The reproductive system in females is responsible for producing gametes (called eggs or ova), certain sex hormones, and maintaining fertilized eggs as they develop into a mature fetus and become ready for delivery. A female’s reproductive years are those between menarche (the first menstrual cycle) and menopause (cessation of menses for 12 consecutive months). During this period, cyclical expulsion of ova from the ovary occurs, with the potential to become fertilized by male gametes (sperm). This cyclic expulsion of eggs is a normal part of the menstrual cycle.
Female gametes derive from germ cells. In utero, oogonia rapidly divide until approximately 7 million germ cells form by the 7th month of gestation. The number of germ cells then rapidly declines; most oogonia perish while the remaining cells, primary oocytes, begin the first meiotic division. These cells arrest in prophase I and remain dormant as such until menarche. A primordial follicle made up of granulosa and theca cells surrounds each oocyte. When primordial follicles mature, the granulosa cells proliferate to form concentric layers around the oocyte. The oocyte itself undergoes a drastic volume increase. With the onset of menarche, finite groups of oocytes periodically resume meiosis and continue to develop. At the time of fertilization, oocytes are arrested in metaphase II. The oocyte becomes an ovum as it expels its second polar body, and meiosis resumes at when the egg undergoes activation by a sperm cell (a male gamete).
Female reproductive organs
Menarche is a female’s first menstrual cycle, marked by her first episode of menstrual bleeding. Menarche occurs during puberty preceded by breast growth, axillary and pubic hair growth, and a growth spurt. At the initiation of each menstrual cycle, a number of primordial follicles in the female’s ovaries continue development. One becomes the dominant follicle and continues to grow while the other follicles become atretic and cease to develop. The dominant follicle develops into a Graafian follicle, at which point meiosis I has completed, and the ovum is no longer in prophase I arrest. At ovulation, the Graafian follicle expels the ovum from its surrounding tissue, henceforth called the corpus luteum. If no fertilization takes place, the expulsion of the egg occurs from the uterus along with the secretory endometrial lining under the influence of declining levels of progesterone; this presents as menstrual bleeding. If fertilization does take place, the fertilized egg implants in the endometrial wall and the endometrial lining is maintained by progesterone secreted (initially) by the corpus luteum until the placenta takes over.]
The normal menstrual cycle divides into the follicular and luteal phases with ovulation occurring between phases. The follicular phase begins with menstrual bleeding and ends right before the LH (luteinizing hormone) surge. The luteal phase begins with the LH surge and ends with the onset of menses. A typical cycle lasts approximately 28 days; the luteal phase lasts 14 days while the follicular phase is more variable in its time course.
Low serum levels of estradiol and progesterone mark the beginning of the follicular phase. The lack of inhibitory feedback allows for an increase in pulsatile GnRH (gonadotropin-releasing hormone) levels, leading to elevations in FSH (follicular stimulating hormone) and LH. This rise in FSH levels stimulates follicular maturation, resulting in continued growth of a select number of follicles. The growth of these follicles results in increasing FSH and estradiol levels. By the end of the follicular phase, the dominant follicle has emerged and increased to a size of approximately 20-25mm. The increase in estradiol induces thickening of the endometrium to accommodate potential implantation of a fertilized egg.
When estradiol levels reach a critical level, the negative feedback effect of estradiol on LH becomes a positive feedback effect, resulting in a massive surge in LH concentration (and a smaller surge in FSH levels.) Approximately 36 hours following the LH surge, the oocyte is released from the dominant follicle and travels into the uterus via the fallopian tube. The corpus luteum (the remaining follicular tissue following oocyte expulsion) releases progesterone, inhibiting the release of LH and FSH and stimulating the formation of the secretory endometrium. In the absence of fertilization, declining LH levels contribute to a decline in progesterone and estradiol levels. In the presence of fertilization, the oocyte implants into the endometrium and releases chorionic gonadotropin which maintains the corpus luteum and thus progesterone production.
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