Normal Menstrual Cycle Flashcards
What marks the start and end of a menstrual cycle?
The length of a menstrual cycle is the number of days between the first day of menstrual bleeding of one cycle to the onset of menses of the next cycle
What is the range for normal length of menstrual cycle?
Average is 28 days
but normal 21-35
When is the menstrual cycle most likely to be irregular?
The extremes of reproductive life (menarche and menopause) due to anovulation and inadequate follicular development
What are the three events that occur in the ovaries during the menstrual cycle?
(the three phases of the menstrual cycle)
- Follicular phase
- Ovulation
- Luteal phase
What phase of the menstrual cycle is constant?
Which phase of the menstrual cycle is cycle length variability most likely to derive from?
The luteal phase is always 14 days
The variability of cycle length is usually derived from varying lengths of follicular phase which can range from 10-16 days
What physiological axis controls menstruation?
The hypothalamic-pituitary-gonadal (HPG) axis
What hormones control the ovaries?
GnRH is released in a pulsatile fashion from the hypothalamus
which stimulates the anterior pituitary to release LH and FSH
which act on the ovaries
What are the two types of cells in the follicle which are involved in oestrogen and progesterone synthesis?
What hormones do they respond to?
Theca cells…respond to LH
Granulosa cells…respond to FSH
What hormone is released after ovulation occurs and is used as a marker of fertility?
Progesterone
Which change marks the transformation of the primordial follicle into a primary follicle?
Development of the zona pellucida
What is the role of the granulosa cells in the follicle?
Granulosa cells in follicles produce oestradiol (oestrogen)
When the follicle becomes the corpeus luteum, granulosa luetin cells produce progesterone which facilitates implantation
Describe the levels of hormones expected at the start of the follicular phase
- At the start of menses there is little ovarian hormone production and the follicle begins to develop independently of gonadotropins or ovarian steroids
- Due to low steroid and inhibin levels there is little negative feedback at the HPG axis resulting in an increase in FSH and LH levels which stimulate follicle growth and oestrogen production
During the follicular phase oestrogen levels start to increase.
What are the effects of this?
As oestrogen levels rise, negative feedback reduces FSH levels and only one follicle can survive to ovulation
Follicular oestrogen eventually becomes high enough to stimulate the LH surge
Describe and explain the LH surge
Mid-late follicular stage the follicular oestrogen levels are high enough to initiate positive feedback at the HPG axis increasing GnRH
The effect is only reflected in LH levels (the LH surge) due to increased follicular inhibin which selectively inhibits FSH production at the anterior pituitary
Granulosa cells become luteinised and express receptors for LH
What occurs due to the LH surge?
The follicle ruptures and the mature oocyte is assisted to the fallopian tube by fimbria
What happens to the follicle after ovulation?
Following ovulation the follicle remains luteineised, 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
What is the corpus luteum? What does it do?
The corpus luteum is the tissue in the ovary that forms at the site of a ruptured follicle following ovulation
It produces:
- oestrogens
- progesterone
- inhibin
to maintain conditions for fertilisation and implantation
In the absence of fertilisation, what happens to the corpus luteum?
it 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
What are the phases of the uterine cycle?
Proliferative phase
Secretory phase
Menses
Discuss the proliferative phase
- the proliferative phase runs alongside the follicular phase
- prepares the reproductive tract for fertilisation and implantation
- oestrogen initiates:
- fallopian tube formation,
- thickening of the endometrium
- increased growth and motility of the myometrium
- production of a thin alkaline cervical mucus (to facilitate sperm transport)
Discuss the secretory phase of the uterine cycle
- the secretory phase runs alongside the luteul phase
- progesterone 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
Discuss menses of the uterine cycle
Menses marks the beginning of a new menstrual cycle
- it occurs in the absence of fertilisation
- occurs once the corupus luteum has broken down
- menstrual bleeding usually lasts between 2-7 days
