Describe the menstrual cycle in terms of changes in the ovaries and in the lining of the uterus

The menstrual cycle is a complex process that involves changes in both the ovaries and the lining of the uterus, coordinating hormonal signals to prepare for potential pregnancy. This cycle can be divided into two main components: the ovarian cycle and the uterine cycle. Below is a detailed description of these changes.

Ovarian Cycle

The ovarian cycle consists of three phases: the follicular phase, ovulation, and the luteal phase.

1. Follicular Phase (Days 1-14):
   • Initiation: The cycle begins on the first day of menstruation. Low levels of estrogen and progesterone stimulate the release of follicle-stimulating hormone (FSH) from the pituitary gland, promoting the growth of ovarian follicles.
   • Follicle Development: Several follicles begin to develop, but typically only one becomes dominant (the Graafian follicle). As these follicles mature, they produce increasing amounts of estrogen, which stimulates the thickening of the uterine lining (endometrium) in preparation for a possible pregnancy.
2. Ovulation (Around Day 14):
   • LH Surge: A peak in luteinizing hormone (LH) triggers ovulation, which is the release of a mature egg (oocyte) from the dominant follicle into the fallopian tube. This event occurs approximately 10-12 hours after the LH surge.
3. Luteal Phase (Days 15-28):
   • Corpus Luteum Formation: After ovulation, the ruptured follicle transforms into the corpus luteum, which secretes high levels of progesterone and some estrogen.
   • Preparation for Implantation: Progesterone further thickens and maintains the endometrial lining, making it rich in blood vessels and nutrients to support a fertilized egg if implantation occurs.

Uterine Cycle

The uterine cycle is also divided into three phases: menstruation, the proliferative phase, and the secretory phase.

1. Menstruation (Days 1-5):
   • If fertilization does not occur, hormone levels drop sharply (especially progesterone), leading to the shedding of the thickened endometrial lining. This results in menstrual bleeding.
2. Proliferative Phase (Days 6-14):
   • Following menstruation, estrogen levels rise due to the developing follicles in the ovaries. This stimulates regeneration and thickening of the endometrium, preparing it for potential implantation. The lining becomes increasingly vascularized and glandular.
3. Secretory Phase (Days 15-28):
   • Under the influence of progesterone from the corpus luteum, the endometrium continues to thicken and becomes more secretory, producing substances that nourish a developing embryo.
   • If implantation does not occur by the end of this phase, hormone levels decline, leading to menstruation and restarting the cycle.

Summary

• The ovarian cycle involves follicle maturation, ovulation, and corpus luteum formation, regulated by hormones like FSH and LH.
• The uterine cycle includes menstruation, proliferation of the endometrial lining, and secretion of nutrients in preparation for implantation.
• These cycles are interconnected; changes in one directly influence changes in the other, ensuring that if fertilization occurs, conditions are optimal for embryo development. If not, it leads to menstruation and a new cycle begins.