blastomeres

A blastomere is any cell produced by the cleavage divisions of a zygote during early embryogenesis. Cleavage partitions the cytoplasm into smaller cells without growth between divisions, yielding progressively smaller cells that inherit the zygote’s genetic material.

During early development, successive cleavages create 2-cell, 4-cell, 8-cell, and 16-cell stages, among others. In many animals, early blastomeres are totipotent, meaning each cell has the potential to form all embryonic and extraembryonic lineages; as development proceeds, their developmental potential becomes progressively restricted.

As cleavage continues, the cells form a morula, a compact ball of blastomeres. Further development leads to the formation of a blastocyst (blastula in some species), which contains a fluid-filled cavity and a lineage-specified arrangement: an inner cell mass that will give rise to the embryo proper and an outer layer called the trophectoderm that contributes to extraembryonic tissues such as the placenta.

The study of blastomeres has clinical and research relevance. In humans and other animals, blastomere biopsy from early embryos is used in preimplantation genetic testing to screen for genetic abnormalities. Blastomere behavior and lineage allocation also inform cloning research and basic questions about totipotency, cellular differentiation, and mosaicism within developing embryos.

Cleavage patterns vary among species: holoblastic cleavage involves division of the entire embryo, while meroblastic cleavage involves partial division due to yolk distribution. Blastomeres are the basic units of these early developmental processes.