Bilaminar Blastocyst - Formation of The Yolk Sac and Chorionic Cavity

Formation of The Yolk Sac and Chorionic Cavity

The formation of the chorionic cavity (Extra-embryonic coelom) and the yolk sac (umbilical vesicle) is still up for debate. The thought of how the yolk sac membranes are formed begins with an increase in production of hypoblast cells, succeeded by different patterns of migration. On day 8, the first portion of hypoblast cells begin their migration and make what is known as the primary yolk sac, or Heuser's membrane (exocoelomic membrane). By day 12, the primary yolk sac has been disestablished by a new batch of migrating hypoblast cells that now contribute to the definitive yolk sac.
While the primary yolk sac is forming, extraembryonic mesoderm makes its way into the blastocyst cavity to fill it with loosely packed cells. When the extraembryonic mesoderm is separated into two portions, a new gap arises called the chorionic cavity, or the extra-embryonic coelom. This new cavity is responsible for detaching the embryo and its amnion and yolk sac from the far wall of the blastocyst, which is now named the chorion. When the extraembryonic mesoderm splits into two layers, the amnion, yolk sac, and chorion follow its lead and also become double layered. The chorion and amnion are composed of extraembryonic ectoderm and mesoderm, where as the yolk sac is made of extraembryonic endoderm and mesoderm. When day 13 rolls around, the connecting stalk, a dense portion of extraembryonic mesoderm, restrains the embryonic disc in the chorionic cavity.

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Bilaminar Blastocyst - Formation of The Yolk Sac and Chorionic Cavity - Yolk Sac During Development
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