Intra-testicular signals regulate germ cell progression and production of qualitatively mature spermatozoa in vertebrates

Abstract

Spermatogenesis, a highly conserved process in vertebrates, is mainly under the hypothalamic-pituitary control, being regulated by the secretion of pituitary gonadotropins, follicle stimulating hormone, and luteinizing hormone, in response to stimulation exerted by gonadotropin releasing hormone from hypothalamic neurons. At testicular level, gonadotropins bind specific receptors located on the somatic cells regulating the production of steroids and factors necessary to ensure a correct spermatogenesis. Indeed, besides the endocrine route, a complex network of cell-to-cell communications regulates germ cell progression, and a combination of endocrine and intra-gonadal signals sustains the production of high quality mature spermatozoa. In this review, we focus on the recent advances in the area of the intra-gonadal signals supporting sperm development.

Keywords: GnRH; estrogens; kisspeptins; sperm quality; spermatogenesis; spermatozoa; testis.

Figure 1

A schematic view of the effects of AEA incubation on GnRH-1 (G1), GnRH-2 (G2), GnRH-R1 (R1), and GnRH-R2 (R2) expression in frog R. esculenta diencephalon. Animals were collected in June and testes were incubated in vitro for 1 h. Via CB1 activation, AEA treatment significantly increased the expression of GnRH-R1 and GnRH-R2 whereas it decreased the expression of both GnRH-1 and GnRH-2; no effect on GnRH-R3 was observed.

Figure 2

A schematic view of the effects of AEA treatment on GnRH-1 (G1), GnRH-2 (G2), GnRH-R1 (R1), GnRH-R2 (R2), and GnRH-R3 (R3) expression in frog R. esculenta testis. Animals were collected in June (A) and February (B) and testes were incubated in vitro for 1 h. In June, AEA treatment significantly increased the expression of GnRH-R1 and GnRH-2 whereas it decreased those of GnRH-1 and GnRH-R2, and had no effect on GnRH-R3; in February, AEA treatment increased GnRH-2 and GnRH-R3 expression, decreased GnRH-R2, and had no effect on GnRH-1 or GnRH-R1. In both periods, AEA-dependent effects occurred via CB1 activation.

Figure 3

Sections of R. esculenta testis, collected in February, analyzed by in situ hybridization for GPR54. GPR54 mRNA was detected in the interstitial compartment (A,B), in primary spermatogonia (B), in secondary spermatogonia cysts (B) as well as in myoid peritubular cells (B). The specificity of signals was tested through the reaction with a sense riboprobe (C). i, Interstitial compartment; white arrow head, ISPG; dark arrow head, IISPG; m, myoid peritubular cells; scale bar: 20 μm.