Why does oestrogen go from inhibitory to stimulating LH/FSH?
The relationship between estrogen and luteinizing hormone (LH) and follicle-stimulating hormone (FSH) is complex and involves a negative feedback system within the female reproductive system.
At the start of the menstrual cycle, the levels of estrogen in the blood are relatively low. This low estrogen acts in a negative feedback loop on the hypothalamus and anterior pituitary gland. The hypothalamus secretes gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary to release LH and FSH. However, in the presence of low estrogen levels, this release is relatively low.
As the ovarian follicles mature and begin to produce estrogen, the levels of estrogen in the blood start to rise. Once estrogen levels reach a certain threshold, estrogen begins to exert a positive feedback on the secretion of LH and FSH.
Estrogen binds to specific receptors in the hypothalamus, causing a surge of GnRH release. This surge of GnRH then leads to a substantial increase in LH and FSH secretion from the anterior pituitary gland. This positive feedback mechanism is responsible for triggering ovulation, where a mature egg is released from the ovary.
The exact molecular mechanisms underlying this shift from estrogen’s inhibitory to stimulatory effects on LH and FSH secretion are not completely understood. However, it is believed that the switch from inhibitory to stimulatory effects involves changes in the hypothalamic sensitivity to estrogen, as well as interactions with other hormones and neurotransmitters.
It’s important to note that this positive feedback mechanism is temporary and occurs only prior to ovulation. Once ovulation has occurred, estrogen levels decrease, resulting in negative feedback that inhibits further LH and FSH secretion. This negative feedback helps to maintain a relatively constant hormonal environment during the subsequent luteal phase of the menstrual cycle.
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