Question

Describe the changes that occur in ovaries and uterus in human female during the reproductive cycle.

Answer 1

The duration of the reproductive cycle is divided into four phases:

• the menstrual phase,
  
• the preovulatory phase,
  
• the ovulatory phase, and
  
• the postovulatory phase.
  

In the menstrual phase, the endometrium of the uterus is shed and the discharge occurs because of the declining levels of hormones, especially progesterone, stimulating the release of prostaglandins. During this phase, some 20 secondary follicles in each ovary begin to enlarge and continue to do so through the pre-ovulatory phase, the time between menstruation and ovulation, under the influence of FSH. By about day six, one follicle has outgrown the others and becomes the dominant follicle or the Graafian follicle. Estrogens and inhibin secreted by the follicle decrease the secretion of FSH and the other follicles stop growing. The Graafian follicle continues to enlarge until it is ready for ovulation. It continues to produce estrogen under the influence of LH. At day 14, the follicle ruptures and releases an oocyte. This process is known as ovulation. After ovulation, the mature follicle collapses. The post-ovulatory phase of the female reproductive cycle lasts approximately from day 15 to 28 and represents the time between ovulation and the onset of the next menses. In the ovary, after ovulation, the LH stimulates the remnants of the mature follicle to develop into the corpus luteum, which secretes increasing quantities of progesterone and some estrogens. This is called the luteal phase of the ovarian cycle. Subsequent events in the ovary that ovulated an oocyte depend on whether or not the oocyte becomes fertilized. If the oocyte is not fertilized, the corpus luteum has a lifespan of only two weeks, after which it degenerates into a corpus albicans. As the levels of progesterone, estrogens, and inhibin decrease during this phase, GnRH, FSH, and LH release increases. Then follicular growth resumes and a new ovarian cycle begins.

If, however, the oocyte is fertilized and begins to divide, the corpus luteum persists. The level of the human chorionic gonadotropin (HCG), a hormone produced by the chorion of the embryo after fertilization increases. HCG acts like LH in stimulating the secretory activity of the corpus luteum and the presence of hCG in maternal blood or urine is an indicator of pregnancy