Female Reproduction Menstrual Cycle
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.