✅ SEGUNDA SEMANA | EMBRIOLOGÍA 📚 DESARROLLO EMBRIONARIO | EMBARAZO | DISCO BILAMINAR
Implantation and Development of the Blastocyst
Overview of the Second Week of Gestation
- The second week of gestation involves significant morphological changes in the embryoblast, leading to the formation of the bilaminar embryonic disc.
- By this time, the blastocyst has begun to invaginate into the maternal endometrium, with erosion facilitated by syncytiotrophoblast.
Differentiation of Trophoblast Layers
- On day eight, the trophoblast differentiates into two layers: cytotrophoblast (inner mononuclear layer) and syncytiotrophoblast (outer multinucleated layer).
- The embryoblast also differentiates into two layers: hypoblast (cuboidal cells) and epiblast (cylindrical cells), forming the bilaminar germinative disc.
Developmental Changes on Day Nine
- By day nine, implantation deepens within the endometrium, creating a fibrin clot at its surface.
- Syncytiotrophoblastic vacuoles merge to form trophoblastic lagoons; these structures are crucial for establishing maternal blood flow.
Formation of Extraembryonic Structures
- As trophoblastic erosion continues, maternal capillaries become congested and dilated, leading to uteroplacental circulation establishment.
- A new connective tissue called extra-embryonic mesoderm forms between cytotrophoblast and exocoelomic cavity surfaces.
Chorionic Cavity Development
- Cavities develop within extraembryonic mesoderm that merge to create an extraembryonic cavity surrounding both yolk sac and amniotic cavity.
- This leads to differentiation into somatic mesoderm covering cytotrophoblast and splanchnic mesoderm covering yolk sac.
Maternal Endometrium Changes
- By day 13, superficial epithelial scars from implantation heal; however, some may experience hemorrhage due to increased blood flow around lagoon spaces.
Formation of Yolk Sac Structures
- Cytotrophoblast cells proliferate locally and penetrate syncytiotrophoblast forming primary villi; additional cells migrate within exocoelomic membrane creating a secondary yolk sac.
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 and Hypoblast
- Within the epiblast, amnioblasts are present, which play a role in forming protective membranes around the embryo.
- In contrast, some cells in the hypoblast become cylindrical, leading to a thickened area known as the prechordal plate.
Significance of the Prechordal Plate