Embryonic Development

  • inner cell mass of the blastocyst begins to differentiate into three primary germ layers: ectoderm, endoderm, and mesoderm
  • within 8 days after fertilization the cells of the cytotrophoblast proliferate and form a fetal membrane ( amnion)
  • about the 12th day after fertilization a yolk sac is forming
  • about the 14th day cells differentiates into three distinct layers: ectoderm, endoderm, and mesoderm
  • as the embryo develops, the endoderm becomes the epithelial lining of the GI tract, respiratory tract and several other organs. The mesoderm forms muscle, bone, and other connective tissue, the ectoderm develops into the skin and nervous system
  • formation of the embryonic membranes, the membranes lie outside the embryo and protect and nourish the embryo and later the fetus
  • the membranes are the yolk sac ( the primary source of nourishment for the embryo), amnion (protective membrane) chorion (the structure for exchange of materials between mother and fetus, produces hCG) , and allantois ( serves as an early site of blood formation)
  • placenta is developed, it is formed by the chorion of the embryo and a portion of the endometrium of the mother
  • the placenta allows oxygen and nutrients to diffuse into fetal blood from maternal blood: carbon dioxide and wastes diffuse from fetal blood into maternal blood
  • the umbilical cord is a vascular connection between mother and fetus
  • the cord consist of 2 umbilical arteries , and surrounded by a layer of amnion