Oestradiol and Progesterone Switch On Genes Inside Their Target Cells
Both oestradiol (an oestrogen) and progesterone are lipophilic steroid hormones.
Because they are lipid-soluble, they diffuse across the plasma membrane and bind intracellular receptors, and the hormone-receptor complex acts as a transcription factor to change gene expression (the general mechanism is covered in C2.1.12).
Each hormone acts on specific target cells that carry its receptor, so the same signal produces different effects in different tissues.
Tip
Steroid hormones cross the membrane freely because they are lipid-soluble.
This is why their receptors sit inside the cell, not on its surface.
Oestradiol Drives Growth of the Uterine Lining and Sexual Characteristics
Definition
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.
Its main target cells are in the endometrium (uterine lining) and in tissues that show secondary sexual characteristics.
Thickening and repairing the endometrium
In endometrial cells oestradiol switches on genes that drive cell division, so the lining thickens and repairs after menstruation.
This rebuilds the lining during the proliferative phase, ready for a possible pregnancy.
Driving secondary sexual characteristics
In target tissues oestradiol switches on genes that produce female secondary sexual characteristics at puberty, such as breast development.
How Oestradiol Acts on a Target Cell
Oestradiol diffuses through the plasma membrane of the target cell.
It binds an intracellular receptor to form a hormone-receptor complex.
The complex enters the nucleus and acts as a transcription factor, changing the expression of target genes such as those for cell division.
Note
Oestradiol also feeds back to the hypothalamus and pituitary to help regulate the cycle.
That feedback control is covered in C2.1.14.
Progesterone Maintains the Endometrium for Pregnancy
Progesterone is a steroid hormone produced by the corpus luteum after ovulation.
Its main role is to maintain the uterine lining (endometrium) for a possible pregnancy.
Keeping the lining nutrient-rich
Progesterone switches on genes that keep endometrial cells active and secreting nutrients, so the lining stays thick and ready for a fertilised egg.
Quietening the uterine muscle
It inhibits contractions of the uterine muscle, so an implanted embryo can grow undisturbed.
Analogy
Progesterone is like a caretaker keeping the endometrium ready to host an embryo.
If no embryo arrives, the lining is dismantled.
How Progesterone Acts on a Target Cell
Progesterone diffuses through the plasma membrane of endometrial cells.
It binds an intracellular receptor in the cytoplasm.
The complex enters the nucleus and acts as a transcription factor, changing the expression of genes for cell maintenance and nutrient secretion.
Example
Progesterone switches on the gene for insulin-like growth factor.
This factor supports growth and upkeep of the endometrial lining.
What Happens Without Pregnancy
If no pregnancy occurs, progesterone levels drop.
Without progesterone to maintain it, the endometrium is shed during menstruation.
Common Mistake
Progesterone does not act only on the uterus.
It also targets other tissues such as the breasts.
Active recall
Why can oestradiol and progesterone bind receptors inside the cell rather than on its surface?
What effect does oestradiol have on gene expression in endometrial cells?
How does progesterone maintain the endometrium after ovulation?
What happens to the endometrium when progesterone levels fall?
Once inside the nucleus, what role does the hormone-receptor complex play?
