Menstrual Cycle Interaction with the Growth Hormone Axis
The periodicity of the menstrual cycle results from interactions among the hypothalamus, pituitary, ovaries, and genital tract. Each normal cycle culminates in menstrual bleeding, the first day of which is defined as the beginning of a menstrual cycle. The human menstrual cycle is divided into three phases: a follicular, or proliferative, phase; an ovulatory phase; and a luteal, or secretory, phase. The initiation of follicular growth begins during the late luteal phase of the preceding menstrual cycle, during which serum levels of progesterone (P) and estrogen (E2) decline, because of termination of the corpus luteum lifespan, and levels of FSH (follicle-stimulating hormone) rise. The rise in FSH initiates follicular development, which continues after the onset of menstrual bleeding. However, FSH then declines as a result of negative feed- back from the increasing E2 secreted by the granulosa cells of the growing follicle. During the follicular phase, E2 levels rise in parallel to the growth of the follicle and number of granulosa cells. LH levels increase at the midfollicular phase as a result of the positive feedback caused by increased E2 release. Just before ovulation, E2 secretion increases dramatically, which initiates the LH surge. LH promotes the process of luteinization of the granulosa cells and consequently enhances P secretion, which in turn facilitates the midcycle surge. After ovulation the remaining follicle undergoes striking changes resulting in the formation of the corpus luteum, which secretes a large amount of P and, to a lesser extent, E2. Accordingly, healthy young women exhibit different gonadal hormone milieus during the normal menstrual cycle, and such variations provide a model to investigate the impact of endogenously secreted estrogen and progesterone on GH secretion.
KeywordsObesity Estrogen Cortisol Arginine Progesterone
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