Segmentación y Nidación

Segmentation and Nidation

• In identical twins, cell segmentation occurs very early, while in Siamese twins, it occurs later.
• The zygote, after becoming a two-cell structure, undergoes repeated mitotic divisions, increasing the number of cells (blastomeres).
• The initial cytoplasm is shared among the cells, but as division continues, the cytoplasm becomes less abundant.
• Cell division stops unless implantation occurs due to insufficient cytoplasm.
• The distance traveled from the fallopian tube is critical in determining the timing.
• Zygote segmentation into blastomeres begins approximately 30 hours post-fertilization.

Stages

• Day 0: Zygote (single cell)
• Days 1-4: Morula (16-32 cells)
• Days 4-5: Blastula/Blastocyst/Blastocoele

Blastocyst Formation

• The morphology changes due to fluid accumulation.
• Blasto = immature, Cysto = cavity/space
• The blastula appears around the fifth day, free-floating in the uterine cavity.
• The outer layer (MCE) is responsible for implantation and uterine wall penetration.
• The MCE eventually develops into the trophoblast, the initial nutritive support system.
• The inner layer (MCI) becomes the developing embryo itself. (embryonic disk)
• The MCE is essential but not part of the embryo, it's temporary
• The embryo is primarily MCI.

Second Week Development

• The blastocyst implants on the endometrial lining around day 6.
• The blastocyst differentiates into two structures, the trophoblast (MCE) and inner cell mass (MCI) or embryoblast.)
• The inner cell mass (MCI) or embryoblast eventually develops into the embryonic disc.
• The MCI forms the embryonic disk, the structure that becomes the embryo itself.
• Day 8-10: Gastrulation
• The growing trophoblast differentiates into two layers: the cytotrophoblast and the syncytiotrophoblast.
• The inner cell mass (MCI) differentiates further into two layers: hypoblast and epiblast, forming a bilaminar embryonic disc.
• Two cavities develop in the gastrula: the yolk sac (from the blastocoele) lined by the hypoblast, and the amniotic sac (new cavity) lined by the epiblast.

Este cuestionario explora el proceso de segmentación y nidación en los embriones, centrándose en la formación de blastocistos y las etapas clave del desarrollo. Aprenderás sobre la diferenciación entre gemelos idénticos y siameses, así como el tiempo crítico de la implantación en el útero.