Information about the estrous cycle in sheep

The estrous cycle in female sheep is the reproductive rotation that allows repeated opportunities for the female to become pregnant. It consists of the physiologic events that occur between successive periods of sexual receptivity and/or ovulations. There are two phases involved during each estrous cycle: a follicular and luteal phase. The follicular phase is dominated by ovarian follicles which produce the hormone estrogen whereas the luteal phase is dominated by the corpus luteum (CL) structure and the hormone progesterone (Senger, 1997). The follicular phase produces the matured follicle with an oocyte surrounded by the granulosa and theca cells. The granulosa and theca cells are responsible for producing estrogen during the follicular phase; after ovulation, when the oocyte escapes these two cells types form the CL after cellular and structural changes of the postovulatory follicle in a process called lutenization. The luteal phase occurs after ovulation and ends with the regression of the CL, also known as luteolysis (Grazul-Bilska, 2006).
The CL is a temporary endocrine gland and is one of the few adult organs that exhibit periodic growth, differentiation, and regression. In sheep, the major luteotropic factor which controls luteal function is luteinizing hormone (LH) and the major luteolytic factor is prostaglandin F2a (PGF). Once fully functional, the CL only regresses if pregnancy does not occur by the secretion of PGF. (Vonnahme et al., 2006). The corpus luteum life cycle goes as follows: the corpus hemorrhagicum (the bloody structure formed from the follicle after ovulation), functional corpus luteum (produces progesterone), and corpus albicans (if no pregnancy occurs, CL is regressed) (Grazul-Bilska, 2006). The CL, along with most organs, has receptors on its membrane in order to bind substances, such as hormones, in order to initiate a cellular response. A major receptor necessary for follicular maturation, ovulation, and more importantly luteal function is the luteinizing hormone receptor (LHR). This is a transmembrane receptor present on granulosa, theca, and luteal cells of the ovary and its expression requires appropriate hormonal stimulation by follicle-stimulating hormone (FSH) and estrogen (Grazul-Bilska, 2006). LHR is the receptor for luteinizing hormone (LH) and this binding is vital because LH stimulation of the CL causes secretion of the dominant hormone progesterone of the luteal phase which maintains pregnancy and brings about healthy offspring (Knobil et al., 1994).
These studies stimulate superovulation in sheep and localize LHR in the luteal tissues in order to quantify LHR protein expression using image analysis on the CL collected on days 5, 10, and 15 of the estrous cycle and after PGF-treatment. We hypothesized that the expression of LH receptors will change during the estrous cycle and during PGF-induced luteal regression.