MENSTRUAL CYCLE

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 MENSTRUAL CYCLE



1. a) Definition of Menstrual Cycle (5%)

The menstrual cycle refers to the regular natural process that occurs in a woman's reproductive system, preparing the body for pregnancy each month. It is a complex series of hormonal changes that facilitate the maturation and release of an egg from the ovaries, along with the preparation of the uterine lining to receive and nurture a fertilized egg. If fertilization does not occur, the lining is shed, resulting in menstruation, and the cycle begins anew. The average menstrual cycle lasts about 28 days, though it can vary from 21 to 35 days depending on the individual.

1. b) Phases of the Ovarian Cycle (15%)

The ovarian cycle is divided into three distinct phases, each playing a crucial role in the development and release of an ovum. These phases are regulated by hormones and occur within the ovaries:

  1. Follicular Phase
    This phase begins on the first day of menstruation and lasts until ovulation. It typically spans about 10 to 14 days. During this phase, the hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary gland to secrete follicle-stimulating hormone (FSH). FSH promotes the growth and maturation of ovarian follicles, each containing an immature egg. As the follicles grow, they release estrogen, which helps thicken the endometrial lining in preparation for a potential pregnancy.

  2. Ovulation
    Ovulation marks the midpoint of the ovarian cycle and usually occurs around day 14 in a 28-day cycle. A surge in luteinizing hormone (LH), triggered by the rising estrogen levels from the dominant follicle, causes the mature follicle to rupture and release an egg (ovum) into the fallopian tube. This phase lasts about 24 hours, and the egg remains viable for fertilization during this time. If sperm is present, fertilization can occur; otherwise, the egg disintegrates.

  3. Luteal Phase
    Following ovulation, the ruptured follicle transforms into the corpus luteum, a temporary endocrine structure that secretes progesterone and small amounts of estrogen. These hormones maintain the thickened uterine lining and prepare it for implantation in case of fertilization. If fertilization does not occur, the corpus luteum degenerates after about 14 days, leading to a drop in progesterone and estrogen levels. This hormonal decline signals the onset of menstruation and the start of a new cycle.

1. c) Phases of the Uterine Cycle (15%)

The uterine cycle, also known as the endometrial or menstrual cycle, refers to the changes that occur in the uterine lining (endometrium) in response to hormonal fluctuations. This cycle is divided into three main phases:

  1. Menstrual Phase
    This phase corresponds to the beginning of the menstrual cycle (days 1 to 5), when the previous cycle's endometrial lining is shed due to a drop in progesterone and estrogen levels. The shedding results in menstrual bleeding, or menstruation, as the body expels blood, mucus, and tissue through the vagina. This phase marks the start of a new cycle and typically lasts about 3 to 7 days, depending on the individual.

  2. Proliferative Phase
    The proliferative phase begins immediately after the menstrual phase and continues until ovulation (days 6 to 14 in a 28-day cycle). During this phase, rising estrogen levels, produced by the growing ovarian follicles, stimulate the regeneration and thickening of the endometrial lining. The cells in the uterine lining proliferate, forming a new functional layer that will be capable of supporting a fertilized egg. The endometrium also becomes rich with blood vessels, in preparation for potential pregnancy.

  3. Secretory Phase
    Following ovulation, the secretory phase (days 15 to 28) begins under the influence of progesterone secreted by the corpus luteum. The endometrial lining reaches its maximum thickness and becomes more glandular and vascularized. During this phase, the endometrium secretes nutrients and fluids that would nourish a fertilized egg if implantation occurs. If fertilization does not take place, the corpus luteum regresses, progesterone and estrogen levels drop, and the endometrial lining starts to break down, leading to the next menstrual phase.

References:

  • American College of Obstetricians and Gynecologists (ACOG), 2022.
  • Tortora, G. J., & Derrickson, B. (2017). Principles of Anatomy and Physiology.
































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