The uterine tubes are paired tubes that extend bilaterally from the uterus toward the ovaries (see Fig. 23.1). Also commonly referred to as the Fallopian tubes, the uterine tubes transport the ovum from the ovary to the uterus and provide the necessary environment for fertilization and initial development of the zygote to the morula stage. One end of 846 changes that involve two phases:  Follicular phase  Luteal phase Ovulation occurs between the two phases (Fig. F23.3.1). The follicular phase begins with the development of a small number of primary follicles (10 to 20) under the influ-ence of FSH and LH. Selection of dominant follicles occurs by days 5 to 7 of the menstrual cycle. During the first 8 to 10 days of the cycle, FSH is the principal hormone influenc-ing the growth of the follicles. It stimulates the granulosa and thecal cells, which begin to secrete steroid hormones, principally estrogens, into the follicular lumen. As estrogen production from the dominant follicle increases, FSH is in-hibited by a negative feedback loop from the pituitary gland. Estrogens continue to accumulate in the follicular lumen, fi-nally reaching a level that makes the follicle independent of FSH for its continued growth and development. Late in the follicular phase, before ovulation, progesterone levels begin to rise under the influence of LH. The amount of estrogens in the circulating blood inhibits further production of FSH by the adenohypophysis. Ovulation is induced by a surge in the in the FSH level. It occurs approximately 34 to 36 hours after the start of the LH surge or about 10 to 12 hours after the peak of the LH surge. The luteal phase begins immediately after ovulation as the granulosa and thecal cells of the ruptured follicle undergo rapid morphologic transformation to form the cor-pus luteum. Estrogens and large amounts of progesterone are secreted by the corpus luteum. Under the influence of both hormones, but primarily progesterone, the en-dometrium begins its secretory phase, which is essential for the preparation of the uterus for implantation in the event that the egg is fertilized. LH appears to be responsi-ble for the development and maintenance of the corpus lu-teum during the menstrual cycle. If fertilization does not occur, the corpus luteum degenerates within a few days as the hormonal levels drop. If fertilization does occur, the corpus luteum is maintained and continues to secrete pro-gesterone and estrogens. hCG, which is initially produced by the embryo and later by the placenta, stimulates the corpus luteum and is responsible for its maintenance dur-ing pregnancy. menstrual phase proliferative phase secretory phase 0 5 10 14 20 25 28 0 5 10 15 20 0 10 20 30 40 50 menstrual cycle ovarian hormones (in blood) estrogen progesterone ovarian cycle pituitary hormones (in blood) LH FSH FIGURE F23.3.1 • Relationship of morphologic and physiologic events that occur in the men -strual cycle. This diagram illustrates the relation of the morphologic changes in the endo metrium and ovary to the pituitary and ovarian blood hormone levels that occur during the menstrual cycle. The pituitary and ovarian hormones and their plasma con centrations are indicated in arbitrary units. LH, luteinizing hormone; FSH, follicle-stimulating hormone.
 FOLDER 23.3 Functional Considerations: Summary of Hormonal Regulation of the Ovarian Cycle