✅ SEGUNDA SEMANA | EMBRIOLOGÍA 📚 DESARROLLO EMBRIONARIO | EMBARAZO | DISCO BILAMINAR
Understanding the Second Week of Gestation
Overview of Embryonic Development
- The second week of gestation involves significant morphological changes in the embryoblast, leading to the formation of the bilaminar embryonic disc.
- The blastocyst begins to invaginate into the maternal endometrium, with erosion facilitated by syncytiotrophoblast, allowing deeper implantation.
Differentiation of Trophoblast Layers
- By day eight, the trophoblast differentiates into two layers: cytotrophoblast (inner mononuclear layer) and syncytiotrophoblast (outer multinucleated layer).
- The embryoblast also differentiates into hypoblast (cuboidal cells) and epiblast (cylindrical cells), forming the bilaminar germinative disc.
Implantation Progression
- On day nine, as implantation deepens, a fibrin clot occludes the surface epithelium; vacuoles in syncytiotrophoblast merge to form trophoblastic lagoons.
- By days 11 and 12, the blastocyst is fully immersed in endometrial stroma; trophoblastic lacunae create an intercommunicating network.
Maternal Blood Flow and Extraembryonic Structures
- Syncytium cells penetrate deeper into stroma, eroding maternal capillaries to establish uteroplacental circulation through maternal sinusoids.
- A new connective tissue called extra-embryonic mesoderm forms between cytotrophoblast and exocoelomic cavity.
Formation of Cavities and Yolk Sac Development
- Cavities develop within extra-embryonic mesoderm, merging to create chorionic cavity surrounding primitive yolk sac and amniotic cavity.
- By day 13, superficial epithelial scars disappear; however, hemorrhage may occur at implantation sites due to increased blood flow.
Cell Proliferation and Yolk Sac Changes
- Cytotrophoblast cells proliferate locally and penetrate syncytiotrophoblast forming primary villi; additional cells migrate within exocoelomic membrane.
Development of the Umbilical Cord and Germ Layers
Formation of the Umbilical Cord
- The fixation pedicle is crucial in suspending structures during early development, eventually evolving into the umbilical cord as blood vessels develop.
Structure of the Bilaminar Germinative Disc
- The bilaminar germinative disc consists of two layers: the epiblast and hypoblast.
Components of the Epiblast
- Within the epiblast, amnioblasts are present, which play a role in forming protective membranes around the embryo.
Characteristics of the Hypoblast
- In contrast, some cells in the hypoblast become cylindrical and thicken to form an area known as the prechordal plate.
Significance of Prechordal Plate