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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Menstrual Cycle Hormones
The menstrual cycle is a complex, carefully orchestrated series of events controlled by hormones. These chemical messengers coordinate changes in the ovaries and uterus to prepare the female body for potential pregnancy each month. Understanding these hormones helps us make sense of the entire reproductive process!
Key Definitions:
Follicle Stimulating Hormone (FSH) is released by the pituitary gland at the start of the cycle. It stimulates the development of follicles in the ovary, each containing an immature egg. As the follicles grow, they produce oestrogen.
Luteinizing Hormone (LH) is also released by the pituitary gland. It triggers ovulation around day 14 when it surges dramatically. After ovulation, LH stimulates the empty follicle to develop into the corpus luteum, which produces progesterone.
Oestrogen is produced by developing follicles in the ovary. It causes the lining of the uterus (endometrium) to thicken in preparation for a potential pregnancy. It also triggers the LH surge that causes ovulation.
Progesterone is produced mainly by the corpus luteum after ovulation. It maintains the thickened uterine lining and prepares it for implantation of a fertilised egg. If pregnancy doesn't occur, progesterone levels fall, triggering menstruation.
The menstrual cycle can be divided into distinct phases, each controlled by different hormones. Let's explore how these hormones work together throughout the cycle.
The cycle begins with menstruation, when the uterine lining sheds. This happens because progesterone and oestrogen levels have dropped at the end of the previous cycle. During this time, FSH levels begin to rise, stimulating new follicle development in the ovaries.
The pituitary gland that produces FSH and LH is only about the size of a pea, but it controls many vital body functions! It's often called the "master gland" because it controls other hormone-producing glands.
As FSH stimulates follicle development, these growing follicles produce increasing amounts of oestrogen. This oestrogen has several important effects:
Usually, one follicle becomes dominant and continues to develop while the others degenerate.
The high levels of oestrogen trigger a sudden surge in LH around day 12-13. This LH surge causes the dominant follicle to rupture and release its egg into the fallopian tube – this is ovulation. The egg can survive for about 24 hours if not fertilised.
Women tracking their fertility often measure LH levels using home testing kits. A positive result indicates that ovulation will likely occur within the next 24-36 hours, which can help with family planning.
After ovulation, the ruptured follicle transforms into a structure called the corpus luteum. Under the influence of LH, the corpus luteum produces progesterone and some oestrogen. Progesterone maintains the thickened endometrium and prepares it for a potential pregnancy.
If the egg is not fertilised, the corpus luteum degenerates after about 10-12 days. This causes progesterone and oestrogen levels to drop sharply, which triggers the shedding of the endometrium – menstruation – and the cycle begins again.
If the egg is fertilised and implants in the uterus, the developing embryo produces human chorionic gonadotropin (hCG), which maintains the corpus luteum. This ensures continued progesterone production to support the pregnancy.
The menstrual cycle is regulated by both positive and negative feedback mechanisms that ensure hormones are released at the right time and in the right amounts.
During most of the cycle, oestrogen and progesterone exert negative feedback on the pituitary gland, keeping FSH and LH levels relatively low. This prevents multiple follicles from developing simultaneously.
Around day 12-13, high oestrogen levels switch to a positive feedback effect on the pituitary, causing a surge in LH production. This LH surge triggers ovulation, demonstrating how positive feedback creates a rapid, dramatic response.
When progesterone and oestrogen levels fall at the end of the cycle, their negative feedback effect is removed. This allows FSH levels to rise again, initiating the development of new follicles for the next cycle.
Understanding the hormonal control of the menstrual cycle has led to the development of various contraceptive methods that work by manipulating these natural hormones.
Contains synthetic versions of oestrogen and progesterone. These maintain a constant hormone level that:
Includes the mini-pill, implants and injections. These work by:
These methods are suitable for women who cannot take oestrogen.
Polycystic Ovary Syndrome (PCOS) affects about 1 in 10 women of reproductive age. It involves an imbalance of reproductive hormones, particularly elevated levels of androgens (male hormones). This can disrupt the menstrual cycle, preventing follicles from developing properly and often stopping ovulation. Women with PCOS may have irregular or absent periods and can experience difficulty getting pregnant. Treatment often involves hormonal contraceptives to regulate the cycle, even for women not seeking contraception.
Let's recap the key roles of each hormone in the menstrual cycle:
Understanding these hormones and their interactions is crucial for comprehending not just the menstrual cycle, but also fertility, contraception and many reproductive health conditions. The elegant feedback systems demonstrate how precisely the human body regulates its reproductive functions.
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