Placenta Formation

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Description

From the earliest stages, a growing embryo requires nutrition, oxygen, and a disposal system for metabolic wastes and a way to prevent some harmful substances from getting into its blood stream. These requirements are met by the placenta and its blood barrier. Around day 8 of development the blastocyst, a hollow ball of cells, moves into the uterus. The blastocyst's outer layer begins to extend and it implants in the uterine lining, seeking uterine blood vessels to nourish it. As the blastocyst moves deeper, a single layer of cells from the uterine lining surrounds it to protect it from harm. On day 9, as the blastocyst grows larger and more complex, it becomes an embryo. Now the blastocyst is about the size of a pinhead. On day 13 small projections from the embryo's chorionic layer reach out into the uterine lining. The chorionic layer is a membrane that surrounds the embryo and helps it implant.