Follicular Phase and Ovulation

The follicular phase begins on the first day of menstruation and lasts until approximately Day 14, based on a 28-day cycle. During this phase, a number of ovarian follicles, each typically containing a single ovum, develop under the influence of follicular stimulating hormone (FSH) produced by the anterior pituitary, a deep, midline endocrine organ in the brain. This hormone is produced and delivered in response to the pulsatile release of a neurohormone, gonadotropin releasing hormone (GnRH), which is produced in the medial basal region of a second brain structure, the hypothalamus. The developing follicles, in turn, produce estrogen, which has three main effects: it dampens the further release of FSH by the anterior pituitary; it, along with GnRH, stimulates the production and gradual release of luteinizing hormone (LH) by the anterior pituitary; and it stimulates the growth of the uterine lining or endometrium. Over the course of 2 weeks, one of the ovarian follicles matures more than the others in that it is larger, has evidence of more mitotic and biosynthetic activity, and has greater vascularization. This "dominant" follicle progresses through three phases (preantral, antral, and preovula-tory) and manufactures increasing amounts of estrogen and, to a lesser extent, progesterone. The other follicles gradually and irreversibly decline.

Toward the end of the follicular phase, a rise in progesterone and surges of estrogen, LH, and FSH take place, resulting in ovulation. Estrogen peaks 24 to 36 hours prior to ovulation, whereas LH and FSH peak roughly within 10 to 12 hours of ovulation. Lu-teinizing hormone stimulates the initiation of oocyte meiosis, leutinizes the granulosa cells of the follicle,

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Figure 2 The normal menstrual cycle. Hormonal ovarian, endometrial, and basal body temperature changes throughout the normal menstrual cycle. P, progesterone; E2, estradiol; LH, luteinizing hormone; FSH, follicle-stimulating hormone. From Carr, B. R., Wilson, J. D. ''Disorders of the Ovary and Female Reproductive Tract,'' in Harrison's Principles of Internal Medicine, 13th Edition. Edited by Isselbacher, K. J., Braunwald, E., Wilson, J. D., et al, p. 2022. New York, McGraw-Hill, 1994. Copyright © 1994 McGraw-Hill, Inc. Used with permission.

Figure 2 The normal menstrual cycle. Hormonal ovarian, endometrial, and basal body temperature changes throughout the normal menstrual cycle. P, progesterone; E2, estradiol; LH, luteinizing hormone; FSH, follicle-stimulating hormone. From Carr, B. R., Wilson, J. D. ''Disorders of the Ovary and Female Reproductive Tract,'' in Harrison's Principles of Internal Medicine, 13th Edition. Edited by Isselbacher, K. J., Braunwald, E., Wilson, J. D., et al, p. 2022. New York, McGraw-Hill, 1994. Copyright © 1994 McGraw-Hill, Inc. Used with permission.

and promotes progesterone and prostaglandin synthesis within the follicle. Progesterone potentiates estrogen effects and also triggers the FSH surge and activates prostaglandins and enzymes present in the follicle. This allows the follicular wall at the edge of the ovary to rupture. Influenced by hormones and chemicals in the immediate locale, the ovum then detaches from its anchor within the ovarian follicle. The release of the ovum from the ovary, allowing it to travel down one of the two fallopian tubes for possible fertilization, is defined as ovulation.

A number of physical and psychological findings have been noted during the follicular and ovulation phases of the menstrual cycle, including endometrial breakdown and sloughing during the initial 2 to 8 days of the follicular phase (i.e., menses), followed by proliferation, vascularization, and differentiation of the endometrium for implantation; increased production of thin, relatively alkaline cervical and vaginal mucus in response to raising estrogen levels; an initial sustained decrease in basal body temperature, followed by a temperature rise shortly after ovulation; regular alterations of electrolyte composition of urine and saliva; and a heightened sense of personal well-being, enhanced sensory perceptions, and, perhaps, somewhat improved cognitive task performance throughout the follicular phase.

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