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face="Arial">Physiology of the Menstrual Cycle
Menstrual flow normally begins between the ages 8 and 16 years (average
age of menarche is 13 years). Menstruation usually stops by age 52 years
(normal range 45 to 55 years). The median cycle length between periods
changes with age, being 28.87 days at age 20 and 26.8 days at age 40.
Cycle length is the most unpredictable in the years following menarche
and just before menopause.
Cycle lengths less than 24 days are considered polymenorrhea. Cycle lengths
more than 35 days are considered oligomenorrhea. Normal menstrual flow
ranges from 3 to 7 days. Blood loss is usually 80 ml or less. Excessive
menstrual blood loss is called menorrhagia.
Diagrammatic representation of the menstrual cycle. From
Ryan
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The hypothalamus releases gonadotropin-releasing hormone (GnRH) in a
pulsatile fashion. Decreased GnRH pulse frequency results in oligomenorrhea
or amenorrhea. Increased pulse frequency is found in polycystic ovary
syndrome. Pulses of GnRH stimulate the pituitary to release luteinizing
hormone (LH) and follicle-stimulating hormone (FSH) in a pulsatile fashion.
Estradiol from the ovary inhibits pituitary release of FSH and LH by negative
feedback. In the ovary, oocytes are resting until stimulated by the LH
surge to enter meiosis. Estradiol produced by the oocyte follicle causes
the endometrium of the uterus to proliferate. Estradiol stimulates vascular
growth in the endometrium.
Proliferative endometrium early in menstrual cycle. Glands are straight
(magnification top 150×; bottom 400×). Reprinted by permission
from the American Society for Reproductive Medicine. From Noyes
RW et al: Dating the endometrial biopsy, Fertil Steril 1:3-20,
1950.
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Proliferative endometrium just prior to ovulation. Glands are now
tortuous (magnification top 150×; bottom 400×). Reprinted
by permission from the American Society for Reproductive Medicine.
From Noyes RW et al: Dating the endometrial biopsy, Fertil Steril
1:3-20, 1950.
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After ovulation the ovarian follicle reorganizes into a corpus luteum.
The corpus luteum produces progesterone. Progesterone induces a secretory
change in the proliferating endometrium. The endometrial vasculature organizes
into tortuous spirals. In the absence of pregnancy the corpus luteum becomes
less sensitive to LH. Estrogen and progesterone levels fall. Prostaglandin
F2a, found in
the endometrium, causes vasoconstriction and myometrial contraction. The
spiral arterioles vasoconstrict. Ischemia of the endometrium results in
tissue breakdown and bleeding. Postovulatory endometrial changes are very
predictable. The first day of menstrual bleeding is designated as day
1 of the menstrual cycle. The appearance of the endometrium on biopsy
correlates with the day of the menstrual cycle to within 2 days.
Click to enlarge. Day 17 of the cycle. Three days after ovulation,
the nuclei of the glands are in the center of the cells with cytoplasm
above and vacuoles below (magnification top 150×; bottom 400×).
Reprinted by permission from the American Society for Reproductive
Medicine. From Noyes RW et al: Dating the endometrial biopsy, Fertil
Steril 1:3-20, 1950.
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Click to enlarge. Day 25 of the cycle. Eleven days after ovulation,
the glands are very tortuous with deep serrations in their walls
("saw tooth glands"). Round cells infiltrate the stroma (magnification
top 150×; bottom 400×). Reprinted by permission from the
American Society for Reproductive Medicine. From Noyes RW et al:
Dating the endometrial biopsy, Fertil Steril 1:3-20, 1950.
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Click to enlarge. Day 27 of the cycle. In the absence of a
pregnancy, the endometrium degenerates. White cells infiltrate the
stroma (magnification top 150×; bottom 400×). Reprinted
by permission from the American Society for Reproductive Medicine.
From Noyes RW et al: Dating the endometrial biopsy, Fertil Steril
1:3-20, 1950.
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If ovulation does not occur, progesterone is not produced. The endometrium
proliferates unchecked. At some point, the endometrium cannot be maintained
and starts to slough. Anovulatory bleeding occurs unpredictably and varies
in amount. Anovulatory bleeding can be quite heavy. (Ryan,
Mishell)
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