Problem 3

Background and context

The menstrual cycle consists of the development and release of a mature egg from the ovaries and the thickening of the uterus lining. If pregnancy does not occur, the uterine lining is shed resulting in menstruation. This process is under the control of the endocrine system based on changing hormone levels. The length of the menstrual cycle varies but on average lasts for 28 days. It consists of the ovarian cycle and uterine cycle that are divided into distinct phases as shown in Figure 43.17. These cycles occur concurrently and are tightly regulated by hormones.

Ovarian cycle – This indicates the changes in the ovaries involving the maturation and release of the egg and is divided into 2 phases.

• Follicular phase: This phase starts from the first day of the period and ends with ovulation. The hypothalamus secretes the gonadotropin-releasing hormone (GnRH) that stimulates the pituitary to release the follicle-stimulating hormone (FSH) and the luteinizing hormone (LH). These hormones signal the ovaries to produce follicles. The follicles develop and finally, only one follicle containing the mature egg will be released from the ovary in a process called ovulation. 
• Luteal phase: The corpus luteum is formed which secretes progesterone and a small amount of estrogen. The uterine lining thickens in preparation to receive the fertilized egg. The increase in progesterone stops the production of FSH and LH from the pituitary gland. 

Uterine cycle – This indicates the changes in the uterus with respect to progesterone and estrogen levels. This is divided into 3 phases.

• Menstrual phase: If the egg is not fertilized after ovulation, the corpus luteum degenerates and progesterone levels decrease. This causes the shedding of the thick uterine lining that passes out of the vagina as a menstrual period. This phase lasts for about 5 days.
• Proliferative phase: After the end of the menstrual phase, the estradiol levels rise during this period for the maturation and release of the egg. The endometrium lining becomes thicker in anticipation of fertilization.
• Secretory phase: The progesterone levels increase in this phase making the endometrium ready for implantation of the fertilized egg. If the egg is not fertilized, the corpus luteum degenerates and the progesterone levels decline.  This causes the endometrium lining to shed leading to menstruation and a new cycle begins.