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Introduction –

Overview of the Brain–Hypophyseal–Gonadal Axis Environmental signals, in conjunction with social cues, are conveyed by sensory signals to various brain centers culminating in the hypothalamus.In the absence of hypophyseal portal system, hypothalamic nerve fibers branch throughout the adenohypophysis to regulate the synthesis and release of the gonadotropic hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Dopamine secreted by the hypothalamus may inhibit LH release. FSH and LH, in turn, bind to their respective receptors in the gonads (FSH-R and LH-R, respectively). The gonads respond by secretion of sex steroid hormones; estradiol 17� (E2) in females that promotes oogonial proliferation and vitellogenesis, and progestogens such as 17�,20� dihydroxy-4-pregnen-3-one (DHP) that promotes initiation of germ cell meiosis and follicular maturation and ovulation (Figure 1(a)). In males, these are androgens, mainly 11 ketotestosterone (11-KT), that regulate spermatogenesis and spermiogenesis, as well as DHP that initiates the meiotic division of spermatogonia and controls the spermatozoa maturation and spermiation. DHP in its free or conjugated forms serve as pheromones as well. Generally, fertilization occurs in the water, either in specialized nests guarded by one or both parents or in the open water. Several teleost fish such as the guppy (Poecilia reticulata) are viviparous.