Abstract

URIBE-VELASQUEZ, Luis Fernando; RESTREPO-CADAVID, Ramiro  and  OSORIO, José Henry. CHANGES IN THE SECRETION OF OVARIAN STEROIDS AND LUTEINIZING HORMONE DURING THE ESTROUS CYCLE IN THE EWE: A REVIEW. Biosalud [online]. 2010, vol.9, n.1, pp.64-78. ISSN 1657-9550.

Ewes and rams from temperate latitudes show seasonal variations in reproductive activity. Sheep sexual activity is stimulated by alterations in the photoperiod, so that animals displaying reproductive seasonality can reduce their response to seasonality as latitude decreases. A functional hypothalamic pituitary axis is essential for mammalian reproduction, since the hypothalamus secretes the decapeptide, gonadotropin-releasing hormone (GnRH) that is responsible for initiating the cascade of events that regulate gonadal function. During the estrous cycle of the ewe, circulating LH and P₄ levels are inversely related, thus providing circumstantial evidence that progesterone may inhibit tonic LH secretion. Progesterone (P₄), in terms of the cumulative duration of its effects, is the most important ovarian steroid secreted during the lifetime of the female mammal and it’s central to the complex regulation of normal reproductive function. During the luteal phase of the estrous cycle, the P₄ produced by the corpus luteum inhibits hypothalamic GnRH secretion, and consequently, peripheral gonadotrophin concentrations are low. The follicular phase, which is initiated by the decline in circulating P₄ concentrations after luteolysis, is characterized by increased gonadotrophin and Estradiol (E₂) secretion. This rise in circulating E₂ induces the preovulatory LH surge, which is caused by a robust, abrupt, and continuous increase in GnRH secretion. Serial measurements of E₂, P₄ and LH during ovulation time in the ewe have shown that maximum secretion of estrogen from pre-ovulatory follicles precedes the LH surge. Recent data show that other steroids secreted by the pre-ovulatory follicle may act synergistically with oestradiol in inducing ovulation. The present review was undertaken to observe discuss the temporal relationship between circulating LH and the secretion of the ovarian steroids, estradiol 17-β and progesterone, during the oestrous cycle in the ewe.

Keywords : GnRH; luteinizing hormone (LH); oestrous cycle; ovulation; progesterone (P4).

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