Endocrine control of spermatogenesis

5.7.1 Synergistic and differential action of androgens and FSH on testicular functions

It can be stated safely that the combination of androgen and FSH consistently produced a better stimulatory effect on spermatogenesis than either factor alone. What might represent a functional distinction between testosterone and FSH is the fact that testosterone induces differentiation of somatic cells in the immature primate testis (Schlatt etal. 1993). This does not seem to be the case for FSH. Hence, devel-opmentally, testosterone may have some primacy in providing somatic testis cell differentiation and maturation prior to FSH action. This view is supported by observations in rats, that elimination of Leydig cells by ethane dimethane sulphonate or prevention of androgen action by flutamide significantly reduced the stimulatory effects of FSH on spermatogenesis (Chandolia etal. 1991), i.e. FSH testicular action is improved by prior androgen exposure. Hence, androgens might be - in relative terms - the predominant differentiation factor and FSH the predominant proliferative factor (Schlatt etal. 1995).

In primates, testosterone and FSH act to stimulate spermatogonial multiplication (Figs 5.6 and 5.7). Testosterone was suggested to be responsible for providing o if) ö tD

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Fig. 5.6 Testicular germ cell numbers (expressed per Sertoli cell) in cynomolgus monkeys (upper panel) and healthy volunteers (lower panel) during steroid-induced suppression of gonadotropin secretion. Note the marked reduction of numbers of type-B spermatogonia. Numbers of type A-pale spermatogonia are also reduced but not those of type A-dark spermatogonia (not shown). Data taken from O'Donnell et al. (2001a) and McLachlan et al. (2002b).

adequate numbers of renewing A-type spermatogonia and FSH - on top of androgen action - ensures quantitatively normal production of differentiated B-type spermatogonia (Marshall et al. 1995). According to other studies in adult (O'Donnell etal. 2001b; van Alphen etal. 1988; Weinbauer etal. 1991) and immature nonhuman primates (Arslan etal. 1993; Schlatt etal. 1995) FSH also stimulated the number of renewing A-type spermatogonia. It is interesting to note that FSH also stimulates renewing A-type spermatogonial numbers and subsequent germ cell populations in normal and mature cynomolgus and rhesus monkeys (van Alphen et al. 1988), whereas this is not the case for hCG (Teerds etal. 1989a). In the immature monkey,

Spermatogoniogenesis

Fig. 5.7 Sites of action of testosterone and FSH on the spermatogenic process in primates. The precise interrelationships between type A-pale and type A-dark spermatogonia are not entirely clear. The majority of available data indicated that testosterone and FSH stimulate spermatogenesis via increasing numbers of type A-pale spermatogonia followed by in increase of subsequent germ cell populations. Meiotic transitions appear to be independent of testosterone/FSH action. FSH has been reported to play a role in chromatin condensation during spermiogenesis. Whether testosterone is needed for spermiogenesis has not been studied yet. Spermiation is affected by gonadotropin sufficiency but it is currently unclear whether this is related to diminished actions of testosterone or FSH or both.

Fig. 5.7 Sites of action of testosterone and FSH on the spermatogenic process in primates. The precise interrelationships between type A-pale and type A-dark spermatogonia are not entirely clear. The majority of available data indicated that testosterone and FSH stimulate spermatogenesis via increasing numbers of type A-pale spermatogonia followed by in increase of subsequent germ cell populations. Meiotic transitions appear to be independent of testosterone/FSH action. FSH has been reported to play a role in chromatin condensation during spermiogenesis. Whether testosterone is needed for spermiogenesis has not been studied yet. Spermiation is affected by gonadotropin sufficiency but it is currently unclear whether this is related to diminished actions of testosterone or FSH or both.

testosterone also significantly increased the numbers of Ap-type spermatogonia (Schlatt etal. 1993).

Spermiogenesis regulation has been either linked to androgens (McLachlan etal. 2002a; 2002) or to FSH (Krishnamurthy etal. 2000a; 2000b; Moudgal and Sairam 1998) whereas spermiation seems to be dependant on both testosterone and FSH. The progression of spermatocytes into spermatids may be independent of LH/FSH as indicated by kinetic studies in gonadotropin-deficient cynomolgus monkeys using 5-bromodeoxyuridine incorporation (Aslam et al. 1999; Weinbauer et al. 1998).

Apart from stimulating cell numbers via cell multiplication, testosterone and FSH also promote germ cell survival. This was clearly demonstrated in the gonadotropin-deficient rat (El Shennawy et al. 1998). Both testosterone and FSH were able to attenuate germ cell loss and were the most effective combination. Since germ cell loss in the hormone-deficient testes also involves apoptosis (Sinha Hikim etal. 1997), it can be speculated that both hormones interfere with apoptotic pathways.

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