Final answer:
Embryonic stem cells are totipotent cells from the blastocyst's inner mass, capable of differentiating into any human body cell. Their totipotency is temporal, lasting only a few days post-fertilization. These cells are key for the development and growth of the embryo through the eighth week, a period characterized by significant changes and organ formation.
Step-by-step explanation:
Understanding Embryonic Stem Cells
Embryonic stem cells are derived from the inner mass of cells found within a blastocyst, which forms within 4 to 5 days following fertilization. At this embryonic stage, the stem cells are totipotent, meaning they have the remarkable ability to differentiate into any cell type of the human body. This period of totipotency lasts for only a few days before the cells commit to specific lineages, eventually giving rise to the various tissues and organs of the body. This stage plays a crucial role in the growth and development of the embryo, which, after implantation, is referred to as the embryo until the eighth week post-fertilization.
In the United States, during procedures like in vitro fertilization (IVF), embryos are typically cultured until they reach the blastocyst stage. This practice is associated with a higher pregnancy rate compared to when embryos are transferred at the eight-cell stage. Once the blastocyst stage is reached, the embryos are transferred to the uterus for implantation. During the embryonic stage, from implantation through the eighth week, the embryo undergoes dramatic changes and significant growth, forming most organs and beginning to take on a more complex structure.