General Embryology

• Embryology series


  1. Fertilization

  2. Cleavage

  3. Blastulation

  4. Trophoblast and embryoblast formation

  5. Bilaminar disk formation
• Uterine anatomy


  • Vagina (sheath)
  • Cervix of uterus
  • Body of uterus (fundus)
  • Fallopian tubes (ampulla where fertilization occurs)
  • Ovaries
• Ovulation


  1. Hypothalamus releases GnRH

  2. Anterior pituitary releases LH

  3. LH stimulates ovary to release oocyte
• Secondary oocyte


  Arrested in metaphase II, awaiting fertilization
• Fertilization


  1. Sperm capacitation

  2. Sperm binds to ZP3 receptor

  3. Sperm releases enzymes to penetrate oocyte

  4. Sperm nucleus enters oocyte cytoplasm

  5. Maternal and paternal chromosomes fuse to form zygote
• Zygote


  Diploid cell with 46 chromosomes
• Cleavage


  1. Zygote divides to 2 cells

  2. 2 cells divide to 4 cells

  3. 4 cells divide to 8 cells

  4. 8 cells divide to 16 cells
• Morula


  Solid ball of 16+ blastomeres
• Blastulation


  1. Morula forms fluid-filled blastocyst

  2. Blastocyst has trophoblast and inner cell mass
• Trophoblast


  Outer cell layer of blastocyst, forms placenta
• Inner cell mass


  Inner group of cells in blastocyst, forms embryo
• Differentiation of trophoblast and inner cell mass

  1. Trophoblast becomes cytotrophoblast and syncytiotrophoblast

  2. Inner cell mass becomes bilaminar disc (epiblast and hypoblast)
• Bilaminar disc will further develop into trilaminar disc to form the entire embryo
• This particular lecture will be covering a topic known as embryo Genesis
• Embryo Genesis


  Looking at the first eight weeks after gestation, where we're looking at the embryo itself before it matures into a fetus
• Learning outcomes for today

  • Processes that occur in the first week
  • Processes in the second week
  • Processes in the third week
• Processes in the first week

  1. Fertilization

  2. Cleavage

  3. Implantation
• Processes in the second week


  1. Further implantation

  2. Bilaminar formation
• Processes in the third week

  1. Gastrulation

  2. Embryo starts to fold
• Frontal section of female reproductive system

  • Fallopian tube
  • Uterus
  • Ovaries
• Fertilization


  1. Egg released from ovary

  2. Sperm travels through vagina, cervix

  3. Sperm capacitation

  4. Acrosome reaction

  5. Sperm fuses with egg
• Zygote


  One cell embryo formed after fertilization
• Cleavage


  1. One cell divides into two, then four, then more cells

  2. Zona pellucida keeps size constant as cell number increases
• Morula


  16 cell stage at 3 days, resembles a mulberry
• Implantation


  1. Blastocyst hits uterine wall

  2. Trophoblast merges with endometrium

  3. Fluid enters blastocyst
• Trophoblast


  Outer cell mass that will nourish the embryo
• Embryoblast


  Inner cell mass that will become the embryo
• Second week


  1. Trophoblast splits into cytotrophoblast and syncytiotrophoblast

  2. Cytotrophoblast and syncytiotrophoblast invade endometrium to form placenta
• Epiblast


  Top layer of embryoblast, will form head end of embryo
• Hypoblast


  Bottom layer of embryoblast
• Amniotic cavity


  Space where fetus and embryo will develop, filled with amniotic fluid
• Sagittal section


  A cut through the body from front to back, showing a reflection of the layers
• Epiblast


  • Top layer of cells
  • Continuous with trophoblast cells on superior aspect
• Hypoblast


  • Bottom layer of cells
  • Continuous with the layer of cells that will become the hypoblast
• Formation of amniotic cavity

  1. Epiblast layer pushed up into the side to form a cavity

  2. Where the fetus and embryo will start to develop
• Yolk sac


  • Space formed by the reflection of the hypoblast cells
  • Will be taken back into the gut
• The amniotic sac and cavity will stay permanent for the whole duration of the embryo/fetus's pregnancy
• The second week is called the "week of the twos" because there are many things that come in pairs
• Gastrulation


  The formation of the three germ layers: ectoderm, mesoderm, and endoderm
• Gastrulation


  1. Epiblast cells become ectoderm

  2. Hypoblast cells become endoderm

  3. Mesoderm forms between ectoderm and endoderm