✅ SEGUNDA SEMANA | EMBRIOLOGÍA 📚 DESARROLLO EMBRIONARIO | EMBARAZO | DISCO BILAMINAR
Morphological Changes During the Second Week of Gestation
Overview of Blastocyst Implantation
- The implantation of the blastocyst leads to morphological changes in the embryoblast, resulting in the formation of the bilaminar embryonic disc during the second week of gestation.
Recap from First Week
- The video continues discussing phenomena and changes from the second week, referencing a previous video on the first week where the blastocyst began to invaginate into the maternal endometrium.
Trophoblast Differentiation
- The endometrium is eroded by syncytiotrophoblast, allowing for deeper implantation. By days seven to eight, embryoblast differentiates into hypoblast and epiblast layers.
Development at Day Eight
- On day eight, the blastocyst is partially submerged in endometrium; trophoblast differentiates into cytotrophoblast (inner layer) and syncytiotrophoblast (outer layer).
Formation of Bilaminar Germ Disc
- The embryoblast forms two layers: hypoblast (cuboidal cells) and epiblast (cylindrical cells), together constituting the bilaminar germinative disc.
Amniotic Cavity Development
- An amniotic cavity begins to form within epiblast cells adjacent to cytotrophoblast, known as amnioblasts.
Progression Through Days 9 to 13
Implantation Depth and Fibrin Clot Formation
- By day nine, implantation deepens in endometrial tissue leading to occlusion by a fibrin clot due to immersion on epithelial surface.
Trophoblastic Lacunae Formation
- Syncytiotrophoblast develops vacuoles that merge into larger spaces called trophoblastic lagoons; internal cytotrophoblast forms Heuser's membrane lining exocoelomic cavity or primitive yolk sac.
Complete Immersion by Days 11 and 12
- By days eleven and twelve, blastocyst is fully immersed in endometrial stroma with surface epithelium closing over it. Trophoblastic lacunae create an intercommunicating network.
Maternal Blood Flow Establishment
- Erosion of maternal capillaries allows blood flow into trophoblastic system establishing uteroplacental circulation; new connective tissue called extra-embryonic mesoderm appears between structures.
Extra-Embryonic Cavity Development
- Cavities develop within extra-embryonic mesoderm merging into one space known as chorionic cavity surrounding primitive yolk sac and amniotic cavity except at connection point with trophoblast via fixation pedicle.
Changes Leading Up To Day 13
Mesodermal Layer Differentiation
- Chorionic cavity appearance causes extraembryonic mesoderm to unfold into somatic mesoderm covering cytotrophoblast/amnion and splanchnic mesoderm covering yolk sac.
Decidual Reaction
- Maternal endometrium becomes rich in glycogen/lipids; tissue edema occurs leading to decidual reaction.
Hemorrhage Risks Post Implantation
- By day thirteen, superficial epithelium scar disappears but hemorrhage may occur at implantation site due to increased blood flow which can be mistaken for menstrual bleeding.
Cytotrophoblast Proliferation
- Cytotrophoblast cells proliferate locally penetrating syncytiotrophoblast forming primary villi surrounded by syncytial covering; additional cells migrate along exocoelomic membrane creating secondary yolk sac.
Chorionic Structure Formation
Understanding the Bilaminar Germinative Disc
Components of the Bilaminar Germinative Disc
- The bilaminar germinative disc consists of two layers: the epiblast and hypoblast.
- Within the epiblast, amnioblasts are present, which play a crucial role in embryonic development.
- The hypoblast features some cells that have transformed into cylindrical shapes, indicating differentiation.
- A significant area within the hypoblast is known as the prechordal plate, which becomes thickened during development.