Menstrual cycle Flashcards
How long is the menstrual cycle?
- 21-35 days (average 28 days)
Compare the lengths of the luteal and follicular phases
- Follicular phase variable in length
- Luteal phase is always 14 phases
What are the feedback mechanisms of the menstrual cycle?
- GnRH causes release of LH and FSH
- FSH stimulates maturation of ovarian follicles
- LH stimulates steroid release, ovulation, and progesterone release by corpus luteum
- Oestrogen and inhibin exert negative feedback in FSH and LH release
- Oestrogen in high levels stimulates LH surge (positive feedback)
- This causes ovulation
What are the phases of the ovarian cycle?
- Follicular phase (days 1-14)
- Ovulation on day 14
- Luteal phase (days 15-28)
What are the phases of the uterine cycle?
- Menstrual phase
- Proliferative phase
- Secretory phase
How do the phases of the ovarian and uterine cycles line up?
- Follicular phase lines up with menstrual phase and proliferative phase
- Then ovulation occurs
- luteal phase lines up with secretory phase
Describe GnRH release in females
- Normally released in a pulsatile
- Persistent release would lead to desensitisation of its receptors on gonadotrophs
- FSH and LH production would cease as would gonadal steroid production
What is the purpose of the ovarian cycle?
- Preparation of gamete for release
Outline the early follicular phase of the ovarian cycle
- Days 0-5
- No ovarian hormone production
- Granulosa cells secrete activin
- FSH levels increase - stimulates follicle growth
- Theca interna appears - androgens produced so oestrogen levels increase
- Enhanced aromatase function
- Inhibin produced by dominant follicle granulosa cells
What is the function of activin?
- Increase FSH production
- Increase FSH receptors on granulosa cells
- Reduce production of androgens by theca cells
What is the function of aromatase?
- Follicle capable of enhanced oestrogen production
Outline the late follicular phase of the ovarian cycle
- Days 5-14
- Dominant follicle (Graafian Follicle) granulosa cells produce oestrogen and inhibin
What is the function of oestrogen in the ovarian cycle?
- Increases FSH receptors on follicle
- Initially exerts negative feedback to reduce FSH production
- Increases LH receptors on granulosa cells - prep for ovulation
What is the function of inhibin in the ovarian cycle?
- Decreases FSH production so no other follicles can develop
- Helps LH to increase androgen production by theca cells
Outline the ovulatory phase of the ovarian cycle
- Oestrogen continues to rise
- Exerts positive feedback on hypothalamus and anterior pituitary
- LH production surges
- Ovulation occurs
- FSH levels do not rise due to inhibin presence
- Granulosa cells start secreting progesterone
Describe the state of the oocyte at ovulation
- Meiosis I completed
- Meiosis II begins but not yet completed
- Mature oocyte travels through capsule of ovary into fallopian tube
Outline the luteal phase of the ovarian cycle
- LH stimulates remaining follicle to develop into corpus luteum
- Corpus luteum secretes oestrogen and progesterone in large quantities
- Inhibin also secreted
- Prepares endometrium for implantation
- Oestrogen and progesterone exert negative feedback on LH
- FSH levels remain low
How long does the corpus luteum stay until it degenerates?
- 14 days +/- 2
- If fertilisation occurs hCG production maintains corpus luteum
- Otherwise it degenerates to form corpus albicans
What are the effects of oestrogen on the endometrium?
- Drives proliferative phase of uterine cycle
- When oestrogen and progesterone are both present, they drive secretory phase of uterine cycle
What are the different layers of the endometrium?
- Endometrium is epithelium and stroma
- Functional layer is responsive to hormones and is shed during menstruation
- Basal layer develops into a new functional layer
- Myometrium
- Perimetrium
Outline how the endometrium changes as it passes through the uterine cycle
- Early proliferative - sparse glands, straight
- Late proliferative - thicker functional layer, coiled glands
- Secretory - endometrial thickness at maximum, very coiled glands, coiled arteries
What happens at the end of the menstrual cycle if fertilisation does not occur?
- Corpus luteum becomes corpus albicans
- Dramatic fall in oestrogen, progesterone and inhibin
- Loss of negative feedback so FSH begins rising slowly
- Decrease in progesterone triggers menstruation
- New cycle starts
What happens at the end of the menstrual cycle if fertilisation occurs?
- Syncytiotrophoblast produces hCG
- Extends lifespan of corpus luteum before placenta forms
What is the action of oestrogen during the menstrual cycle?
- Key in follicular phase
- Thickening of endometrium
- Fallopian tube function
- Growth and motility of myometrium
- Thin alkaline cervical mucus
- Vaginal changes
- Changes in skin, hair, metabolism
What is the action of progesterone during the menstrual cycle?
- Key in luteal phase
- Further thickening of endometrium in secretory phase
- Thickening of myometrium and reduction of motility
- Thick acidic cervical mucus
- Development of breast tissue
- Increased body temperature
- Metabolic/electrolyte changes
What can cause disruptions to the menstrual cycle?
- Pregnancy, lactation
- Emotional stress
- Body weight
- Infertility
