Endocrine control of spermatogenesis: Role of FSH and LH/ testosterone

Abstract

Evaluation of testicular functions (production of sperm and androgens) is an important aspect of preclinical safety assessment and testicular toxicity is comparatively far more common than ovarian toxicity. This chapter focuses (1) on the histological sequelae of disturbed reproductive endocrinology in rat, dog and nonhuman primates and (2) provides a review of our current understanding of the roles of gonadotropins and androgens. The response of the rodent testis to endocrine disturbances is clearly different from that of dog and primates with different germ cell types and spermatogenic stages being affected initially and also that the end-stage spermatogenic involution is more pronounced in dog and primates compared to rodents. Luteinizing hormone (LH)/testosterone and follicle-stimulating hormone (FSH) are the pivotal endocrine factors controlling testicular functions. The relative importance of either hormone is somewhat different between rodents and primates. Generally, however, both LH/testosterone and FSH are necessary for quantitatively normal spermatogenesis, at least in non-seasonal species.

Keywords: Leydig cells; Sertoli cells; dog; germ cells; gonadotropins; nonhuman primates; rodent; testis; testosterone; toxicology.

Figure 1.

Testicular histology in control and GnRH antagonist-treated cynomolgus and rhesus monkeys. Upper left: Control; upper right: Following 16 d of treatment–note the lack of B-type spermatogonia compared to control while other cell types are still present; middle left: following 25 d of treatment–same magnification–note lack of B-spermatogonia and depletion of pachytene spermatocytes and round spermatids and reduction of tubular diameter; middle right–same duration of treatment–note retained elongated spermatid heads at the base of the germinal epithelium (arrowheads); bottom left–following 31 d of treatment–note progressive depletion of germ cells, presence of degenerating cells (asterisks) and tubular shrinkage but also presence of elongated spermatids; bottom right–following treatment for approx. 6 months (bar indicates 100 μm) - note that only spermatogonia (asterisks) and Sertoli cells remain; Sertoli cells are stained for GATA4 (brown) (image courtesy of Dr. Plant TM, unpublished data). B = B-type spermatogonia, A = A-type spermatogonia, PS = pachytene spermatocytes, RS = round spermatids, SC = Sertoli cells Bar in middle right indicates indicates 100 μm and in bottom left 50 μm.

Figure 2.

Percent change (mean ± SEM, n = 5) of testicular size (open symbols) and sperm numbers in the ejaculate (solid symbols) of cynomolgus monkeys treated with GnRH antagonist for 18 weeks. The open triangle indicates testosterone supplementation at doses to provide normal range serum testosterone but not testicular testosterone levels in order to restore ejaculate behavior. Asterisks denote occasion when no ejaculate was obtained. Modified from ref 95.

Figure 3.

Flow cytometric analysis of testicular tissue in control and GnRH antagonist-treated cynomolgus monkeys. Animals were treated for 31 d It is of interest to note that the proportion of HC (haploid spermatids with reduced chromatin staining due to chromatin condensation hence considered to representelongated spermatids) is increased compatible with retained spermatids and the proportion of 4C (spermatogonia and primary spermatocytes in G2 phase) cells is decreased compatible with a reduced proliferation of differentiated spermatogonia and subsequently spermatocyte numbers. However, the proportion of 2C (mostly spermatogonia and spermatocytes in G1 phase) cells, representing early spermatogonia is unchanged. The 1C cell population comprises haploid round and elongating spermatids. These observations are concordant with those obtained from quantitative histological analysis. Data represent mean ± SEM of 4–6 animals per group.

Figure 4.

Testicular histology in rats. Upper left–control; upper right–following 14 d of treatment with GnRH antagonist–note degenerating germ cells (arrows) and vacuoles (asterisks); lower left–following 14 d of treatment with GnRH antagonist–note retained elongated spermatids heads at the base of the germinal epithelium (arrowheads); lower right–long-term endocrine suppression with a 17,20-lyase inhibitor–note that pachytene spermatocytes and round spermatids are still present but reduced in number, while elongating and elongated spermatids are absent. PS = pachytene spermatocytes, RS = round spermatids .

Figure 5.

Schematic description of spermatogenic stages and identification of stages and cell types affected initially by gonadotropin deficiency (yellow shaded area) in the cynomolgus monkey (upper panel) and in the rat (lower panel). Suppression of gonadotropin secretion by a GnRH antagonist or by administration of exogenous androgens or selective depletion of intratesticular testostone induces comparable stage- and cell-dependent effects. Note that primate and rodent testis initially respond quite differently to reproductive hormone deficiency: In the rodent occasional pachytene spermatocytes and round spermatids in stage VII undergo apoptosis and step 19 spermatids fail to be released (spermatid retention), whereas in primates germ cell loss initially affects a population of differentiating and dividing spermatogonia in tubules throughout the spermatogenic cycle .

Figure 6.

Testicular histology in the dog. Upper left–control; upper right–following acute testosterone depletion–note pronounced loss of germ cells, multinucleated cells (double asterisks) and germ cell with abnormal morphology (asterisk); lower left–following 14 d treatment with a neurokinase inhibitor–note detached germ cells in the lumen or in epithelium (asterisk); lower right–following chronic endocrine suppression–note that only spermatogonia and Sertoli cells are present. Sp = spermatogonia, SC = Sertoli cells.

Figure 7.

Testicular histology in the dog. Upper left–following GnRH agonist treatment for about 5 months–note that few spermatocytes (arrows) are the most advanced germ cells; upper right–following 3–9 weeks of recovery–note that round spermatids (arrow) have reappeared; lower left–following another few weeks of recovery and elongating spermatids are present (arrows); lower right–within 3–6 months spermatogenesis is completely restored and mature elongated spermatids are present (arrows). Modified from Goericke-Pesch.

Figure 8.

Testis size in dogs before and during 25 weeks of treatment with a GnRH agonist (mean ± SD of 8 animals) and following removal of the GnRH agonist implant (5 animals) for another 25 weeks. Data compiled from Ludwig.

Figure 9.

Comparison of testis size determinations either by sonography (left panel) or caliper (right panel) with testis weight in the cynomolgus monkey. Size measurements were collected prior to necropsy for comparison with true testis weight. Caliper measurements tend to underestimate testis weight possible since some pressure needs to be applied for consistency of measurements, while following sonographic size assessments, the data appear closer to testis weight.