Menstrual Cycle
2 cycles: Ovarian & Uterine
Controlled by hormonal changes
Ovarian Cycle
Average ovarian cycle lasts 28 days
Normally interrupted only by pregnancy
Finally terminated by menopause
Consists of two alternating phases
– Follicular phase
• Dominated by presence of maturing follicles
• Approx 14 days
– Luteal phase
• Characterized by presence of corpus luteum
• 14 days
Follicular phase
Operates first half of cycle
–Granulosa cells of some primary follicles
proliferate
–Oocyte inside each follicle enlarges
–Thecal cells in follicle secrete increased
amounts of oestrogen
–Zona pellucida and antrum form
–Rapid follicular growth continues during
follicular phase
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Follicular phase
One follicle usually grows
more rapidly & matures about
14 days after onset of follicular
development
- Graafian or mature follicle
- Oocyte surrounded by zona
pellucida and single layer of
granulosa cells
Follicular phase: hormonal control
FSH and oestrogens stimulate follicle growth and development
Flow chart shows production of oestrogen by follicle
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Ovulation
– Wall of Graffian follicle ruptures
– Antral fluid with oocyte flows to ovarian
surface
– Fluid and oocyte released
– Oocyte enters uterine tube
– Hormonal control: LH surge is trigger
– 2 or more follicles may become dominant and
released at ovulation
– If both are fertilized – fraternal twins
– Also called dizygotic twins (develop from 2 zygotes)
Ovarian Cycle: Luteal phase
– Last 14 days of ovarian cycle
– Old follicular cells undergo structural transformation to form
corpus luteum (gland)
– Becomes highly vascularized
– Fully functional within four days after ovulation
– Continues to increase in size for another 4-5 days
– If released ovum is not fertilized and does not implant, corpus
luteum degenerates within about 14 days after formation
– Hormonal control: LH causes development of corpus luteum
from empty Graafian follicle
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Ovarian cycle: summary
Uterine Cycle
• Reflects hormonal changes during ovarian cycle
• Averages 28 days
• Consists of three phases
– Menstrual phase (decreased oestrogen & progesterone)
• Uterine lining is shed. 5-7 days
– Proliferative phase (oestrogen)
• Endometrium renewed in preparation for possible pregnancy
– Secretory or progestational phase (progesterone)
• Coincides with luteal phase. Endometrium develops
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Menstrual phase
– Characterized by discharge of blood and endometrial
debris from vagina
– First day of menstruation is considered start of new cycle
– Coincides with end of ovarian luteal phase and onset of
follicular phase
– Triggered by decreased oestrogen and progesterone:
hormones decrease when CL degenerates
– Release of uterine prostaglandin
• Causes vasoconstriction of endometrial vessels
– Disrupts blood supply
– Causes death of endometrium
• Stimulates mild rhythmic contractions of uterine
myometrium
– Helps expel the menstrual flow
Proliferative phase
Begins concurrent with last portion of ovarian follicular phase
Uterus prepares for fertilized ovum:
Endometrium starts to repair itself and proliferate under
influence of oestrogen from newly-growing follicles
Oestrogen-dominant proliferative phase lasts from end of
menstruation to ovulation
Peak oestrogen levels trigger LH surge responsible for
ovulation
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Secretory phase
Endometrium prepared for implantation
– Blood supply increased
– Glands enlarge and secrete glycogen-rich fluids
– Secretory phase conditions promoted by progesterone
– Uterus enters this phase after ovulation when new corpus
luteum is formed
– Corpus luteum secretes large amounts of progesterone
and oestrogen
• Progesterone converts endometrium to highly
vascularized, glycogen-filled tissue
– Endometrial glands actively secrete glycogen
– If fertilization and implantation do not occur
• Corpus luteum degenerates
• New follicular phase and menstrual phase begin again
Timing of ovarian and uterine cycles
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Hormonal control of the cycle
GnRH
Gonadotropins (FSH and LH)
Oestrogen and progesterone
FSH:
follicle stimulating hormone
growth and development of follicles
LH:
luteinising hormone
causes ovulation
development of corpus luteum
Inhibin:
decreases FSH, not LH
Hormonal control of the cycle
Oestrogens
Progesterone
Stimulate oogenesis and follicle
development
Promotes secretory-phase uterine
conditions
Promote proliferative-phase uterine
conditions
Suppresses uterine
during pregnancy
Induce expression of
progesterone receptors
Promotes growth of glandular
breast tissue but suppresses milk
production
uterine
contraction
Induce expression of granulosa
cell LH receptors
Secondary sexual characteristics
Breast growth during pregnancy
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– Oestrogen secreted first
from the follicle, then from
the corpus luteum
– Progesterone secreted
from the corpus luteum
– LH and FSH secreted from
the anterior pituitary
– Oestrogen and
progesterone inhibit LH and
FSH secretion
Regulation of Hormone Secretion:
Mid-Follicular Phase
Permissive Effects of
Hormones during Mid-Follicular
Phase
– FSH
•Granulosa cells  theca cells
•Theca cells have LH receptors
– Oestrogens
•LH receptors on granulosa
cells
•Progesterone receptors on
endometrial cells
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Regulation of Hormone Secretion –
Late Follicular Phase
Oestrogens stimulate LH
secretion (opposite of usual
effect)
This is one of the rare
examples of positive
feedback in Physiology
Hormonal control: ovulation
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Regulation of Hormone Secretion: Luteal
Phase
Long-Term Regulation of Female
Reproductive Function
Puberty
• Oestrogen –
secondary sex
characteristics
Reproductive years
• Oestrogen levels
high to maintain
secondary sex
characteristics
Menopause
• Cessation of woman’s
menstrual cycle
• Age 45 to 55
• Preceded by period of
progressive ovarian failure
– Increasingly irregular cycles
– Dwindling oestrogen levels
• Loss of oestrogen
primarily affects skeleton
and cardiovascular
system (increased risk of
heart disease and
osteoporosis)
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