Can Probiotics Affect Reproductive Processes of Aquatic Animals? - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 329

1996; Ohta et al. 1997), are usually more serious in female broodstocks, and may affect the
reproductive system at different levels.
This chapter provides an overview of the fish reproductive system, broodstock reproductive
dysfunctions, the existing interplay between reproduction and metabolism, and the effects of
probiotics on the reproduction and fecundity of cultured fish.

12.2 THE FISH REPRODUCTIVE SYSTEM

Similar to other vertebrates, the reproductive functions of fish are regulated by the interplay
of the three major organs - hypothalamus, pituitary and gonads - which form the so-called
hypothalamus-pituitary-gonadal axis (HPG axis) (Peter and Yu 1997; Yu et al. 1997). The
hypothalamus, integrating internal and external stimuli, releases the gonadotrophin releas-
ing hormone (GnRH) (Zohar et al. 2010). Under the influence of the GnRH, the pituitary
secretes follicle stimulating hormone (FSH) and luteinizing hormone (LH) which act upon
the ovary, regulating oogenesis (Holland et al. 2001; Moles et al. 2007) (Figure 12.1). During
oogenesis, the oocytes grow, differentiate, undergo meiosis and incorporate macromolecules,
such as proteins, lipids, sugars, enzymes, vitamins, hormones, growth factors and transcrip-
tion factors, required to support embryo development (Brooks et al. 1997). Oogenesis con-
sists of two distinct stages: (1) the growth stage and (2) the maturation phase. The growth
stage, or vitellogenesis, is controlled by the FSH (Figure 12.2). In all oviparous vertebrates,
during vitellogenesis, the liver synthesizes and releases vitellogenin (Vtg), a bulky and com-
plex calcium-binding phospho-glycoprotein, under the stimulation of estradiol (E 2 ) produced
by the follicular cells (Wallace and Selman 1990; Carnevali et al. 1999; Calp et al. 2003;
Polzonetti-Magni et al. 2004). Once synthesized in the liver, vitellogenin is released into
the bloodstream and is sequestered by the growing oocytes via specific receptors (VtgRs)

EXTERNAL STIMULI
(temperature, photoperiod, etc.)

INTERNAL STIMULI
(metabolic state, etc.)

+ / ‒ + / ‒

Brain

GnRH

+

Pituitary

GtH

FSH
LH

Egg

+

Ovary

Sex
Steroids

Fig. 12.1 Diagrammatic presentation of the actions of the HPG axis in the control of reproductive
function. The hypothalamus, integrating internal and external stimuli, releases the gonadotrophin releasing
hormone (GnRH). Under the influence of the GnRH, the pituitary secretes follicle stimulating hormone (FSH)
and luteinizing hormone (LH) which act upon the ovary, regulating oogenesis by activating steroidogenesis.
For colour detail see Plate 12.