menstrual cycle Flashcards
hormone biosynthesis and mechanism of action
Receptors for hormones can be divided into 2 groups
Present on cell surface
Intracellular and interact with lipophilic hormones
Present in serum and tissues in low concentrations
Receptors must have high affinity and specificity for ligands
Mainly metabolised in the liver, and to a lesser extent the kidney
steroid hormone biosynthesis, storage, release and transport
Synthesised in a series of reaction pathways from cholesterol
Process occurs mainly in the mitochondria and the smooth ER
Not stored prior to secretion (lipid soluble and permeable to membranes), they are released upon their synthesis
Circulate in blood primarily bound to plasma proteins
aromatase
converts testosterone into oestrogen in the ovary
oestrogens
Circulating oestrogens are a mix of oestrone and oestradiol
- Oestrone secreted directly from ovary or converted from androstenedione
- Oestradiol produced by the ovary, derived by direct synthesis in developing follicles or through
conversion of oestrone - The enzyme aromatase is responsible for this conversion
Oestrogens are involved in the development of female secondary sex characteristics, control of the menstrual
cycle and in pregnancy
androgens
Synthesised in the testes, the ovary and the adrenal gland
Adrenal cortex contributes approx half the daily production of androgens (males and females)
Regulate the development of male primary sex organs, secondary sex characteristics and are important in libido and sexual arousal
Examples of androgens are testosterone, dihydrotestosterone and androstenedione
progestogens
Synthesised from cholesterol via pregnenolone
Produced primarily in corpus luteum of ovary, the adrenal glands and in the placenta during pregnancy
- Endometrial development - smooth muscle control
- Maintenance of pregnancy and placenta
- Mammary gland development
hypothalamic-pituitary-gonadal axis
The menstrual cycle is regulated by interactions between the hypothalamic-pituitary-ovarian axis and the uterus
The hypothalamus secretes gonadotropin releasing hormone (GnRH) which stimulates the anterior pituitary to release follicle stimulating hormone (FSH) and luteinising hormone (LH)
The ovary (and placenta) respond to levels of the gonadotropins and secrete steroid sex hormones
hypothalamic control
Neurosecretory cells produce GnRH (Gonadotrophin Releasing Hormone)
GnRH – 10 amino acid peptide with short half life
Secreted into the portal vessels
Secreted in a pulsatile manner
Activates its receptor (GnRHR) within the anterior pituitary which stimulates release of gonadotrophins
pituitary gland
2nd level of hormonal control of reproduction
Anterior pituitary secretes peptide hormones - Gonadotrophins
follicle stimulating hormone (FSH)
luteinising hormone (LH)
Act on the ovary
Posterior pituitary secretes oxytocin
Involved in childbirth and lactation
ovaries
Ovaries (and placenta) represent the 3rd level of hormonal control
Levels of FSH and LH trigger follicle maturation and regulate steroid hormone production in the ovary
These hormones act on target tissues in the reproductive tract
follicle stimulating hormone
Initiates recruitment of follicles
Supports growth of the follicle, especially the granulosa cells
luteinising hormone
Supports theca cells
Receptors expressed on maturing follicle
LH surge triggers ovulation
stages of menstrual cycle
When thinking about ovarian function, the menstrual cycle can be defined as a preovulatory follicular phase and postovulatory luteal phase
Corresponding phases in the endometrium are termed the proliferative and secretory phases
The menstrual phase represents the time of menstruation
follicular phase
Varies in length (10-14 d)
Characterised by growth of dominant follicle
Progesterone
production is low
Oestrogen is rising
due to conversion of
androgens to oestrogens
via aromatase
development of the secondary follicle
FSH secretion increases slightly, stimulating further growth of recruited follicles.
Circulating LH levels increase slowly, beginning 1 to 2 days after the increase in FSH.
Theca develops – follicle gains an independent blood supply
Granulosa cells develop FSH, oestrogen and androgen receptors
development of secondary follicle- recruited follicles
Recruited follicles increase production of oestradiol via conversion of androgens produced in the theca interna into oestrogens by the granulosa cells – aromatase
Stimulates LH and FSH synthesis but inhibits their secretion
FSH levels decrease. FSH and LH levels diverge partly becauseoestradiol inhibits FSH secretion more than LH secretion.
Developing follicles produce the hormone inhibin, which inhibits FSH secretion but not LH secretion.
Levels of oestrogen, particularly oestradiol, increase exponentially.
uterine changes in response to hormones in the proliferative phase
Oestrogens from the ovary act on the endometrium
Thickening of the stroma
Elongation of uterine glands
Growth of the spiral arteries
ovulation
Towards the end of the proliferative phase, rising oestrogens
Increase responsiveness of pituitary to GnRH
- Surge in hypothalamic secretion of GnRH
Oestradiol peaks and progesterone levels begin to increase.
High levels ofoestradioltrigger LH secretion by gonadotropes (positive feedback).
Stored LH is released in massive amounts (LH surge), usually over 36 to 48 h, with a smaller increase in FSH.
Ovulation occurs around day 14
Oestradiol decreases
LH and ovulation
The LH surge stimulates enzymes that initiate breakdown of the follicle wall and release of mature oocyte within about 16 to 32 h.
The LH surge also triggers completion of the first meiotic division of the oocyte within about 36h of ovulation.
luteal/secretory phase
The length of this phase is the most constant, averaging 14 days
Formation of the corpus luteum from the follicle
The corpus luteum secretes primarily progesterone in increasing quantities, peaking at about 6 to 8 days after ovulation
Progesterone stimulates development of the secretory endometrium
LH and FSH levels in luteal phase
Because levels of circulatingoestradiol, progesterone, and inhibin are high during most of the luteal phase, LH and FSH levels decrease.
Oestradiol and progesterone levels decrease late in this phase.
If implantation occurs, the corpus luteum does not degenerate but remains, supported by human chorionic gonadotropin that is produced by the developing embryo.
role of corpus luteum
The corpus luteum plays a critical role in the establishment and maintenance of pregnancy
If pregnancy occurs, cells within the developing embryo begin to produce a hormone called Human Chorionic Gonadotrophin (hCG) at around 9 days.
hCG is the hormone that a pregnancy test will detect
hCG signals to the corpus luteum to continue to secrete progesterone, which in turn maintains the lining of the endometrium
menstruation
Regression of the corpus luteum and a reduction in
the secretion of progesterone
Leukocyte infiltration of endometrium
Constriction and breakdown of spiral arteries – ischemia
Menstruation begins
hormonal effects of vagina
Early follicular phase, oestrogen is low
Vaginal epithelium is thin and pale
Late follicular phase, oestrogen increases
Squamous cells mature, causing
epithelial thickening
Luteal phase
Mature squamous cells shed as cellular debris
