Week 2 Embryonic Development

Week 2 Embryonic Development

• The bilaminar germ disc forms from two layers: the epiblast and hypoblast.

• Trophoblast differentiates into two layers:

  • Cytotrophoblast: Inner layer of mononucleated cells.
  • Syncytiotrophoblast: Outer layer of multinucleated cells, involved in implantation and producing hCG.

• Embryoblast differentiates into:

  • Epiblast: Forms the amniotic cavity.
  • Hypoblast: Migrates to form the exocoelomic membrane and lines the primary yolk sac.

• Extraembryonic mesoderm develops from yolk sac cells, differentiates into:

  • Somatic mesoderm: Lines the cytotrophoblast and amnion.
  • Visceral mesoderm: Covers the yolk sac.

• Amniotic cavity forms within the epiblast, filled with amniotic fluid.

• The exocoelomic cavity (primary yolk sac) forms from hypoblast cells and exocoelomic membrane lined by extraembryonic mesoderm.

• The chorionic cavity forms from spaces in the extraembryonic mesoderm.

• The connecting stalk is composed of extraembryonic mesoderm and will become the umbilical cord.

• Allantois, a diverticulum from the yolk sac, is involved in early blood formation.

hCG and Developmental Abnormalities

• Human Chorionic Gonadotropin (hCG) is produced by the syncytiotrophoblast, maintaining corpus luteum, progesterone production, and used in pregnancy tests.

• Low hCG levels indicate risk of spontaneous abortion or ectopic pregnancy.

• High hCG levels may indicate multiple pregnancies or trophoblastic diseases.

• Gestational trophoblastic diseases (GTD) include:

  • Hydatidiform mole: Benign trophoblastic proliferation with high hCG and no embryo development.
  • Choriocarcinoma: Malignant trophoblastic tumor, potentially metastasizing.

• Ectopic pregnancy is implantation outside the uterus, most often occurring in the fallopian tubes, and can lead to severe complications like bleeding.