HORMONES 2015, 14(4):468-478 Review A short evolutionary history of FSH-stimulated spermatogenesis Ilpo Huhtaniemi Institute of Reproductive and Developmental Biology, Department of Surgery and Cancer, Imperial College London, Hammersmith Campus, London, UK; and Department of Physiology, Institute of Biomedicine, University of Turku, Turku, Finland ABSTRACT It is well established in various experimental models that luteinizing hormone (LH) stimulated testosterone (T) production of Leydig cells is the key endocrine stimulus of spermatogenesis. The role of the other gonadotrophin, follicle-stimulating hormone (FSH), is as yet somewhat unclear given that several clinical conditions and experimental models, including men with inactivating FSH receptor (R) mutation and male Fshb and Fshr knockout mice, maintain fairly normal spermatogenesis and fertility. Furthermore, FSH treatment of male infertility has produced at best modest results. On the other hand, there are animal species (e.g. teleost fishes and the Djungarian hamster) where spermatogenesis is primarily FSH-dependent. The purpose of this article is to briefly review the gonadotrophin dependence of spermatogenesis in several model species and examine how it has shifted during evolution from FSH to LH dominance. The information may provide new insight into the role of FSH treatment of male infertility. Key words: Evolution, Follicle-stimulating hormone, Gonadotrophin, Luteinizing hormone, Pituitary gland, Spermatogenesis, Testis INTRODUCTION hormone (LH), are the pivotal endocrine regulators of these testicular functions. LH stimulates T production The adult testis has two functions: the production by Leydig cells of testicular interstitial tissue. T, on of sex steroids, in particular testosterone (T), and sper- the one hand, is secreted into the circulation where it matogenesis. Two pituitary gonadotrophic hormones, exerts its e xtragonadal sexual (potency, libido) and follicle-stimulating hormone (FSH) and luteinizing anabolic (muscle strength, bone density) actions. On Address for correspondence: the other hand, T has an intratesticular paracrine role Ilpo Huhtaniemi, Institute of Reproductive and Developmental in the maintenance of spermatogenesis, which occurs Biology, Department of Surgery and Cancer, Imperial College indirectly through stimulation of Sertoli cells present London, Hammersmith Campus, Du Cane Road, London in the seminiferous tubules adjacent to germ cells. The W12 0NN, UK; and Department of Physiology, Institute of Biomedicine, University of Turku, Kiinamyllynkatu 10, Sertoli cells then send T-dependent paracrine stimuli 20520 Turku, Finland; to spermatogenic cells. Sertoli cells are also the target E-mail: [email protected] of FSH, and thus T and FSH regulate spermatogenesis Received: 27-08-2015, Accepted: 05-10-2015 in a similar indirect paracrine fashion. A short evolutionary history of FSH-stimulated spermatogenesis 469 While it is textbook knowledge that LH-stimulated studies have demonstrated the same LH/T dominance T production is quintessential for the maintenance of in mouse spermatogenesis (see below). qualitatively and quantitatively complete spermato- Before puberty, T is essential for male genital genesis, the specific role of FSH is not to date so differentiation and growth, including testicular de- clearly delineated. FSH apparently synergizes with scent and Sertoli cell differentiation. The main steps T in the regulation of Sertoli cell function, but there of rodent spermatogenesis that are under T regula- are several examples from a variety of mammalian tion are meiosis and postmeiotic spermiogenesis. T species showing that spermatogenesis can be initi- seems to have no role in spermatogonial proliferation ated and maintained with T alone and that sufficient, and maturation,4,5 but it supports spermatocyte and though quantitatively suppressed, spermatogenesis is spermatid survival in immature rats,6 probably by possible without FSH. Moreover, considerable spe- antiapoptotic mechanisms (summarized by Ruwan- cies differences exist with respect to the role of FSH pura et al7). Androgen receptor (AR) knockout mice in spermatogenesis in comparison with the almost have clearly demonstrated that spermatogenesis does universal dominance of LH/T action. not proceed to complete meiosis without indirect T 8,9 The purpose of this review is to reflect on the action through Sertoli cells. role of FSH in the LH/T-dominated maintenance of Male Lhb and Lhr knockout mice are azoosper- spermatogenesis. Evidence will be presented that the mic,10-12 indicating that LH stimulated T production involvement of the two gonadotrophins has taken is indispensable for spermatogenesis. However, old different directions in the course of evolution. We