Lecture 8- The Menstrual Cycle Flashcards
The Hypothalamic-Pituitary-Gonadal (HPG) Axis
The hypothalamus, anterior pituitary gland and gonads (ovaries) work together to regulate the menstrual cycle.
Gonadotropin releasing hormone (GnRH) from the hypothalamus stimulates
luteinising hormone (LH) and follicular stimulating hormone (FSH) release from the anterior pituitary gland.
LH and FSH are gonadotropins that act primarily on the ovaries in the female reproductive tract: FSH binds to
FSH binds to granulosa cells to stimulate follicle growth, permit the conversion of androgens (from theca cells) to oestrogens and stimulate inhibin secretion
LH and FSH are gonadotropins that act primarily on the ovaries in the female reproductive tract: LH binds to
acts on theca cells to stimulate production and secretion of androgens
Moderate oestrogen levels exert a ………….feedback on the HPG axis
negative
High oestrogen levels (in the absence of progesterone (due to increased inhibin) ………..feedback on the HPG axis
positively
Oestrogen in the presence of progesterone exerts …………….feedback on the HPG axis
negative
Inhibin selectively inhibits …………at the anterior pituitar
FSH
summary of the feedback systems acting on the HPG axis
the menstrual cycle is
Menstrual cycle
- 2 cycles happening in parallel- ovarian and uterine
- Ovarian cycle- 2 phases
- Pre ovulation- follicular phase
- Post ovulation- luteal phase
- Uterine cycle- 2 phases
- Pre-ovulation
- Period
- Proliferative
- Post-ovulation
- secretory
- Pre-ovulation
- Ovarian cycle- 2 phases
average length of menstrual cycle
- Average length =28 days
GnRH release in females
*
- Female HPG axis is pulsatile
- Persistent presence of GnRH would lead to desensitisation of its receptors on gonadotrophs
- FSH and LH production cease
- Gonadal steroid production ceases
Clinical importance
- GnRH analogous e.g. Decapeptyl aka Triptorelin pamoate- used in treatment of endometriosis and infertility
the ovarian cycle involves the
follicular phase- pre-ovulatory
Luteal phase- post ovulatory
The follicular phase marks the
beginning of a new cycle as follicles (oocytes surrounded by stromal cells) begin to mature and prepare to release an oocyte.
At the start of a new cycle (menses) there is little
ovarian hormone production and the follicle begins to develop independently of gonadotropins or ovarian steroids.
Due to the low steroid and inhibin levels at the begining of the follicular phase….
there is little negative feedback at the HPG axis, resulting in an increase in FSH and LH levels. These stimulate follicle growth and oestrogen production.
how many follicles can contunue to maturity and compelte each menstrual cycle
Only one dominant follicle can continue to maturity and complete each menstrual cycle.
As oestrogen levels rise,
negative feedback reduces FSH levels (inhibin), and only one follicle can survive, with the other follicles forming polar bodies.
when is positive feedback at the HOG axis initiated
when oestrogen eventually becomes high enough
This…..increaseslevels of GnRH and gonadotropins.
why do we only see a surge in LH when oestrogen levels initiate a positive feedback on the hypothalamus and pituitary
due to the increased follicular inhibin, selectively inhibiting FSH production at the anterior pituitary.
Granulosa cells become luteinised and express receptors for LH.
(LH SURGE= ovulation)
Ovulation is in response to
to the LH surge , the follicle ruptures and the mature oocyte is assisted to the fallopian tube by fimbria. Here it remains viable for fertilisation for around 24 hours.
what happens to the empty follicle after ovulation
the follicle remains luteinised, secreting oestrogen and now also progesterone, reverting back to negative feedback on the HPG axis. This, together with inhibin (inhibits FSH) stalls the cycle in anticipation of fertilisation.
luteal phase represents the
corpus luteum development and maintainence
what is the corpus luteum and what does it produce
is the tissue in the ovary that forms at the site of a ruptured follicle following ovulation.
It produces:
- oestrogens
- progesterone
why is the corpus luteum important
maintains conditions for fertilisation and implantation.
At the end of the cycle, in the absence of fertilisation
the corpus luteum spontaneously regresses after 14 days.
There is a significant fall in hormones, relieving negative feedback, resetting the HPG axis ready to begin the cycle again.
If fertilisation occurs, what stimulates (lutenises) the CL
the syncytiotrophoblast of the embryo produces human chorionic gonadotropin (HcG), exerting a luteinising effect, maintaining the corpus luteum. It is supported by placental HcG and it produces hormones to support the pregnancy. At around 4 months of gestation, the placenta is capable of production of sufficient steroid hormone to control the HPG axis.
summary of the ovarian cycle
- initially low oestrogen, inhibin and progesterone will send negative feedback to the H and AP –> inhibitng release of LH
- as the follicle grows it will produce more and more oestrogen and inhibin (inhibits FSH) which will at a set point change the negative feedback to positive feedback
- causing a surge in LH
- LH surge casuses rupture of the ovum from the follicle
- the follicle will turn into a corpus luteum, which is stimulated by HcG and produces oestrogen, progesterone to help maintain conditions for fertilisation and implantation
The uterine cycle
- Pre-ovulation
- Proliferative
- Post-ovulation
- secretory
proliferative phase
Following menses, the proliferative phase runs alongside the follicular phase, preparing the reproductive tract for fertilisation and implantation.
oestrogens role in the proliferative phase
initiates fallopian tube formation, thickening of the endometrium, increased growth and motility of the myometrium and production of a thin alkaline cervical mucus (to facilitate sperm transport).
Secretory Phase
The secretory phase runs alongside the luteal phase.
progesterone in the secretory phase
stimulates further thickening of the endometrium into a glandular secretory form, thickening of the myometrium, reduction of motility of the myometrium, thick acidic cervical mucus production (a hostile environment to prevent polyspermy), changes in mammary tissue and other metabolic changes.
- endometrial thickness at maximum, very coiled glands, coiled arterioles
Menses
Menses marks the beginning of a new menstrual cycle. It occurs in the absence of fertilisation once the corpus luteum has broken down and the internal lining of the uterus is shed.
Menstrual bleeding usually lasts between
Oestrogen
- Highest in
-
Follicular phase
- Thickening of endometrium
- Increases fallopian tube function- cilia which waft oocyte along fallopian tube
- Growth and motility of myometrium
- Produces thin alkaline cervical mucus- for sperm
- Vaginal changes
- Changes skin, hair, metabolism
- Progesterone is Highest in
-
Luteal phase
- Further thickening of endometrium (secretory)
- Thickening of myometrium and reduction of motility- to stop implanted embryo from being expelled
- Thick acidic cervical mucus
- Development of breast tissue
- Increased body temp
- Metabolic changes
- Electrolyte changes
Cycle length- what is normal
- *
- Normal duration 21-35 days
- Variation is due to length of follicular phase
- Luteal phase
Disruption to cycle
- Physiological factors
- Pregnancy
- Lactation
- Emotional stress
- Body weight
- Infertility
