The Menstrual Cycle Consists of Two Interconnected Cycles
- The ovarian cycle, which focuses on egg development and release.
- The uterine cycle, which prepares the uterus for a potential pregnancy.
The Menstrual Cycle
The menstrual cycle is a recurring sequence of physiological changes in the female reproductive system, typically lasting about 28 days, that prepares the body for potential pregnancy.
Remember: FSHand oestradiol work together to prepare the egg and the uterus for potential fertilization.
Hormones Are The Key Players in the Menstrual Cycle
- To understand the menstrual cycle, it’s essential to know the primary hormones involved and their roles.
- These hormones interact through positive and negative feedback mechanisms to regulate the ovarian and uterine cycles:
Follicle-Stimulating Hormone (FSH)
Stimulates the growth of ovarian follicles and oestradiol production.
Luteinizing Hormone (LH)
Triggers ovulation and supports the formation of the corpus luteum.
Oestradiol
A form of estrogen that promotes the growth of the uterine lining (endometrium) and regulates FSH and LH via feedback loops.
Progesterone
Maintains the endometrium, preparing it for implantation, and inhibits FSH and LH to prevent new follicle development.
Don't let "oestradiol" throw you off, it is used officially (and more specifically) to mean "estrogen"
The Ovarian Cycle Is A Three-Stage Process Involving Egg Development and Release
1. Follicular Phase (Days 1–14)
- FSH Stimulates Follicle Growth: Multiple follicles begin to develop in the ovary, but only one becomes dominant.
- Rising Oestradiol Levels: The maturing follicle produces increasing amounts of oestradiol, which promotes follicle development and thickens the uterine lining.
- Positive Feedback: High oestradiol levels trigger a surge of LH.
- Associate positive feedback with a "green light" (LH surge leading to ovulation)
- Associate negative feedback with a "red light" (progesterone preventing new follicle development).
2. Ovulation (Day 14)
- LH Surge: The surge in LH causes the dominant follicle to rupture, releasing the egg into the oviduct (fallopian tube).
- Completion of Meiosis: The oocyte completes the first stage of meiosis, preparing it for fertilization.
3. Luteal Phase (Days 15–28)
- Corpus Luteum Formation: The ruptured follicle transforms into the corpus luteum, which secretes progesterone and oestradiol.
- Role of Progesterone: Thickens the endometrium, preparing it for embryo implantation, and inhibits FSH and LH secretion (negative feedback).
- If No Fertilization Occurs: The corpus luteum degenerates, causing a drop in progesterone and oestradiol levels. This leads to the breakdown of the endometrium and the start of a new cycle.
Use mnemonics, like "FOCUS" for FSH: Follicle growth, Oestradiol production, and preparation for Cellular development Until Surge.
Common Mistake- Don’t confuse ovulation with menstruation.
- Ovulation occurs around Day 14, while menstruation marks the start of the cycle.
- Imagine a gardener preparing soil for planting.
- During the luteal phase, progesterone acts like nutrients enriching the soil (endometrium) to support a seed (embryo).
- If no seed is planted, the soil is cleared for the next cycle.
The Uterine Cycle Prepares For Shedding the Endometrium
1. Menstrual Phase (Days 1–5)
- Shedding of the Endometrium: Low levels of oestradiol and progesterone cause the uterine lining to break down and be expelled as menstrual blood.
2. Proliferative Phase (Days 6–14)
- Endometrial Growth: Rising oestradiol levels stimulate the repair and thickening of the endometrium in preparation for a potential pregnancy.
3. Secretory Phase (Days 15–28)
- Preparation for Implantation: Progesterone from the corpus luteum further thickens the endometrium and increases its blood supply.
- If No Implantation Occurs: Progesterone levels drop, the endometrium breaks down, and the menstrual phase begins.
- Think of the uterine cycle as preparing a guest room.
- During the secretory phase, the room is furnished and decorated (endometrium thickens) in anticipation of a guest (embryo).
- If no guest arrives, the room is cleared (menstruation).
Feedback Mechanisms
- Positive Feedback
- High oestradiol levels during the follicular phase stimulate the LH surge, leading to ovulation.
- Negative Feedback
- Progesterone and oestradiol inhibit FSH and LH secretion during the luteal phase, preventing new follicle development.
- Students often confuse positive and negative feedback.
- Remember: positive feedback amplifies a process (e.g., LH surge), while negative feedback reduces it (e.g., inhibition of FSH and LH).
Why the Menstrual Cycle Matters
The menstrual cycle is essential for reproduction, but its hormonal regulation also affects other aspects of health, such as bone density and mood.
Theory of KnowledgeHow does the balance of positive and negative feedback in the menstrual cycle reflect broader principles of homeostasis in biology?
