Prenatal Development

• Germinal stage: Fertilisation


  • Millions of sperm
  • 50–100 reach egg
  • One penetrates egg
  • Chemical process stops other sperm from penetrating
  • Egg is now a zygote

  Monozygotic twins = from one zygote

  Dizygotic twins = from two zygotes
• Germinal stage: Implantation


  • Starts around 6 days after fertilization
  • By 2 weeks, it is fully implanted
  • Fully implanted egg is called an embryo → start of embryonal stage
• Early Divisions


  • First division at 2 hours
  • Day 4 = morula ⎼ 16 cells
  • Day 5 = outer and inner cells differentiate
  • → Now blastocyst
  • → Outer cells will be placenta
  • Day 16 = gastrulation
  • → Blastocyst inner cell mass differentiates into ectoderm, mesoderm, endoderm

  
• Cell Division and Differentiation

  
• Embryonic stage


  Weeks 3 – 8

  • Large changes in body structure
  • Large changes in size
  • Cells are differentiating

  
• Foetal stage


  Weeks 9 – birth

  • Characterised by heartbeat and facial features
  • By 22 weeks: some can survive with neonatal intensive care
  • By 28 weeks: fully developed lung

  
• Homeobox genes


  Homeobox genes = encode homeodomain proteins that regulate genetic switches affecting general structure and organisation of body parts

  • Homeobox genes are highly specific, resulting in species-specific characteristics
  • Homeobox gene sequence found in many different species

  
• Adverse influences on prenatal development

  Teratogens

  • harmful environmental factors that can disturb prenatal development (i.e prescription medication or non-prescription drugs taken by mother; environmental factors, e.g. pollution)
  • exert effects largely during critical periods of development
  • have specific effects, depending on type of teratogen and timing of exposure
• Smoking during pregnancy

  
• Smoking and birth weight

  In England and Wales

  • 7.0% (47,257) of live births were low birthweight (<2500g) in 2017, unchanged since 2011 •
  • Babies born to women who smoke weigh on average 200 gram less than babies born to non-smokers
• Foetal alcohol syndrome


  • Caused by (excessive) alcohol use during pregnancy
  • Distinctive facial features
  • Intellectual disability, emotional, and behavioural problems

  
• Foetal alcohol syndrome and foetal alcohol effects

  
• Thalidomide


  • Drug used to relieve morning sickness in the 1960s
  • Caused stunted growth of limbs
  • Interfered with activity of genes that regulate normal limb growth and/or with timing signals of the Homeobox genes
  • Specific effects of Thalidomide depended on when in pregnancy drug was used

  
• Timing of teratogens’ influence on development

  
• Adverse prenatal influence: Zika virus

  
  

  Consequences of prenatal Zika exposure:

  • Follow-up study of 219 children prenatally exposed to Zika, up to 42 months
  • 53 children (24%) had microcephaly
  • Even in the group with normal head size, many had neurological abnormalities (68%) or atypical brain imaging results (29%)
  • 27% had low, and 9% had very low, cognitive ability scores

  
• Teratogens


  • Teratogens exert effects largely during critical periods of development
  • Teratogens have specific effects, depending on type of teratogen and timing of exposure
  • Some teratogens may produce same effect (e.g., reduced birth weight)
  • The longer and greater the intensity of the exposure, the more likely the foetus will be harmed, and the greater the effect