Embryology week 1 to 8

• Embryonic period


  First 8 weeks, all major organs formed

  
• Fetal period


  Remaining 30 weeks, organs grow larger and become more complex

  
• Preembryonic period


  1st week to end of 2nd week, fertilization to formation of bilaminar germ disc

  
• Embryonic period


  3rd week to end of 8th week

  
• Fetal period


  9th week to birth, growth

  
• Perinatal stage


  26th week to 4 weeks after birth

  
• Ovulation


  Egg released into the peritoneal cavity, captured by fimbria, fertilization in ampulla, travels down fallopian tube
• Conception


  Maternal and paternal genetic material join to form a new human life (zygote)
• Cell division


  Zygote divides repeatedly while moving down tube toward uterus (cleavage)
• Blastomeres


  Daughter cells from cleavage
• Morula


  Solid cluster of 12-16 blastomeres at about 72 hours
• Blastocyst formation


  Day 4: late 60 cell morula enters uterus, taking up fluid to become the blastocyst
• Inner cell mass


  Forms the embryo
• Trophoblast


  Layer of cells surrounding the cavity which helps form the placenta
• Implantation


  Trophoblast erodes uterine wall, takes 1 week to complete
• If inner cell mass of a single blastocyst divides

  Monozygotic (identical) twins
• Cleavage


  Early division of fertilized egg
• Morula


  12 to 16-cell stage, enclosed in the zona pellucida
• Blastocyst


  About 100 blastomeres, with blastocoele, inner cell mass and trophoblast
• Cleavage phases


  Fertilized egg -> 2-cell stage (18-36h) -> 4-cell stage (36-48h) -> 8-cell stage (48-60h)

  
• Bilaminar germ disc formation


  Inner cell mass divides into epiblast and hypoblast, forming amniotic sac and yolk sac
• Amnion formation


  Epiblast -> amnioblast -> amniotic membrane -> amniotic cavity -> amniotic fluid
• Primary yolk sac formation

  Hypoblast -> extraembryonic endoderm -> primary yolk sac
• Trilaminar germ disc formation

  Primitive streak, primitive groove, primitive node, and primitive pit form, leading to endoderm, mesoderm, and ectoderm
• Bilaminar to trilaminar disc establishes 3 germ layers, all body tissues develop from these
• Three primary "germ" layers

  • Ectoderm
  • Endoderm
  • Mesoderm
• Gastrulation


  Invagination of epiblast cells, replacing hypoblast to become endoderm, mesoderm formed in between
• Mesenchyme


  Star-shaped cells that do not attach to one another, able to migrate freely
• Notochord formation


  Primitive node epiblast cells invaginate and migrate anteriorly with some endoderm cells, forming a rod defining the body axis
• Neurulation


  Notochord signals overlying ectoderm, formation of spinal cord and brain begins
• Neural tube closure


  Begins at end of week 3, complete by end of week 4, extends cranially (brain) and caudally (spinal cord)
• Neural crest


  Lateral ectodermal cells pulled along, form sensory nerve cells and other structures
• Mesoderm differentiation


  Lateral to notochord, extends cranially and caudally, divides into somites, intermediate mesoderm, and lateral plate
• Somites


  40 pairs of body segments
• Somatic mesoderm

  Apposed to the ectoderm
• Splanchnic mesoderm


  Apposed to the endoderm
• Coelom


  Space between somatic and splanchnic mesoderm, forms serous cavities
• Major derivatives of the embryonic germ layers

  • Endoderm: gastrointestinal tract lining, respiratory tract lining, urinary bladder lining, thyroid/parathyroid/liver/pancreas parenchyma, tympanic cavity/auditory tube lining
  • Ectoderm: central nervous system, peripheral nervous system, sensory epithelia, epidermis, glands
  • Mesoderm: splanchnic, intermediate, somatic, somites (sclerotome, dermatome, myotome)