Development of the Zygote (Cambridge (CIE) O Level Biology): Revision Note

Development of the Zygote

• After fertilisation in the oviduct, the zygote travels towards the uterus

• This takes about 3 days, during which time the zygote will divide several times to form a ball of cells known as an embryo

• In the uterus, the embryo embeds itself in the thick lining (implantation) and continues to grow and develop

• The gestation period for humans is 9 months

• Major development of organs takes place within the first 12 weeks, during which time the embryo gets nutrients from the mother by diffusion through the uterus lining

• After this point, the organs are all in place, the placenta has formed and the embryo is now called a fetus

• The remaining gestation time is used by the fetus to grow bigger in size

The foetus in the uterus

• The fetus is surrounded by an amniotic sac which contains amniotic fluid (made from the mother’s blood plasma)

• This protects the fetus during development by cushioning it from bumps to the mother’s abdomen

• The umbilical cord joins the fetus’s blood supply to the placenta for exchange of nutrients and removal of waste products

Virus Transmission Across the Placenta

• During the gestation period the fetus develops and grows by gaining the glucose, amino acids, fats, water and oxygen it needs from the mother’s blood

• The blood runs opposite each other, never mixing, in the placenta

• The fetus’s blood connects to and from the placenta by the umbilical cord

• The mother’s blood also absorbs the waste from the fetus’s blood in the placenta; substances like carbon dioxide and urea are removed from the fetus’s blood so that they do not build up to dangerous levels

• Movement of all molecules across the placenta occurs by diffusion due to differences in concentration gradients

• The placenta is adapted for this diffusion by having a large surface area and a thin wall for efficient diffusion

• The placenta acts as a barrier to prevent toxins and pathogens from getting into the fetus’s blood

• Not all toxin molecules or pathogenic organisms are stopped from passing through the placenta (this usually depends on the size of the molecule)

• This is why pregnant women are advised not to smoke during pregnancy, as molecules like nicotine can pass across the placenta

• Certain viruses are small enough to cross the placenta and reach the bloodstream of the fetus

  • Examples of such viruses include rubella and HIV

• Once circulating in the blood of the fetus, these viruses can cause very serious health complications

  • Rubella may lead to problems such as deafness, heart disease and eye problems

  • HIV could be fatal to a foetus

The structure of the placenta diagram

Certain molecules and viruses are small enough to cross the placental barrier and reach the bloodstream of the fetus