Embryology L2: 2nd Week of Development

Embryology L2: 2nd Week of Development

• The blastocyst implants in the endometrial stroma during the 2nd week.
• The trophoblast differentiates into two layers: cytotrophoblast and syncytiotrophoblast.
• Mitotic figures are found in cytotrophoblast but not in syncytiotrophoblast.
• Cells from the inner cell mass (embryoblast) differentiate into epiblast and hypoblast, forming the bilaminar disc.
• A cavity, the amniotic cavity, forms within the epiblast.
• Amnioblasts, part of the epiblast, line the amniotic cavity.
• The endometrial stroma surrounding the implantation site becomes edematous and highly vascular.
• The trophoblast shows progress in development at the embryonic pole, forming lacunae.
• Flattened cells from the hypoblast form the exocoelomic (Heuser) membrane lining the cytotrophoblast.

Day 8

• The blastocyst is partially embedded in the endometrial stroma.
• The trophoblast differentiates into cytotrophoblast and syncytiotrophoblast layers.
• Mitotic figures are present in cytotrophoblast cells, but not in syncytiotrophoblast cells of the outer layer.
• The cells of the inner cell mass (embryoblast) differentiates into epiblast and hypoblast.

Day 9

• The blastocyst implants more deeply in the endometrium and the defect in the surface epithelium is closed by a fibrin coagulum.
• Vacuoles appear in the syncytiotrophoblast that fuse to form lacunae (lacuna stage of development).
• Flattened cells from the hypoblast form the exocoelomic membrane.

Days 11 and 12

• The blastocyst is completely embedded and a slight protrusion occurs in the uterine lumen.
• The trophoblast has a network of lacunar spaces in the syncytiotrophoblast (especially evident at the embryonic pole), and the main part of cells still consists of the cytotrophblastic cells.
• Cells of the syncytiotrophoblast penetrate deeper into the stroma, eroding maternal capillaries (which become known as sinusoids).
• Maternal blood enters the lacunar system,establishing the uteroplacental circulation.
• Extraembryonic mesoderm forms between the cytotrophoblast and the exocoelomic cavity.
• Large cavities develop in the extraembryonic mesoderm, forming the extraembryonic cavity/chorionic cavity.

Day 13

• The surface defect in the endometrium is healed.
• Bleeding might occur at implantation site due to increased blood flow, which could be mistaken for normal menstrual bleeding.
• The trophoblast develops villous structures.
• Cytotrophoblast cells proliferate and penetrate the syncytiotrophoblast, forming primary villi.
• The hypoblast forms additional cells that migrate along the exocoelomic membrane, forming the secondary yolk sac (definitive yolk sac).
• The extraembryonic coelom becomes the chorionic cavity and the extraembryonic mesoderm lining the cytotrophoblast is known as the chorionic plate.
• The connecting stalk develops blood vessels.

Clinical Correlates

• The syncytiotrophoblast produces human chorionic gonadotropin (hCG), detectable in the blood by radioimmunoassays.
• It's crucial to understand that roughly 50% of the embryo genome is derived from the father, resulting in a foreign body that should be rejected by the mother's immune system, which must adapt to tolerate the pregnancy.

Abnormal Implantation

• Normal implantation is typically in the anterior or posterior wall of the uterus.
• Abnormal implantation can occur close to the internal os of the cervix, leading to placenta previa.
• Ectopic pregnancies are implantations outside the uterus.
• The most common ectopic site is the uterine tube (ampulla).

This quiz covers the key events that occur during the second week of embryonic development, focusing on the blastocyst's implantation and the differentiation of trophoblast layers. Participants will explore the formation of the bilaminar disc and associated structures while understanding the changes in the endometrial environment. Test your knowledge on these critical processes in human embryology!