Quiz

• After floating for 2-3 days in the lumen in the uterus, the embryo implants
• Formation of bilaminar embryo in the second week
• Amniotic cavity: a small space that appears between the inner cell mass and the invading trophoblast during implantation
• Amnion:

  • Lines the amniotic cavity
  • Holds the amniotic fluid
  • Forms as a layer of epiblast cells expands towards the embryonic pole and differentiates into a thin membrane
• Chorion:

  • A double-layered membrane formed by the trophoblast
  • Surrounds the amnion, embryo, and other membranes
  • Gives rise to the fetal part of the placenta
• Allantois:

  • An extension from the embryo
  • Together with the remnants of the yolk sac, it forms the umbilical cord
  • Functions to collect liquid waste from the embryo and exchange gases
• Secondary yolk sac:

  • Forms when the primary yolk sac decreases in size
  • Responsible for the initial hematopoietic and absorptive function of the embryo
• Primitive streak:

  • Formation marks the beginning of gastrulation
  • Hypoblast becomes endoderm
  • Epiblast becomes ectoderm
  • Cells in the epiblast die in the primitive streak, forming the middle layer called mesoderm
• Hensen node:

  • A primitive knot of cells at the cephalic end of the primitive streak
  • Gives rise to the cells that form the notochordal process
• Neurulation:

  • Begins after gastrulation
  • First develops the nervous system
  • Ectoderm layer forms the nervous system
• Notochord formation:

  • Provides skeletal support for the developing embryo
  • Eventually disappears but is represented by the nucleus pulposus in adulthood
• Neural tube:

  • From which the brain and spinal cord arise
• Development of somites:

  • Paraxial mesoderm thickens on each side of the notochord
  • End of the third week, paraxial mesoderm breaks up into segmented pairs of blocks called somites
  • Somites give rise to the vertebral column, associated musculature, and adjacent dermis of the skin
• Lateral plate mesoderm:

  • In the third week of human development, small gaps merge to form the intraembryonic coelom
  • Splits the lateral plate mesoderm into splanchnic mesoderm and somatic mesoderm
• Component of somites:

  • Sclerotome contributes to two adjacent vertebrae and their disks
  • Myotome gives origin to a segmented mass of muscles
  • Dermatome gives rise to connective tissues of the skin overlying the somite