and non-normalized data [42], contradictory results depending on the reference gene used for the same data set [56], as well as the validity of nonnormalized expression data to explain the molecular changes associated with oocyte/ovarian development in teleost fish [22,26].…”

“…mRNA and protein expression sites for both factors were furthermore localized in the ovary by in situ hybridization and immunohistochemistry, respectively. Finally, the changes in ovarian mRNA and protein levels of both factors in response to unilateral ovariectomy (ULO), a surgical procedure implying the removal of one the gonads [17], were analyzed in order to evaluate a potential involvement of Bmp15 and Gdf9 in the growth compensation observed in the remaining ovary.…”

Biological Impact of Unilateral Oophorectomy: Does the Number of Ovaries Really Matter?

“…Plasma steroid levels of fishes subjected to oophorectomy were found to be similar to those of a control group with two intact ovaries [8]. In this case, GhRH/FSH hormones appeared to be the basic regulators of the compensatory process [8].…”

“…The growth of the remaining ovary seems to be related to an increase in the number and size of mid-to-late stage follicles without any recruitment of new primary oocytes [7]. Plasma steroid levels of fishes subjected to oophorectomy were found to be similar to those of a control group with two intact ovaries [8]. In this case, GhRH/FSH hormones appeared to be the basic regulators of the compensatory process [8].…”