Introduction
In vertebrates, reproduction is primarily controlled by the hypothalamus-pituitary-gonad axis. The gonadotropin-releasing hormone (GnRH) neurons in the hypothalamus are key regulators of the hypothalamic neuroendocrine system mainly though the synthesis of GnRH (Moenter et al., 2003). The decapeptide GnRH is released from the hypothalamus in a pulsatile manner, and the amplitude and frequency of these pulses change throughout the estrous cycle (McCartney et al., 2002; Moenter et al., 2003). High amplitude and low frequency pulses of GnRH secretion occur during the luteal phase of the estrous cycle with pulse frequency increasing and amplitude decreasing during the follicular phase (Savio et al., 1988; Clarke and Pompolo, 2005). …show more content…
The initiation of estrous cycles in heifers occurs at the time of puberty. A preovulatory dominate follicle (DF) and corpus luteum (CL) are the two primary ovarian structures that play a role in regulating the estrous cycle. The normal estrous cycle averages 21 d in cows and 20 d in heifers with a normal range of 18 to 24 d (Werner et al., 1938; Olds and Seath, 1951; Beal et al., 1980). Each cycle consist of two phases: the luteal phase (14 to 18 d) and the follicular phase (4 to 6 d). The luteal phase is often further separated as metestrus and diestrus while the follicular phase is further separated as proestrus and estrus. During metestrus of the luteal phase P4 concentrations begin to rise due to CL formation from the collapsed ovulated follicle (corpus haemorragicum) in which the granulosal and thecal cells of the ovulated follicle luteinize and produce P4 (Ireland et al., 1980). Progesterone concentrations remain elevated during diestrus of the luteal phase due to the presence of the CL (Ireland et al., 1980). Follicular development continues and is initiated throughout diestrus by the release of FSH from the anterior pituitary. Dominant follicles that grow during the luteal phase do not ovulate. Through negative feedback, P4 only allows the secretion of greater amplitude but lesser frequency LH pulses that are inadequate for ovulation of the DF (Ireland et al., 1980; Rahe et al., 1980). During proestrus of the follicular phase, PGF2α secretion from the uterus causes the regression of the CL and progesterone concentrations rapidly decrease (Hansel and Convey, 1983). Luteinizing hormone binds to theca cells of the DF resulting in the synthesis of androgens that diffuse into the granulosa cells of the DF. Follicle stimulating hormone binds to the granulosa cells and increases aromatase
In vertebrates, reproduction is primarily controlled by the hypothalamus-pituitary-gonad axis. The gonadotropin-releasing hormone (GnRH) neurons in the hypothalamus are key regulators of the hypothalamic neuroendocrine system mainly though the synthesis of GnRH (Moenter et al., 2003). The decapeptide GnRH is released from the hypothalamus in a pulsatile manner, and the amplitude and frequency of these pulses change throughout the estrous cycle (McCartney et al., 2002; Moenter et al., 2003). High amplitude and low frequency pulses of GnRH secretion occur during the luteal phase of the estrous cycle with pulse frequency increasing and amplitude decreasing during the follicular phase (Savio et al., 1988; Clarke and Pompolo, 2005). …show more content…
The initiation of estrous cycles in heifers occurs at the time of puberty. A preovulatory dominate follicle (DF) and corpus luteum (CL) are the two primary ovarian structures that play a role in regulating the estrous cycle. The normal estrous cycle averages 21 d in cows and 20 d in heifers with a normal range of 18 to 24 d (Werner et al., 1938; Olds and Seath, 1951; Beal et al., 1980). Each cycle consist of two phases: the luteal phase (14 to 18 d) and the follicular phase (4 to 6 d). The luteal phase is often further separated as metestrus and diestrus while the follicular phase is further separated as proestrus and estrus. During metestrus of the luteal phase P4 concentrations begin to rise due to CL formation from the collapsed ovulated follicle (corpus haemorragicum) in which the granulosal and thecal cells of the ovulated follicle luteinize and produce P4 (Ireland et al., 1980). Progesterone concentrations remain elevated during diestrus of the luteal phase due to the presence of the CL (Ireland et al., 1980). Follicular development continues and is initiated throughout diestrus by the release of FSH from the anterior pituitary. Dominant follicles that grow during the luteal phase do not ovulate. Through negative feedback, P4 only allows the secretion of greater amplitude but lesser frequency LH pulses that are inadequate for ovulation of the DF (Ireland et al., 1980; Rahe et al., 1980). During proestrus of the follicular phase, PGF2α secretion from the uterus causes the regression of the CL and progesterone concentrations rapidly decrease (Hansel and Convey, 1983). Luteinizing hormone binds to theca cells of the DF resulting in the synthesis of androgens that diffuse into the granulosa cells of the DF. Follicle stimulating hormone binds to the granulosa cells and increases aromatase