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Fertilisation

Fertilisation

  • Egg cells and sperm cells are gametes and are heavily specialised for their functions.
  • The main functions of the egg is carry the female DNA and nourish the developing embryo:
    • 1) The egg contains nutrients in the cytoplasm to feed the embryo.
    • 2) As soon as the egg is fertilised with a sperm, the egg’s membrane quickly changes its structure to prevent any more sperm getting in. This is to ensure the offspring have the correct amount of DNA.
    • 3) The egg contains a haploid nucleus, like sperm, so when they join together the zygote will have the diploid number of chromosomes.
  • The function of the sperm is to transport the male’s DNA to the female’s egg so that their DNA can combine.
    • 1) Sperm are small and have tails so they can swim to the egg.
    • 2) Sperm have a lot of mitochondria in their middle section to provide the energy needed to swim this distance.
    • 3) Sperm also have an acrosome at the front of the head where they store the enzymes they need to digest their way through the membrane of the egg cell.
    • 4) They contain a haploid nucleus – they contain one copy of each chromosome.
  • The two nuclei from the sperm and egg fuse to form a zygote. The zygote is diploid and divides repeatedly to form an embryo which embeds in the uterus lining to grow and develop.

Fertility treatment

  • Some women have levels of FSH that are too low to cause their eggs to mature. This means that no eggs are released and the woman can’t get pregnant.
  • Hormones: Women can be given extra hormones such as FSH and LH to stimulate egg release in their ovaries. However, it doesn’t always work and too many eggs could be stimulated, resulting in multiple pregnancies. The babies also tend to be born earlier than usual, increasing the risk of problems at birth or later.
  • IVF: This stands for in vitro fertilisation. Some of the woman’s egg cells are taken from her ovaries and fertilised in a dish with her partner’s sperm cells. One or two embryos are then put into her uterus to develop. IVF babies are born early more often than naturally conceived babies which may causes problems at birth or later.
  • Surrogate mothers: If the woman cannot grow an embryo in her own uterus, another woman can grow it for her using her own eggs or the surrogate mother’s eggs with the man’s sperm. The surrogate mother gives birth to the baby. Handing the baby over the couple may cause problems if the surrogate mother has developed a strong bond with the baby and does not want to give it up.
  • Egg donation: If the woman’s ovaries aren’t producing eggs, eggs can be used from a woman who agrees to donate hers. This woman will be given hormones to make her ovaries release the eggs. The IVF is carried out using sperm from the first woman’s partner. A few women who donate eggs react really badly to the hormones used to collect them.