Ultrasonography has permitted close visualisation of follicular development in cattle and has revealed that three waves of follicles normally develop during the oestrous cycle. Follicle stimulating hormone is important for the recruitment of follicles but it is not known how the recruited follicles are selected to become dominant over other follicles and eventually to ovulate. Hypothalamic, pituitary and ovarian hormones are secreted and released episodically. During the luteal phase of the oestrous cycle, the negative feedback of progesterone prevents any rise in luteinising hormone (LH). As progesterone levels fall, LH increases, prompting an increase in oestradiol which triggers the ovulatory LH surge and behavioural oestrus. LH causes ovulation and luteinisation of follicular cells. Luteal phase progesterone may be responsible for later increases in prostaglandin F2a (PG)F2a, a luteolytic hormone in cattle and sheep which is transferred from the uterus, its site of synthesis, to the corpus luteum. Oestradiol and oxytocin, synthesised in the ovary, may be involved in regulation of uterine synthesis of PGF2a, hastening luteolysis. At this time it is not known how essential these two hormones are to the process of luteolysis. The discovery and synthesis of new ovarian hormones should permit superovulation to become a more exact and a less expensive procedure and for embryonic mortality to be reduced.
Proceedings of the New Zealand Society of Animal Production, Volume 49, , 39-42, 1989
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