Blastomere vs. Blastocyst — What's the Difference?

A blastomere is an individual cell resulting from the cleavage, or division, of the zygote after fertilization. These cells are totipotent in the earliest stages, meaning each can develop into a complete organism. On the other hand, a blastocyst is a later stage of development that occurs about five days after fertilization in humans, featuring differentiated cells that will form either the embryo or part of the placenta.

In the development process, blastomeres undergo several rounds of division without the embryo increasing in size, which leads to the formation of a morula. This eventually transitions into the blastocyst stage, where the structure becomes hollow, filling with fluid, and the cells start to specialize. Whereas blastomeres are essentially uniform in their early stages, blastocysts exhibit the first major differentiation of cells into specific types and structures.

The primary role of blastomeres is to contribute to the initial increase in cell number and to start the process of differentiation. In contrast, the blastocyst has the critical function of implantation into the uterine wall, a step that is vital for establishing a successful pregnancy. The blastocyst’s inner cell mass will develop into the embryo, while the outer layer, known as the trophoblast, will form the placenta and embryonic membranes.

Blastomeres are found in the earliest stages of embryonic development, making them crucial for the fundamental aspects of cell division and the embryo’s initial formation. Meanwhile, the blastocyst represents a more advanced developmental stage, crucial for pregnancy continuation and the establishment of key embryonic structures.

The study and manipulation of blastomeres are fundamental in research and assisted reproductive technologies, such as in vitro fertilization (IVF), where embryo quality is assessed at the blastocyst stage for a higher chance of successful implantation. This contrasts with the blastocyst, which is used in similar contexts but evaluated for different criteria, such as the development of the inner cell mass and the trophoblast, which are indicative of the potential for successful implantation and pregnancy.