Can Probiotics Affect Reproductive Processes of Aquatic Animals? - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 330

BRAIN

MATURATION

GROWTH

PITUITARY

FSH LH

ESTRADIOL

MIH

MPF

LIVER

THECA CELLS

GVBD

GRANULOSA CELLS

VITELLOGENIN

Fig. 12.2 Control of follicle growth and maturation in teleosts. The action of follicle stimulating hormone
(FSH) and luteinizing hormone (LH) is depicted in teleost oocytes during growth and maturation phases. A
teleost follicle contains an oocyte, one layer of granulosa cells and one layer of theca cells. The basement
membrane separating the granulosa and thecal cells and the zona radiata are omitted in the figure. Under
the influence of FSH, granulosa cells aromatize testosterone to estradiol, which initiates vitellogenin
production in the liver, leading to follicle growth. Under the influence of LH, granulosa cells produce MIH,
leading to the oocyte maturation. (Source: The model is based primarily on the work of Clelland and Peng
2009.) For colour detail see Plate 13.

clustered in endocytic clathrin coated pits, giving rise to the formation of Vtg-containing
coated vesicles that move into the peripheral oolema (Hiramatsu et al. 2006). Vesicles fuse
with lysosomes leading to the formation of multivesicular bodies (MVB). The presence of
lysosomal enzymes, such as cathepsins, leads to the cleavage of Vtg into the yolk components
(lipovitellins, phosvitin, phosvettes and β-components) necessary for embryonic development
(Nagahama et al. 1995; Okumura et al. 2002; Carnevali et al. 2006; Le Menn et al. 2007).
The maturation phase occurs at the end of vitellogenesis, when the accumulation of mRNAs
and the necessary yolk proteins has been completed (Patino et al. 2001). Maturation of fish
oocytes is a complex phenomenon that is characterized by the resumption of meiosis and is
accompanied by several nuclear and cytoplasmic events, including migration of the germinal
vesicle towards the micropylar cell followed by germinal vesicle breakdown (GVBD), spindle
formation, chromosome condensation and secondary cleavage of yolk proteins to free amino
acids (FAAs) which leads to the clearing of the ooplasm as well as to the hydration of the
oocyte (Kondo et al. 1997; Yamashita 1998; 2000; Le Menn et al. 2007; Carnevali et al. 2006;
2008; Nagahama and Yamashita 2008). Oocyte maturation in teleosts is stimulated by the
release of LH from the pituitary gland (Figure 12.2). In particular, at the ovary level, LH stim-
ulates the production of maturation inducing hormone (MIH) through its receptor (lhr) (Patino
et al. 2001). The ability of the follicle cells to produce MIH and the responsiveness of oocytes
to MIH represent the acquiring of maturational competence which is a very important stage
of oogenesis that signals the transition from the growth phase to the maturation phase. The
binding of MIH to its membrane receptors activates the maturation processes (Hanna and Zhu