Menstrual cycle Flashcards
Give a very brief overview of the stages of the reproductive cycle in females
Ovulation occurs intermittently
Preceded by a period of preparation
Followed by a period of waiting.
Give a brief overview of the preparation stage
Follicles grow in ovary
Uterus prepared for sperm transport and implantation of conceptus
Changes to facilitate sexual interactions
Known as the Follicular phase in ovaries
Known as the proliferative phase in uterus
Briefly describe Ovulation
Brief period of fertility (about 36 hours)
Release of ovum
Formation of corpus luteum
Briefly describe the waiting stage
Corpus luteum is in ovary
Changes in preparation for pregnancy
Luteal phase in ovaries
Secretory phase in uterus
What is different in the reproductive cycle of higher primates compared to other species?
Ovulation spontaneous
Corpus luteum also forms spontaneously
Preparatory and waiting phases of similar duration
Ended by shedding of part of endometrium
How is the menstrual cycle controlled?
Coordinated by hormones
- Gonadotrophins
- Gonadal steroids
- Oestrogen
- Progesterone
What are the actions of gonadotrophins in the follicular phase?
FSH binds to granulosa cells
LH binds to Thecal cells (theca interna)
They stimulate the development of follicles
What are the actions of gonadotrophins in pre-ovulation?
LH surge stimulates ovulation
What are the actions of gonadotrophins in the luteal phase?
LH maintains corpus luteum
What are the actions of gonadal steroids in the follicular phase?
Oestrogens stimulate
- Fallopian tube function
- Thickening of endometrium
- Growth and motility of myometrium
- Thin alkaline cervical mucus
- Vaginal changes
- Changes in skin, hair, metabolism
- Calcium metabolism
What are the actions of gonadal steroids in the luteal phase?
Progesterone acts on oestrogen primed cells
- Further thickening of endometrium into secretory form
- Thickening of myometrium, but reduction of motility
- Thick, acid cervical mucus
- Changes in mammary tissue
- Increased body temperature
- Metabolic changes
- Electrolyte changes
Where are gonadotrophins secreted from?
And how is this stimulated?
Pituitary gonadotrophs
Stimulated by pulsatile release of GnRH from hypothalamus
Describe the start of the menstrual cycle with details of hormones involved
Corpus luteum is absent
Follicles only partly developed
Secrete very little steroid or inhibin
Oestrogen and progesterone levels are low
Little inhibition at hypothalamus or pituitary
FSH levels rise
What are the effects of FSH at the beginning of the menstrual cycle?
FSH binds to granulosa cells Follicular development continues Theca interna develops Secretes oestrogen stimulated by LH Secretes inhibin
Describe the state of hormones at the mid follicular stage.
Oestrogen levels are rising
Inhibin levels are rising
- Selective inhibition of FSH
- No new follicles can develop
Oestrogen exerts positive feedback at hypothalamus and pituitary
LH levels
- But not FSH
What happens in the pre-ovulatory phase?
The rising oestrogen increases chance of an LH surge.
What influences the precise timing of the LH surge
Environmental factors - lots of evidence for effects of stress
A circadian generator which “fires” late afternoon everyday - little evidence for this
It is the coincidence of “firing” and high oestrogen which generates an LH surge
Therefore precise timing of ovulation is affected by the environment
In terms of the menstrual cycle, describe what happens after ovulation.
Corpus luteum forms spontaneously
Begins to secrete progesterone and oestrogen
Maintains suppression of FSH
LH also supressed due to positive feedback inhibited by progesterone
In terms of the menstrual cycle describe the luteal phase.
Corpus luteum grows
Secretes more steroids
After 14 days precisely it dies
Describe the end of the menstrual cycle
Death of the corpus luteum
Leads to a rapid fall in steroid
Stimulates menses
Relieves inhibition on FSH
Tell me some interesting features of the timing of the menstrual cycle.
The interval from ovulation to menses is very constant - always 14 days
The variation in cycle length is due to the variation in the timing of ovulation