Lhr knockout (LuRKO) mice do show low levels of first briefly review the hormonal regulation of sper- qualitatively complete spermatogenesis, apparently matogenesis in the standard experimental models, i.e. due to the residual LH independent T production of the laboratory rat and mouse, and humans. Thereaf- the immature Leydig cells present in LuRKO tes- ter, some evolutionary milestones in this regulatory tes.13 This steroidogenesis is either constitutive or process, as well as notable exceptions, are described. stimulated by paracrine Sertoli cell factors. Because This information may help us better understand the treatment with the antiandrogen flutamide blocks the pathogenesis of spermatogenic failure in humans progression from round to elongating spermatids in and offer novel strategies for the FSH treatment of old LuRKO mice, the process is clearly shown to be male infertility. dependent on T stimulation. Although LuRKO mice have elevated FSH levels, due to reduced feedback ROLE OF LH AND T inhibition of testicular inhibin, the flutamide effect demonstrates that FSH is not able to compensate for Rodents the lack of T action in this model. Much of the experimental work on spermatogenesis Numerous studies have demonstrated that FSH has been carried out in the two commonest rodent alone is not able to rescue full spermatogenesis in models, the rat and mouse. They can be discussed gonadotrophin-deficient animals, except for a small together because the role of gonadotrophins in the increase in the number of spermatogonia and pre- regulation of their spermatogenesis is very similar. meiotic spermatocytes;7 however, androgen action is Earlier studies were mainly conducted on rats, but the absolutely necessary for the completion of meiosis, most recent research, in particular genetic manipula- progression from round to elongating spermatids and tion experiments, has been carried out on mice. Rats spermiogenesis.14-16 and mice have demonstrated that T is essential for Although Leydig cells produce numerous nonsteroi- spermatogenesis but is relatively independent of FSH. dal paracrine factors, most of the evidence indicates Rat spermatogenesis can be initiated by T treat- that T is the important mediator of LH effects on ment alone in animals made gonadotrophin-deficient spermatogenesis, occurring through paracrine effects by immunization against GnRH1 or FSH2,3 or when of Sertoli cell derived factors. The best evidence for the treated with GnRH antagonist.3 Numerous more recent role of Leydig cell factors other than T in spermato- 470 I. Huhtaniemi genesis comes from comparison of T and LH [using testis size in the face of otherwise roughly normal the LH agonist human choronic gonadotrophin (hCG)] spermatogenesis and fertility.28-30 Hence, FSH action treatments in gonadotrophin-deficient hpg mice.17 per se is not necessary for the initiation of spermato- Whereas no difference was found in the production genesis and maintenance of fertility in mice. The main of mature sperm, the number of spermatogonia was role of FSH is, in synergy with T, to maximize the larger after hCG treatment. Hence, some Leydig cell quantity of sperm production. factor(s) other than T can promote the proliferation In adult rat testis, FSH impacts on gonocyte matu- and/or survival of spermatogonia. However, high-dose ration, acts as an antiapoptotic survival factor for T supplementation seems to fully restore the number spermatogonia, spermatocytes and spermatids and of mature spermatids in LuRKO18,19 and hpg16 mice. supports meiosis and spermiogenesis by regulating Furthermore, transcriptome analysis of testes of wild- the adhesion complexes between germ and Sertoli type LuRKO and T-treated LuRKO mice showed that cells (reviewed by Ruwanpura et al7). The findings most of the defects in gene expression in the absence that FSH treatment increases the numbers of all cell of LH action were corrected by T treatment.20 types up to round spermatids may be explained by Another potential Leydig cell factor stimulat- increased supply of spermatogonia and ‘passive’ in- ing spermatogenesis could be insulin-like factor 3 crease in subsequent downstream maturational steps, (INSL3), though it has been shown to be dispensable including the meiotic reduction of late pachytene for mouse spermatogenesis.21 However, recent studies spermatocytes to round spermatids. However, sper- have shown that INSL3 can promote spermatogonial matid elongation was not restored by FSH, indicating differentiation in zebrafish testis,22,23 providing another the need for additional factor(s), most critically T, as example of evolutionary divergence in the hormonal described above.31 regulation of gametogenesis. The spermatogenic effect There is also some evidence of