Testosterone, Spermatogenesis, and Unravelling the Mysteries of Puberty

Testosterone is not essential to life, but it is necessary for natural fertility because, without it, fertile sperm cannot be formed or delivered. The hormonal control of spermatogenesis has been a focus of reproductive biology over the past century. Milestones in its understanding were the histological classification of the spermatogenic cycle identifying mitotic and meiotic replication and discovery of the 2 gonadotropins, followed by the Nobel Prize-winning discoveries of testosterone, hormone immunoassay methods, gonadotropin-releasing hormone and targeted genetic modification of genes/peptides. Using these tools, ingenious investigators have created the present understanding of the hormonal basis of spermatogenesis and male fertility. Starting with the identification in 1977 of the GnRH1 knockout (KO), the hypogonadal (hpg) mouse, its hpg reproductive phenotype (1) of reproductive immaturity and infertility has provided a template for understanding subsequent complementary studies that contribute to the present understanding. Models comprising KOs of LHβ, FSHβ, LH, and FSH receptors, without and with treatment by a pure (dihydrotestosterone) or an aromatizable (testosterone) androgen, singly or in combined customized genetic models replicate and elaborate on various aspects of the hpg phenotype. Although nature fills biology with redundancies manifest as genetic KOs without overt phenotype, for the hormonal regulation of spermatogenesis, mouse models have provided high-fidelity replication of human genetic reproductive pathophysiology. Untreated hpg mice display prepubertal, immature testes with a characteristic meiotic block halting spermatogenesis at the pachytene spermatocyte stage. At that point, tetraploid germ cells accumulate, being unable to complete the testosterone-sensitive reductive division to form haploid germ cells. That hpg histological appearance is identical with that in naturally occurring tfm or targeted androgen receptor (AR) KO testes where AR inactivation abolishes androgen action. However, unlike tfm or AR KO testes, the suite of hpg geno/phenocopies are androgen sensitive. Raising intratesticular androgen concentration to exceed the threshold required to initiate spermatogenesis leads to testicular growth, creating small but well-formed testes. Models of LHβ with androgen add-back and LH receptor KO confirm the role of LH as elucidated in the hpg as being to induce replication and maturation of Leydig cells and maintain mature male testosterone secretion. LH-stimulated testosterone secretion provides androgen action on sperm production and maturation. In addition, LH-driven Leydig cell testosterone secretion maintains systemic androgen effects, including sexual function, by releasing testosterone down a concentration gradient from testes to the peripheral bloodstream. Notably, however, all hpg-like mouse models that lack the neonatal testosterone surge to imprint male copulatory behavior as required to deliver the fertile sperm remain infertile in mating trials. The role of FSH in spermatogenesis is clarified from multiple convergent models of FSH deficiency In the absence of FSH (FSHβ and FSH receptor KO as well as dihydrotestosterone-treated hpg, LHβ, and LH receptor KOs), testes grow to only ~40% the volume of wild-type testes, but their seminiferous tubules display quantitatively normal spermatogenesis and produce sperm with in vitro fertilizing capacity. The constrained testis size is proportionate to the reduction in numbers of Sertoli cells (SC), that have a fixed, finite carrying capacity for germ cells that constitute the bulk of testis parenchyma. Thus, the primary role of FSH in spermatogenesis is to induce SC proliferation to achieve the full complement of SC numbers (2). This synergizes with androgen action which is required for completion of meiosis and germ cell proliferation. Additionally, as an aromatizable androgen, testosterone is partly converted to estradiol, which then stimulates pituitary FSH secretion thereby reinforcing FSH-driven SC proliferation, as an additional indirect mechanism of testosterone effects in stimulating spermatogenesis. Human puberty is initiated by a still mysterious brain process triggering a cascade of at least 35 genes/proteins. This culminates in hypothalamic GnRH secretion into the pituitary portal system as the final common pathway of initiating male puberty, spermatogenesis, and fertility. The next upward rung on that ladder is kisspeptin 1 (KISS1), a peptide secreted by neurons that form the long elusive GnRH pulse generator neurons (3). Hence, the male reproductive phenotype of the KISS1 KO mouse is of great interest and, in the present issue, Goto et al. provide a detailed evaluation with their findings closely replicating the expected hpg reproductive phenotype. They confirm that, in testosterone-treated KISS1 KO males, epididymal sperm can fertilize oocytes in vitro but those males cannot impregnate females in mating trials. Despite exhibiting mounting behavior, the KISS1 KO males do not ejaculate or form copulatory plugs, lacking as they do the neonatal testosterone imprinting of male copulatory behavior. Testosterone treatment of KISS1 KO males increased testicular testosterone though the measurements was inflated by using testes without saline perfusion (to remove blood) and a testosterone immunoassay rather than mass spectrometry (4). Nevertheless, it evidently exceeded the relatively low intratesticular threshold required to overcome the meiotic block and largely restore to normal (relative to SC numbers) postmeiotic spermatids and epididymal sperm numbers. Finally, although Goto et al. speculate that their findings suggest that testosterone may induce spermatogenesis and fertility in gonadotropin-deficient men, this is unlikely given classical clinical studies establishing that, at acceptable doses in humans, testosterone treatment does not initiate spermatogenesis in hypogonadotropic hypogonadism (5) and that gonadotropin stimulation is indispensable (6). Nevertheless, because the threshold to maintain spermatogenesis is much lower than required to initiate it (7), testosterone might maintain spermatogenesis in gonadotropin-deficient individuals after initiation by gonadotrophin treatment (8). Like most incisive research studies, Goto et al. raises new questions warranting investigation including the next upward rung in the pubertal cascade and how that impacts initiation and maintenance of spermatogenesis and male fertility. Thereby, the present study by Goto et al. adds an important new element in our understanding of the cascade that enacts male puberty and initiates spermatogenesis and male fertility. androgen receptor kisspeptin 1 knockout Sertoli cell The authors thank Kirsty Walters for helpful comments on the manuscript. Data Availability: Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.