Development of the Zygote (Cambridge O Level Biology)

• 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

• 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