Development Week 2 - Bilaminar Disc

Second Week of Development - Bilaminar Embryonic Disc

• Implantation and Inner Cell Mass Changes: As the blastocyst implants, the inner cell mass (embryoblast) undergoes significant changes
• Amniotic Cavity Formation: The amniotic cavity appears, separating the embryoblast from the trophoblast. Amnioblasts, derived from the inner cell mass, line the cavity.
• Amniotic Fluid Accumulation: The amniotic cavity expands and fills with amniotic fluid, creating a protective environment for the developing embryo.
• Embryonic Disc Formation: The inner cell mass flattens to form a bilaminar disc, a two-layered structure; the epiblast (ectoderm) and the hypoblast (endoderm).
• Epiblast Characteristics: The epiblast forms the floor of the amniotic cavity and consists of high columnar cells.
• Hypoblast Characteristics: The hypoblast forms the roof of the primitive yolk sac and is composed of small cuboidal cells.
• Primitive Yolk Sac Formation: Hypoblastic cells replace the exocoelomic membrane, forming the primitive (primary) yolk sac.
• Connecting Stalk Development: A connecting stalk forms from the region where no cavity has appeared, connecting the amniotic cavity, yolk sac, and embryonic disc to the outer wall. The stalk's location helps determine the embryo's posterior end.
• Extraembryonic Mesoderm Formation: The endoderm of the yolk sac gives rise to a layer of loose connective tissue called extraembryonic mesoderm, which surrounds the amniotic cavity and yolk sac.
• Extraembryonic Coelome Formation: Isolated spaces within the extraembryonic mesoderm fuse to create a large fluid-filled cavity called the extraembryonic coelome, which surrounds the amniotic cavity and yolk sac.
• Clinical Application: HCG Production: The syncytiotrophoblast secretes human chorionic gonadotropin (hCG), which maintains the corpus luteum and promotes progesterone production essential for the placenta. This hormone can be detected in maternal blood/urine, forming the basis for pregnancy tests.
• Two-Layered Trophoblast: The trophoblast differentiates into two distinct layers: cytotrophoblast and syncytiotrophoblast.
• Inner Cell Mass Differentiation: The inner cell mass (embryoblast) differentiates into two layers: epiblast and hypoblast.
• Primary Mesoderm Splitting: The primary mesoderm splits into two layers: somatic mesoderm (parietal mesoderm) and splanchnic mesoderm (visceral mesoderm).
• Amniotic and Yolk Sac Cavities Formation: The amniotic and yolk sac cavities begin to form.