Thyroid Hormone Stimulates the Proliferation of Sertoli Cells and Single Type A Spermatogonia in Adult Zebrafish (Danio rerio) Testis

Abstract

Thyroid hormones participate in regulating growth and homeostatic processes in vertebrates, including development and adult functioning of the reproductive system. Here we report a new stimulatory role of thyroid hormone on the proliferation of Sertoli cells (SCs) and single, type A undifferentiated spermatogonia (A(und)) in adult zebrafish testes. A role for T3 in zebrafish testis is suggested by in situ hybridization studies, which localized thyroid receptor α (thrα) in SCs and the β (thrβ) mRNA in Sertoli and Leydig cells. Using a primary zebrafish testis tissue culture system, the effect of T3 on steroid release, spermatogenesis, and the expression of selected genes was evaluated. Basal steroid release and Leydig cell gene expression did not change in response to T3. However, in the presence of FSH, T3 potentiated gonadotropin-stimulated androgen release as well as androgen receptor (ar) and 17α-hydroxylase/17,20 lyase (cyp17a1) gene expression. Moreover, T3 alone stimulated the proliferation of both SCs and A(und), potentially resulting in newly formed spermatogonial cysts. Additional tissue culture studies demonstrated that Igf3, a new, gonad-specific member of the IGF family, mediated the stimulatory effect of T3 on the proliferation of A(und) and SCs. Finally, T3 induced changes in connexin 43 mRNA levels in the testis, a known T3-responsive gene. Taken together, our studies suggest that T3 expands the population of SCs and A(und) involving Igf signaling and potentiates gonadotropin-stimulated testicular androgen production as well as androgen sensitivity.

Fsh Stimulates Spermatogonial Proliferation and Differentiation in Zebrafish via Igf3

“…The present results on the localization and effects of Igf3 suggest autocrine stimulation of Sertoli cell proliferation (59) and paracrine stimulation of the proliferation of type A and type B spermatogonia as well as their entry into meiosis. Future studies will have to show what germ cell stage(s) can respond directly to Igf3, eg, by studying Igf receptor localization.…”

“…Also in trout (57) and seabream (58), Igf receptors were found in somatic and germ cells, which is consistent with our analysis of Igf receptor gene expression in wild-type and germ cellfree mutant zebrafish. Genetic evidence as regards the cell type-specific effects of receptor gene ablation is missing in zebrafish, but recent work showed that thyroid hormoneinduced Sertoli cell proliferation involved elevated igf3 transcript levels, and that this proliferation was compromised (but not fully blocked) by an Igf receptor antagonist (59). One way of understanding the results on Igf3-stimulated germ cell development presented here is assuming a direct stimulatory effect of Igf3 on germ cells, as has been shown for Igf1 in primary spermatogonial cell culture studies of trout (60), and possibly also an autocrine stimulation of Sertoli cell proliferation to support the survival of newly formed spermatogonia type A.…”

Crossover of the Hypothalamic Pituitary–Adrenal/Interrenal, –Thyroid, and –Gonadal Axes in Testicular Development

“…The level of both gonadotropins, as well as related gene expression, can be altered by hyper- and hypothyroidic conditions in Mus musculus (8, 9, 68, 71). Moreover, studies have shown that THs can interfere with the regulatory activity of FSH, influencing the rate of proliferation and the functioning of Sertoli cells of Rattus norvegicus (57, 103) and Danio rerio (38). The Sertoli cells are found within the seminiferous tubules and are responsible for spermatogenesis (104).…”

“…For example, thyroidectomized rams ( Ovis aries ) – during their seasonal testicular regression – show an increase in blood FSH concentration and a faster testis growth (77, 108). Moreover, in a testis tissue culture of D. rerio , T3 in combination with FSH increases 11-ketotestosterone (11-KT) synthesis (38); the main bioactive androgen in fish (Figure 2). Thus, it has been proposed that FSH partially mediates the effects of THs in male sexual development in D. rerio .…”

PBDE flame retardants

“…244,245 Recent evidence, in some contrast, also supports a stimulatory role for THs in the proliferation of sertolli cells and spermatogonia in zebrafish testes. 246 Little is known about the role of PBDE-induced TH disruption in potential reproductive toxicity in fishes or other vertebrates. One hypothesis is that PBDEs could be mimicking THs that is in turn leading to altered steroidogenesis and steroidal hormone regulation among exposed animals, although this has not been examined.…”