Estrogen and Progesterone

Gonadotropins

In a sexually mature female a predictable series of changes occurs every month within the ovariesThe female gonads that produce eggs and hormones. and the uterusThe muscular organ where a fertilized egg implants and develops.. These processes are referred to separately as the ovarian cycleThe cycle of follicle development, ovulation, and corpus luteum formation. and the uterine cycle and are regulated by specific hormones. These processes are linked by a series of cascading events.

It all begins when the hypothalamusA small but vital brain region controlling hormones, temperature, and autonomic functions. releases GnRH or the gonadotropin releasing hormone into the hypophyseal portal systemBlood vessel network connecting hypothalamus to anterior pituitary..  This hormone travels to the anteriorThe front of the body or toward the front when standing in the anatomical position. lobe of the pituitary and connects with cellsThe basic structural and functional units of life. that have receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter for GnRH. The acidophils of the anterior pituitary make the hormones FSH, the follicle-stimulating hormone, and LH, the luteinizing hormone(LH): Triggers ovulation and testosterone production..  Like any hormone these chemical messengers are released into systemic circulation.

LH targets the thecal cells surrounding ovarian folliclesStructures in the ovaries that contain developing oocytes.. It prompts them to produce androgensMale sex hormones, including testosterone and DHEA.. These androgens can be converted into estrogensFemale sex hormones responsible for secondary sex characteristics and regulation of the menstrual cy by the granulosa cellsCells in the follicle that support the oocyte and produce estrogen..  FSH finds it target in the granulosa cells of the follicles, causing them to make estrogen

Estrogen

When ovarian follicles are primary follicles, they have very little granulosa and produce only a little bit of estrogen.  This causes slow follicle growthAn increase in size and number of cells. at first and also causes inhibition of GnRH.  As the granulosa cells are slowly added, the level of estrogen produced slowly rises.  At some tipping point, the level of estrogen no longer slows the growth process but switches to accelerating it.

The high level of estrogen causes a positive feedbackA control mechanism that amplifies a change instead of reversing it. mechanism to add granulosa cells to the follicle.  At high levels, estrogen encourages GnRH release, increasing FSH and LH as well.  It’s important to realize that the effects of estrogen on hormones and oocytes is different at high and low levels.  It’s the opposite actually.  The rising level of estrogen also causes some of the primary oocytesImmature egg cells halted in development until puberty. arrested in meiosis I to restart meiosis. 

Estrogen has somatic effects that are important to maintain throughout the female lifetime. Estrogen acts like superaldosterone. It causes the reabsorptionThe process of fluid moving back into capillaries from surrounding tissues due to colloid osmotic p of a lot of salt and waterThe universal solvent essential for life.. This results in the bloating common in the ovarian cycle. Estrogen helps maintain cholesterolA lipid molecule that is a key component of cell membranes and a precursor for bile acids and steroi.  Some women entering menopauseThe natural cessation of menstruation and ovarian function. have horrible cholesterol issues from the loss of estrogen.  Better known are the calcium issue that accompany menopause.  Estrogen encourages osteoblasts to be actively removing calcium from blood and storing it in your bone reservoir.  With a loss of estrogen, osetoclasts and their bone-eating osteolysis start to out-perform the osteoblasts leading to osteoporosis.  Can you just take in more calcium.  No.  You need to bank it before you get to menopause.  Further complicating the accumulation of calcium either in or out of menopause is the need for vitamin D.  No use taking that calcium if you’re not getting your vitamin D and sunlight.

Permissiveness

Permissiveness was a topic that we saw back in the endocrine topic.  At very high levels, the thyroid hormones inserted receptors for epinephrineadrenaline): Fight-or-flight hormone from the adrenal medulla. into cardiac muscle tissue.  This made the myocardium of the heart more responsive to epinephrine although there was no additional epinephrine A catecholamine hormone and neurotransmitter involved in the fight-or-flight response. being produced.  The permission TH gave to epinephrine generated the high heart rate associated with hyperthyroidism.  Estrogen and progesteroneA hormone that supports pregnancy and regulates the menstrual cycle. do the same to each other.  Estrogen spends its time pre-ovulation inserting receptors for progesterone into organs.  Then progesterone spends its time inserting receptors for estrogen into organs.

Progesterone

Progesterone is known as the pregnancy hormone and dominates the phase that comes after ovulationThe release of a mature oocyte from the ovary..  The remaining granulosa in the ovary converts to a corpus luteum, which emits progesterone and a little estrogen.  At this point, the entire body is waiting for a zygote to be implanted into the uterus.  The corpus luteum is like, “Well, I sent forth an ovumA mature egg cell so let’s assume it’s been fertilized and nest.”  The endometriumThe inner lining of the uterus that thickens during the menstrual cycle to support a potential pregn of the uterus starts to thicken. It receives increased blood delivery from those coiled arteriesBlood vessels that carry oxygenated blood away from the heart (except pulmonary arteries, which carr in the basal layerThe deeper layer of the endometrium that remains intact during menstruation and regenerates the func. It also receives mucus from mucus glands in the functional layerThe superficial layer of the endometrium that is shed during menstruation..  Lipid metabolismThe sum of all chemical reactions in the body. preserves as much as it can instead of spending the calories.  Hair is not lost.  You have to remember that we are mammals, define as having hair and mammary glands.  Hair is something that can help conserve heat when living exposed to the elements.

As progesterone reaches its peak, it starts to thicken the cervical mucus.  This forms a cervical mucus plug. Sperm have trouble swimming through this plug. It prevents a second pregnancy and causes competition in the womb.  I once read an article about a surrogate woman. She was implanted with a zygote from another couple.  It was confirmed that she was indeed pregnant with the other couple’s fetus. Then she became pregnant with her own fetus.  Well, a cervical mucus plug should have prevented that from happening.

Birth Control

Estrogen based birth control usually has an increasing dose of estrogen for three weeks. Then there is a week that lets the estrogen level fall. In contrast, the progesterone only pill maintains constant high progesterone levels. This mimics the effect of the corpus luteum during pregnancy. Progesterone only birth control simulates pregnancy by tricking the cervix into making that cervical mucus plug. The plug will be maintained by the constantly high level of progesterone.  What if you miss a day?  The cervical mucus plug washes away and you have to start all over again.  This is what led to the creation of IUDs or intrauterine devices such as the one pictured here.  These devices deliver that constant progesterone.  In the 2010s, these were replaced with shots of progesterone such as the depo-shot.