✅ SEGUNDA SEMANA | EMBRIOLOGÍA 📚 DESARROLLO EMBRIONARIO | EMBARAZO | DISCO BILAMINAR
Morphological Changes During the Second Week of Gestation
Overview of Embryonic Development
- The implantation of the blastocyst leads to morphological changes in the embryoblast, forming the bilaminar embryonic disc during the second week of gestation.
Recap from Previous Week
- The video continues discussing phenomena and changes from the second week of gestation, referencing a previous video on the first week where the blastocyst began to invaginate into the maternal endometrium.
Trophoblast Differentiation
- The syncytiotrophoblast erodes the endometrium, allowing deeper implantation. The trophoblast differentiates into two layers: cytotrophoblast (inner layer) and syncytiotrophoblast (outer layer).
Formation of Bilaminar Germ Disc
- By day eight, both trophoblast layers are established. The embryoblast differentiates into hypoblast (cuboidal cells) and epiblast (cylindrical cells), forming a bilaminar germinative disc.
Amniotic Cavity Development
- An amniotic cavity begins to form within the epiblast as it increases in size. Cells adjacent to cytotrophoblast are called amnioblasts, lining this cavity.
Implantation Progression and Maternal Interaction
Deepening Implantation
- By day nine, the blastocyst is more deeply implanted in the endometrium, leading to an occlusion by a fibrin clot due to its immersion.
Trophoblastic Lacunae Formation
- Vacuoles in syncytiotrophoblast merge to create trophoblastic lagoons. A thin membrane known as Heuser's membrane forms alongside hypoblast lining for exocoelomic cavity development.
Establishment of Uteroplacental Circulation
Complete Immersion and Network Formation
- By days 11 and 12, complete immersion occurs with surface epithelium closure. Trophoblastic lacunae form an intercommunicating network while syncytium penetrates deeper into stroma.
Maternal Blood Flow Initiation
- Erosion of maternal capillaries allows blood flow into trophoblastic lagoons, establishing uteroplacental circulation through congested maternal sinusoids.
Extraembryonic Mesoderm Development
Emergence of Extraembryonic Mesoderm
- A new population of cells forms loose connective tissue called extraembryonic mesoderm between cytotrophoblast and exocoelomic cavity surfaces.
Chorionic Cavity Formation
- Cavities develop within extraembryonic mesoderm that merge into one large space known as chorionic cavity surrounding primitive yolk sac and amniotic cavity except at fixation pedicle connection point.
Changes in Maternal Endometrium
Decidual Reaction
- The maternal endometrium becomes rich in glycogen and lipids; these changes lead to edema known as decidual reaction by day 13 after implantation.
Hemorrhage Considerations
- Occasionally hemorrhage may occur at implantation sites due to increased blood flow around day 28 menstrual cycle; this can be mistaken for normal menstrual bleeding affecting delivery date calculations.
Developmental Structures Post Implantation
Cytotrophoblast Proliferation
- Cytotrophoblast cells proliferate locally penetrating syncytiotrophoblast forming primary villi surrounded by syncytial covering.
Secondary Yolk Sac Formation
- Additional hypoblastic cells migrate inside exocoelomic membrane creating secondary yolk sac which is smaller than primitive yolk sac; portions detach forming exocoelomic cysts.
Chorion Structure Definition
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.