Ontogenic Expression Profiles of Gonadotropins ( fshb, lhb ) and Growth Hormone ( gh ) during Sexual Differentiation and Puberty Onset in Female Zebrafish

摘要

The reproductive process in teleosts is mainlyregulated by the hypothalamus-pituitary-gonad (HPG)axis. As a primary factor in the axis, gonadotropinreleasinghormone (GnRH) from the hypothalamus actsat the pituitary to control the expression and release ofgonadotropins (GTHs), follicle-stimulating hormone(FSH) and luteinizing hormone (LH), which in turnregulate gonadal activities [1]. Growth hormone (GH),another pituitary hormone, has also been demonstrated toplay an important role in reproduction, so has beentermed co-gonadotropin [2-4]. The timing of theappearance of FSH and LH-expressing cells duringreproductive development appeared to be speciesspecific.In most species studied such as the salmonids[5, 6], gilthead seabream [7] and cichlids [8], the FSHcells appeared earlier than LH cells. In the giltheadseabream, both FSH and LH cells appeared prior tosexual differentiation whereas in the cichlids, the LHcells were detected at the onset of sex differentiation [7].Both FSH and LH cells also appeared before sexdifferentiation in the pejerry, while LH cells weredetected earlier than FSH cells [9]. Puberty is the periodin vertebrate life cycle that marks the transition fromsexual immaturity to maturity to enable the animal toacquire adult reproductive function. As an importantpoint in vertebrate reproductive life, puberty has beenunder extensive study in the past decades. Although it iswell accepted that the puberty onset requires anincreased pulsatile release of GnRH which in turnactivates GTHs, the two types of gonadotropins seem tohave different roles in controlling puberty in differentspecies. In mammals, the onset of puberty is associatedwith an increase in the amplitude and frequency of LH[10]. However, evidence in teleosts points to theimportance of FSH signaling in puberty onset.