Embryonic and fetal development

• Embryogenesis


  The process of embryo development
• Fetal development


  The process of fetal development after the embryonic period
• Fertilisation/conception

  Approximately 14 days LMP (last menstrual period)
• Preorganogenesis


  1. 4 weeks LMP. Pre and perimplantation period (0-2 weeks)
• Embryonic period

  1. 10 weeks LMP: placental growth and organogenesis (1-8 weeks)
• Fetal period

  11 weeks: functionality and shaping, final structure formation (9+ weeks)
• Timing of major events

  • Major Malformations
  • Death
  • Functional defect and minor malformations
• Timing of major events

  • Pre-organogenesis
  • Embryonic Period
  • Fetal Period
• Ovulation


  Occurs at day 14 of the menstrual cycle
• Insemination


  Can occur several days pre or post ovulation
• Early pregnancy factor

  Detected in maternal blood at 2-3 days post fertilisation (=week 3 LMP)
• Human Chorionic Gonadotrophin (hCG)


  Detectable early in week 4 LMP (8 days post-fertilisation)
• Placental hormones


  Start supporting the corpus luteum in week 4
• hCG insufficiency may lead to loss of pregnancy (58% attrition)
• Hormones during pregnancy

  • hCG
  • Oestrogen
  • Progesterone
  • Placental Lactogen
  • Relaxin
  • Glucocorticoids
  • Prolactin
  • Corticotrophin RH
• Hormone levels induce the changes required for the maintenance and accommodation of pregnancy. Eg maintenance of placenta, increasing maternal blood glucose levels etc.
• Fertilisation


  Occurs in week 0
• Blastocyst implantation


  Starts in week 3 (day 5-9)
• Gastrulation


  Occurs at the end of week 4 (day 12-14), three embryonic layers begin to form
• HFEA defines the start of life as an embryo as the end of week 4 (day 12), when implantation is complete
• Maternal "spotting"; small amount of blood lost may occur at the end of week 4 (day 12) when implantation is complete
• Early feto-maternal circulation

  Begins at the end of week 4
• Gastrulation


  Sets up the three body layers (ectoderm, mesoderm and endoderm), which allows for the development of the majority of organs and tissue types
• Problems/events in first 4 weeks


  • Embryo loss due to insufficient placental hormones
  • Genetic problems leading to cell replication/survival issues
  • Cell adhesion issues leading to embryonic tissue splitting or twinning
• Conditions arising from early events

  • Caudal dysgenesis: lack of caudal mesoderm
  • Situs inversus: reversal of internal organs
  • Situs ambiguous: partial malrotation
  • Sacrococcygeal teratoma: origins in primitive streak
• Neurulation


  Initiated in weeks 2-3
• Somites


  Develop and will become part of the torso musculoskeletal system
• Blood vessels and cells

  Form in the yolk sac
• Heart tube


  Starts to beat in week 5
• Neural tube closure


  Occurs in week 6
• Heart valves


  Form in week 6
• Limb buds


  Appear by week 6
• The embryo is 5mm and 20mg by week 6
• Cardiac development

  The heart starts as a cardiac tube, which swells and loops to form a more complex, compact structure. Growth in specific areas leads to separation of the tube into 4 chambers.
• Time-lapse of human development from day 24 to 56 (week 5.5 to 10 LMP)
• Brain development


  Progressing, with the brain and spine separating into distinct regions
• Gut tube

  Formed in gastrulation, splits into oesophagus and trachea, with lungs budding from the trachea
• Ureteric bud and Metanephric tissue


  Begin to form the kidney
• Limbs


  Have distinct regions
• Craniofacial development


  Progressing