Zygote

The zygote is a totipotent cell, meaning it has the ability to differentiate into any type of cell in the body, including those that make up the placenta and other extra-embryonic tissues. During the first few days after fertilization, the zygote undergoes a series of cell divisions, forming a ball of cells called a blastocyst. The blastocyst consists of two distinct cell types: the inner cell mass (ICM) and the trophoblast.

The ICM is a cluster of cells that will eventually give rise to the embryo itself, while the trophoblast is a layer of cells that will form the placenta and other supportive tissues. The blastocyst will implant into the lining of the uterus around 5-7 days after fertilization, at which point the trophoblast will begin to invade the uterine wall and establish the maternal-fetal interface.

In the context of in vitro fertilization, zygotes are typically cultured in the laboratory for several days until they reach the blastocyst stage, at which point they may be transferred back into the uterus to establish a pregnancy. Alternatively, the blastocysts may be cryopreserved (frozen) for later use.