Oestradiol and Progesterone Regulate Gene Expression in Specific Target Cells
- Hormones like oestradiol and progesterone play key roles in regulating processes in specific target cells by binding to intracellular receptors.
- Their effects are highly localized to particular cell types, such as cells in the hypothalamus or the endometrium, and involve the regulation of gene expression.
Remember that steroid hormones like oestradiol can pass through cell membranes because they are lipid-soluble.
Oestradiol: Regulating Hormone Release in the Hypothalamus
Oestradiol
A form of estrogen that promotes the growth of the uterine lining (endometrium) and regulates FSH and LH via feedback loops.
- Oestradiol is a steroid hormone produced mainly in the ovaries.
- It plays a central role in the menstrual cycle, influencing reproductive organs and the brain.
- The hypothalamus is a small but critical region in the brain that regulates hormone release.
- Gonadotropin-Releasing Hormone (GnRH): The hypothalamus produces GnRH, which triggers the pituitary gland to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
During the follicular phase of the menstrual cycle, rising oestradiol levels eventually trigger a surge in LH, leading to ovulation.
Mechanism of Action
- Entry into the Cell: Oestradiol diffuses through the plasma membrane of hypothalamic cells.
- Binding to Receptor: It binds to an intracellular receptor in the cytoplasm.
- Activation of Gene Expression: The hormone-receptor complex enters the nucleus and acts as a transcription factor, enhancing the production of GnRH mRNA.
- Don’t confuse positive and negative feedback.
- Oestradiol’s effect depends on its concentration and the timing within the menstrual cycle.
Progesterone Maintains the Endometrium
- Progesterone is another steroid hormone, primarily produced by the corpus luteum in the ovaries after ovulation.
- Its main role is to prepare and maintain the uterine lining (endometrium) for a potential pregnancy.
If pregnancy does not occur, progesterone levels drop, leading to the shedding of the endometrium during menstruation.
How Does Progesterone Affect the Endometrium?
- Thickening the Lining: Progesterone stimulates the endometrial cells to proliferate and thicken, creating a nutrient-rich environment for a fertilized egg.
- Preventing Contractions: It inhibits uterine muscle contractions, ensuring the embryo can implant and grow undisturbed.
Progesterone is like a caretaker, ensuring the "home" (endometrium) is ready to host a guest (embryo) or dismantling it if no guest arrives.
Mechanism of Action
- Diffusion into Cells: Progesterone diffuses through the plasma membrane of endometrial cells.
- Binding to Receptor: It binds to an intracellular receptor in the cytoplasm.
- Gene Activation: The hormone-receptor complex enters the nucleus and activates genes involved in cell proliferation and secretion of nutrients.
Progesterone activates the gene for insulin-like growth factor, which supports cell growth and maintenance of the endometrial lining.
Common Mistake- It’s a common misconception that progesterone only affects the uterus.
- While its primary role is in the reproductive system, it also influences other tissues, such as the breasts and brain.
How do the feedback mechanisms of oestradiol and progesterone reflect broader biological principles, such as homeostasis and regulation?
Self review- What is the role of oestradiol in GnRH regulation during the menstrual cycle?
- How does progesterone affect the endometrial cells during the luteal phase?
