Female Reproduction Menstrual Cycle
Clinical background
The end of puberty in females is
associated with the onset of regular ovulatory menstrual cycles. The menstrual
cycle repre- sents complex hormonal changes involving the ovaries, hypothalamus
and pituitary and is characterized by ovarian follicular maturation, ovulation
of a dominant follicle and formation of a corpus luteum. The first day of
bleeding is day 1 of the cycle and marks the onset of the follicular phase
which culminates in the LH surge (day 12–14), ovulation and the onset of the
luteal phase. During the luteal phase progesterone is secreted by the corpus
luteum causing characteristic changes in the endometrium. In the absence of
pregnancy, endometrial changes regress at the end of the luteal phase causing
breakdown and the onset of bleeding. In normal women, the menstrual cycle lasts
28 days although this may be less regular at either end of the reproductive
years.
Clinically, monitoring of the
menstrual cycle may be useful in the assessment of subfertility. Monitoring of
cycle dates should be performed, including other symptoms such as midcycle
lower abdominal pain and increased cervical secretion. Progesterone secretion
causes basal body temperature to rise in the second half of the cycle and early
morning temperature rises of 0.5° C indicate the onset of the luteal phase. In
the assessment of ovulatory disorders ultrasound scanning may be used to track
follicular development and elevation of the serum pro- gesterone concentration
on day 21 of the cycle confirms ovulation.
Female reproductive organs
The female reproductive organs are the
ovaries, the fallopian tubes, the uterus and the vagina (Fig. 25a). The ovaries
produce the estrogens, progesterone and the ovum. After ovulation, the ovum is
released into the abdominal cavity, where it is swept up by the fimbriae of the
oviducts, and passes into the fallopian tube. Here it may be fertilized, and
the fertilized ovum, or morula, passes into the uterus, where it is implanted
into the uterine endometrium and grows to become the fetus. Usually, a single
ovum is released each cycle from the human ovary.
The menstrual cycle
The principal functions of the female
reproductive system are to produce the ovum and to ensure that it is fertilized,
nurtured and allowed to grow, and to expel it safely into the external
environment. The production of the ovum depends upon the orchestration of a
number of hormone-dependent events which culminate in ovulation (Fig. 25b).
Inside the ovary during each cycle, many follicles or groups of cells are
developing, but only one will de-velop fully and the others will undergo
atresia (degeneration). The follicle develops under the influence of
luteinizing hor-mone (LH), which stimulates estrogen production, and
follicle-stimulating hormone, (FSH) which promotes follicular growth and
induces LH receptors (Fig. 25c). The ovarian granulosa cells produce a protein
hormone, inhibin, which is able to suppress FSH secretion from the pituitary.
It has been found that subunits of inhibin can actually stimulate the release
of FSH, and so the protein may have a complex but important role in the
regulation of follicular maturation.
Estrogen is produced by the ovary
during follicular maturation, and stimulates glandular proliferation of the
inner lining or endometrium of the uterus – the proliferative phase. At the
same time, the hormone stimulates the synthesis of progesterone receptors, thus
preparing the uterus for subsequent large concentrations of progesterone. This
hormone makes the endometrium secretory, in preparation for the fertilized ovum.
The vagina, too, alters cyclically. As estrogen rises, so the vaginal
epithelium proliferates. If fertilization does not occur, then towards the end
of the luteal phase the epithelium is invaded by leukocytes and cast off by the
underlying epithelium, representing new growth at the beginning of the next
cycle.
The characteristics of the cervical
mucus are dependent on the hormonal milieu. During the follicular phase, the mucus
is watery, but progesterone changes the mucus to a more viscous form, with
minute channels through which the spermatozoa pass on their way to the ovum.
During the preovulatory or follicular
phase of the cycle, circulating FSH is low, but as estrogen and inhibin
concentrations rise, they continue feed back to suppress FSH release. Negative
feedback of estrogen keeps LH release low, as in the early follicular phase.
However, rising estrogen concentrations towards the end of the follicular phase
sensitize the pituitary gonadotrophs to GnRH, resulting in the massive
preovulatory LH surge and the triggering of ovulation.
At maturation, the follicle, which is
now termed a Graafian follicle, produces less estrogen and more progesterone,
and these hormones appear to act in concert to produce, together with GnRH, a
massive release of LH into the bloodstream. The LH causes the follicle to
rupture, and the ovum is released. The follicle now becomes the
progesterone-secreting corpus luteum and the postovulatory period is termed the
luteal phase of the menstrual cycle. If fertilization does not occur, the
corpus luteum gradually releases less and less progesterone as it runs its
limited lifespan and becomes the corpus albicans (white body). The spiral
arteries shrivel, the endometrium collapses due to a lack of blood, and the
lining is lost with the menstrual flow. The events described above are termed
the menstrual cycle, and occur approximately monthly for the reproductive life
of women.
The menstrual cycle varies with the
individual, but is taken on average as 28 days, and is numbered from the first
day of vaginal bleeding or menses.
