In summary, the hormone regulation of stallion reproduction and spermato-genesis involves a three-tier system: the hypothalamic-pituitary-gonadal axis. Acting as an overriding influence upon this axis are environmental factors such as daylength. The effect of daylength is mediated via the pineal gland and melatonin secretion, which governs the equine breeding season and, to some extent, also sexual maturity and development (Marusi and Ferroni, 1993). Under appropriate conditions, melatonin secretion declines and its inhibitory effect upon the hypothalamus is removed. In response, the hypothalamus secretes GnRH, which acts on the higher centres of the brain to affect libido and passes to the anterior pituitary via the hypophyseal portal vessels. In response, the anterior pituitary produces LH and FSH. These two hormones in turn act, respectively, upon the Leidig cells, to induce testosterone production, and on the Sertoli cells, to initiate spermatogenesis. A positive correlation between LH and testosterone pulse frequency has been demonstrated.
The functions of the hypothalamus and the pituitary are also affected by testicular hormones. Testosterone has a negative feedback effect on both, reducing the concentration of GnRH released and the sensitivity of the anterior pituitary to GnRH stimulation (Thompson et al., 1979b; Irvine et al., 1986). In response to this long negative feedback loop, the pituitary produces less LH. This negative feedback may also be attributed to the effect of dihydrotestosterone and possibly oestrogens and maybe progesterone (Amann, 1993b). As a result of this decline in LH levels, the stimulation of the Leidig cells is depressed, so reducing the production of testosterone. The decline in circulating testosterone levels reduces the negative feedback loop and allows activation of the hypothalamic-pituitary axis and further pulsatile release of LH and a corresponding increase in testosterone. Testosterone also exerts its effect on spermatogenesis, allowing completion of the process and spermatozoan production and release. Testosterone, therefore, acts as the major control over stallion reproduction, resulting in an equilibrium within a finely balanced system.
FSH is also produced by the anterior pituitary and its production controlled by GnRH. However, it is increasingly apparent that the control of LH and FSH is partly separate, as episodic release of LH does not necessarily correspond with a similar release of FSH (Thompson et al., 1985; Clay et al., 1988, 1989). In other species it has been demonstrated that FSH acts exclusively upon the Sertoli cells during the initial stages of spermatogenesis (Flink, 1988). Sertoli cells also secrete inhibin and activin, which again act upon the hypothalamic-pituitary axis and so also control the release of GnRH and the pituitary's response as far as FSH secretion is concerned. It is evident, therefore, that there are two main feedback loops that control stallion reproductive function: one primarily controlling LH and hence testosterone secretion, and the other controlling FSH and hence spermatogenesis. However, this is a considerable oversimplification of a highly complex and as yet poorly understood area of control.
In addition to the above there is evidence for the involvement of oxytocin, prolactin and oestrogen. It is also evident that there must be a continual cycle of messages between the Leidig cells, the germinal cells and the Sertoli cells and then between the testis and the body as a whole.
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