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Overview_of_Female_Reproductive_Physiology - Overview of...

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Overview of Female Reproductive Physiology The menstrual cycle can be described in reference to the ovary or uterus. Thus, there is an ovarian cycle consisting of three phases: 1) Follicular (days 0-12); 2) Ovulatory (days 13 & 14); and 3) Luteal (days 15-28). The normal length of the menstrual cycle ranges from 21-35 days with an average of 28 days. The uterine cycle consists of 4 phases: 1) menses (days 0-5); 2) Proliferative (days 6-13) and 3) Secretory (days 14-28) and 4) ischemic phase immediately before menses. The timing of these phases is dependent on feedback within the hypothalamic-pituitary-ovarian axis, and the complex interactions between hypothalamic releasing hormones (GnRH), anterior pituitary hormones (FSH, LH), and ovarian hormones (estrogen, progesterone, and androgens). The hypothalamus secretes gonatropin releasing hormone (GnRH) in pulses throughout the day. GnRH stimulates the anterior pituitary to secrete the gonadotropins, follicle stimulating hormone (FSH), and luteinizing hormone (LH). FSH and LH are similar to TSH and hCG in their α subunit, but the different forms of the β subunit confers the unique structure and function to each of these hormones. FSH and LH stimulate the ovary to synthesize estrogen, progesterone, and androgens. The hypothalamic-pituitary-ovarian axis generally functions in a classic negative feedback manner with elevated levels of ovarian hormones inhibiting the release of FSH and LH. The exception to this occurs during the follicular phase of the ovarian cycle when elevated levels of estrogen stimulates the pituitary to increase LH secretion (the midcycle LH surge) via a positive feedback mechanism. The ovarian follicular phase varies in duration from 10-16 days, and is characterized by follicle development and high levels of estradiol secretion. The ovarian luteal phase is more constant with a length of 14 days, and is characterized by formation of the corpus luteum and high levels of progesterone. In summary, the hypothalamic-pituitary-ovarian axis is controlled from the bottom up. Ovarian events determine ovarian development and the hypothalamic response. The ovarian follicle determines ovulation and the uterine cycle. The ovary has 2 functions: folliculogenesis and steroidogenesis. Refer to the text for the description of folliculogenesis. Steroidogenesis refers to the synthesis of estrogens, progesterone and androgens. There are 3 types of estrogen: 1) estradiol (17- β estradiol); 2) estrone and 3) estriol. The androgens are androstenedione, dehydroepiandrosterone (DHEA), and testosterone. Estrone is the dominant form of estrogen in menopause. Estriol is 1/80 th as potent as estradiol, and is formed from the conversion of DHEA. It is elevated in pregnancy. Steroidogenesis occurs in the ovary and is currently described by the 2 Cell, 2 Gonadotropin hypothesis. The 2 Cells are the Theca and Granulosa cells of the follicle.
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