LUTEOTROPHIC CONTROL OF THE CORPUS-LUTEUM

The requirement for maintenance of the corpus luteum varies between species and within the same species in the stage of the luteal phase. In both primates and domestic ruminants luteinizing hormone (LH) is essential for the secretion of normal amounts of progesterone at all stages of the luteal phase. The corpus luteum responds to LH pulses by an increase in the secretion of progesterone in the mid and late luteal phase. Removal of luteotrophic support by hypophysectomy or injection of a potent antagonist of GnRH in the early luteal phase results in a gradual reduction in the secretion of progesterone. In contrast, when luteotrophic support is withdrawn in the mid-late luteal phase of the cycle, luteal regression occurs promptly within 48 h. An inadequate corpus luteum such as occurs during puberty or following seasonal or lactational anoestrus, can result from suboptimal priming of the preovulatory follicle with gonadotrophins or reduced luteotrophic support of the corpus luteum. Both mechanisms may contribute to the periods of relative infertility which occur as full reproductive activity is being (re) established.