The female reproductive system includes the ovaries, which produce the female gametes, the oviducts, where fertilisation usually takes place, and the uterus, where the baby grows and develops (Figure 6.30, overleaf ). The vagina is the passageway for sperm cells to enter the body, and the way out for the baby when it is born.
Production of female gametes is a cyclical process, which lasts
approximately 28 days. During the fi rst half of this menstrual cycle the egg cell is produced, and in the second half the uterus lining thickens to prepare for implantation of a fertilised egg. The cycle involves hormones that are released by the ovaries and the pituitary gland.
Female sex hormones and the menstrual cycle
Ovaries produce two hormones, estrogen and progesterone. These hormones stimulate the development of female characteristics at puberty and also infl uence the changes in the uterus lining during the menstrual cycle and pregnancy. The pituitary gland in the brain produces two further hormones, luteinising hormone(LH) and follicle-stimulating hormone(FSH). FSH stimulates the development of immature follicles in the ovary, one of which will come to contain a mature egg cell. LH stimulates the follicle to release the egg and subsequently to form the corpus luteum.
The sequence of events begins at the start of menstruation, which is often called a period (Figure 6.31, page 161). During the fi rst four or fi ve days of the cycle, the endometrium (lining) of the uterus is shed and leaves the body through the vagina. This indicates that fertilisation has not occurred during the previous month.
In this early part of the cycle, the pituitary gland secretes FSH, which stimulates the development of an immature follicle in the ovary. The follicle then secretes estrogen, which enhances the follicle’s response to FSH. As the level of estrogen rises, it also stimulates the repair of the uterus lining.
Figure 6.30 The female reproductive system. The diagrams also show organs of the urinary system – in the front view the bladder has been drawn to one side, to reveal the uterus.
ureter
spine
cervix rectum urethra vagina
anus ovary
oviduct (fallopian tube)
uterus
bladder
bone of pelvis Side view
right kidney
ureter
bladder
urethra
oviduct
ovary
(produces female gametes)
uterus endometrium (lining of uterus)
cervix
(neck of uterus) vagina
Front view
6 HUMAN HEALTH AND PHYSIOLOGY 1 161 As the follicle grows, estrogen levels rise to a peak at around day 12, when
they stimulate the release of LH from the pituitary gland. As LH levels reach their highest point, ovulation – the release of the egg cell from the follicle – takes place. Ovulation usually occurs at around the day 14 of the cycle. Immediately afterwards, LH stimulates the empty follicle to form the corpus luteum. Levels of estrogen begin to rise and as a result FSH and LH levels fall.
The corpus luteum secretes progesterone, which stimulates the thickening of the endometrium and prepares the uterus to receive an embryo. It also inhibits the production of FSH and LH.
If the egg cell is not fertilised, the LH level falls, the corpus luteum degenerates and progesterone and estrogen levels fall. The fall in progesterone stimulates the breakdown of the uterus lining. FSH is no longer inhibited, so a new follicle is stimulated and the cycle begins again.
In vitro fertilisation
In vitro fertilisation (IVF) is a technique used to help couples who have been unable to conceive naturally. There are many reasons for infertility.
Males may have a low sperm count, blocked or damaged sperm ducts or be unable to achieve an erection. Females may fail to ovulate or have blocked or damaged oviducts, or produce antibodies in cervical mucus that destroy sperm.
The fi rst step in IVF treatment is an assessment of whether the couple are suitable for treatment. If so, the woman is injected with FSH for about 10 days. This hormone causes a number of egg cells to mature at the same time in her ovaries. Just before the egg cells are released from the
Figure 6.31 The menstrual cycle lasts an average of 28 days and involves changes in hormone levels that infl uence the follicles and lining of the uterus.
blood levels of pituitary hormones
blood levels of hormones from the ovary
ovary
estrogen
follicle corpus luteum
LH
FSH
progesterone
thickness of lining of uterus time in days
menstruation menstruation
development of follicle development of corpus luteum
2 4 6 8 10 12 14 16 18 20 22 24 26 28 ovulation start of new cycle
Figure 11.22 on page 276 shows the stages in the development of a follicle in the ovary, right up to ovulation and then the formation of the corpus luteum.
The fi rst successful IVF baby was Louise Brown, born in 1978 following her mother’s IVF treatment. Professor Robert Edwards, who pioneered the technique, was awarded the Nobel prize in Physiology or Medicine in 2010.
follicles, they are collected using a laparoscope. The egg cells are ‘matured’
in culture medium for up to 24 hours before sperm cells are added to fertilise them. Fertilised egg cells are incubated for about three days until they have divided to form a ball of cells. These embryos are checked to make sure they are healthy and developing normally. Either two or three will be selected and placed into the woman’s uterus for implantation. The pregnancy is then allowed to continue in the normal way. Any remaining embryos can be frozen and stored for use later. Figure 6.32 summarises the stages in IVF treatment.
Ethical issues associated with IVF treatment
IVF has enabled men and women who would naturally be infertile to have children but it has also produced some serious ethical issues. Some of these are outlined in Table 6.8. Each society needs to think about these issues and decide what should be done about them.