Growth factors in spermatogenesis

Abstract

The aim of this study was to increase the knowledge of die effects of locally produced growth factors and hormones on germ cell proliferation in rat testis in vitro. Since impaired sperm production and function accounts for half the cases of infertility, it is important to understand the physiological actions in germ cell development. Androgens and follicle stimulating hormone are considered to be the most important factors in the regulation of spermatogenesis, and Sertoli cells hold receptors for these hormones. Previous studies together with the present data suggest that there is an equally important intra-testicular network of hormones and growth factors interacting in a paracrine or autocrine fashion. The transillumination method used in this study, where segments of seminiferous tubules were identified and micro-dissected, is considered to closely mirror the physiological state in vivo, thus keeping the germ cells in their microenvironment enfolded in between Sertoli cells in the seminiferous epithelium.

Several studies show that mitotic and meiotic DNA synthesis are differentially regulated. We have studied the role of interleukin-1alpha, (IL-1alpha) in testis development, where IL-1alpha protein and mRNA were expressed at the onset of puberty in the rat testis and seminiferous epithelium. Further on we have investigated the role of several growth factors and hormones located in the testis, on germ cell proliferation. Insulin-like growth factor (IGF)-I, IGF-II, and ligands of the epidermal growth factor (EGF) family, EGF and transforming growth factor-alpha, are all known to play an important role in fetal and postnatal development. They are also found to be produced in several tissues including the testis, in our system they stimulated spermatogonial proliferation.

Human growth hormone had a slight stimulatory effect on spermatogonial proliferation which can either be a direct effect or mediated via local production of IGF-I in the tissue culture. Betacellulin, also a ligand of the EGF family but not yet explored in the testis, was used to further confirm specificity of EGF action as it a] so binds to the EGF receptor (erb1). In addition, fibroblast growth factor (FGF) -1 and -2 which are shown to be produced in the testis, and their receptors are differentially expressed by the various germ cells, show stimulatory effect on spermatogonial proliferation. In the presence of FGF2, there was no cell loss compared to control, in stage I segments of seminiferous tubules, corresponding to type A spermatogonia, using supravital staining by NM, indicating that FGF-2 is a mitogenic as well as a survival factor in the testis.

Due to several studies indicating that estrogens are important for not only female reproduction but also male fertility we investigated the role of estrogen and androgen metabolites on germ cell proliferation in vitro. 5alpha-androstane 3beta, 17beta-diol (3betaA-diol), a metabolite of testosterone produced via the intermediate dehydrotestosterone (DHT; a potent androgen), has recently been shown to bind to the estrogen receptor-beta (ERbeta), bid not to the androgen receptor (AR). Estrogen, 3betaA-diol and androstendione exerted similar stimulatory effects on spermatogonial proliferation. In contrast, DHT, which unlike androstendione, cannot be aromatised, had no effect in this process. As germ cells carry ERbeta and not AR, we suggest that the stimulatory effect on spermatogonial proliferation is via the estrogen receptor-beta.

Altogether, my findings confirm that the intra-testicular regulatory system is of importance for testicular and germ cell development, and the factors I have studied play a role in the complex process of spermatogenesis. Identifying growth factors and hormones that are required for qualitatively and quantitatively normal spermatogenesis is of importance for the understanding of male infertility. Once these factors are elucidated, it will be possible to identify abnormalities resulting in idiopathic testicular failure. Hopefully in the future we will both understand environmental aspects of endocrine disrupters on male reproduction and also develop specific therapies to restore fertility.