Embryology

• Embryology


  The study of the development of an organism from fertilization through birth

  
• Outline of General Embryo Lectures

  • Lect1 – General to Implantation
  • Lect2 – Bilaminar & Trilaminar Germ Disc
  • Lect3 – Gastrulation & Neurulation
  • Lect4 – Germinal Epithelium
  • Lect5 – Placenta
• Embryonic period


  First 8 weeks, all major organs formed

  
• Fetal period


  Remaining 30 weeks, organs grow larger and become more complex
• Periods of human development

  • Preembryonic period: 1st week to end of 2nd week
  • Embryonic period: 3rd week to end of 8th week
  • Fetal period: 9th week to birth
  • Perinatal stage: 26th week to 4 week after birth

  
• Fertilization


  1. Egg released into peritoneal cavity

  2. Captured by fimbria, fertilization in ampulla

  3. Travels down fallopian tube

  4. Maternal and paternal genetic material join to form zygote

  
• Week 1 (post conception)


  1. Zygote divides repeatedly while moving down tube toward uterus (cleavage)

  2. Daughter cells are called blastomeres

  3. Morula: solid cluster of 12-16 blastomeres at about 72 hours

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

  5. Formation of inner cell mass

  
• Blastocyst


  Two distinct types of cells: inner cell mass forms the embryo, trophoblast layer surrounds cavity and helps form placenta

  
• Implantation


  1. Blastocyst floats for about 3 days

  2. Implantation on about day 6 post conception

  3. Trophoblast erodes uterine wall

  4. Takes 1 week to complete

  5. If inner cell mass divides: monozygotic (identical) twins

  
• Stages of blastocyst formation

  • Cleavage: early division of fertilized egg
  • Blastomere: daughter cells from cleavage
  • Morula: 12 to 16- cell stage, enclosed in zona pellucida
  • Blastocyst: about 100 blastomeres, blastocoele, inner cell mass and trophoblast

  
• Week 2


  1. Inner cell mass divides into epiblast and hypoblast

  2. 2 fluid filled sacs: amniotic sac from epiblast, yolk sac from hypoblast

  3. Bilaminar embryonic disc: area of contact - gives rise to whole body
• Inner cell mass → 2 layers of cells in disc (7th day)
• Embryology


  The study of the development of an organism from fertilization through birth

  
• Epiblast


  Columnar cells adjacent to trophoblast
• Outline of General Embryo Lectures

  • Lect1 – General to Implantation
  • Lect2 – Bilaminar & Trilaminar Germ Disc
  • Lect3 – Gastrulation & Neurulation
  • Lect4 – Germinal Epithelium
  • Lect5 – Placenta
• Hypoblast


  Cuboidal cells adjacent to blastocoele

  
• Embryonic period


  First 8 weeks, all major organs formed

  
• Fetal period


  Remaining 30 weeks, organs grow larger and become more complex
• Periods of human development

  • Preembryonic period: 1st week to end of 2nd week
  • Embryonic period: 3rd week to end of 8th week
  • Fetal period: 9th week to birth
  • Perinatal stage: 26th week to 4 week after birth

  
• Fertilization


  1. Egg released into peritoneal cavity

  2. Captured by fimbria, fertilization in ampulla

  3. Travels down fallopian tube

  4. Maternal and paternal genetic material join to form zygote

  
• Week 1 (post conception)


  1. Zygote divides repeatedly while moving down tube toward uterus (cleavage)

  2. Daughter cells are called blastomeres

  3. Morula: solid cluster of 12-16 blastomeres at about 72 hours

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

  5. Formation of inner cell mass

  
• Blastocyst


  Two distinct types of cells: inner cell mass forms the embryo, trophoblast layer surrounds cavity and helps form placenta

  
• Implantation


  1. Blastocyst floats for about 3 days

  2. Implantation on about day 6 post conception

  3. Trophoblast erodes uterine wall

  4. Takes 1 week to complete

  5. If inner cell mass divides: monozygotic (identical) twins
• Stages of blastocyst formation

  • Cleavage: early division of fertilized egg
  • Blastomere: daughter cells from cleavage
  • Morula: 12 to 16- cell stage, enclosed in zona pellucida
  • Blastocyst: about 100 blastomeres, blastocoele, inner cell mass and trophoblast

  
• Week 2


  1. Inner cell mass divides into epiblast and hypoblast

  2. 2 fluid filled sacs: amniotic sac from epiblast, yolk sac from hypoblast

  3. Bilaminar embryonic disc: area of contact - gives rise to whole body
• Inner cell mass → 2 layers of cells in disc (7th day)
• Epiblast


  Columnar cells adjacent to trophoblast
• Hypoblast


  Cuboidal cells adjacent to blastocoele

  
• During implantation, the blastocyst attaches itself to the uterine wall through the trophoblast.
• The inner cell mass (ICM) is located inside the blastocoel cavity and gives rise to all other tissues of the embryo.
• The blastocyst consists of an outer layer called the trophoblast, which will form the placenta.
• The blastocyst burrows into the endometrial lining by secreting proteolytic enzymes that break down the extracellular matrix.
• The blastocoel cavity contains fluid that helps with gas exchange between the developing embryo and its environment.
• The outer layer of the blastocyst is called the trophectoderm or trophoblast, which will give rise to extraembryonic structures such as the placenta and umbilical cord.
• Implantation occurs at around 8 days post-fertilization.
• The endometrium is shed during menstruation if pregnancy does not occur.
• The blastocoel cavity contains fluid that fills most of the space within the blastocyst.
• The epiblast forms the ectoderm, while the hypoblast forms the endoderm.
• The mesoderm develops from the middle layer between the epiblast and hypoblast.