Blastomeri

Blastomeri are the cells produced by the cleavage divisions of the zygote after fertilization. Cleavage partitions cytoplasm into smaller cells without a significant overall growth of the embryo, so the total embryo size remains similar to the zygote while the cell number increases and the cell size diminishes. Early cleavage yields two, four, and eight blastomeres. Around the eight-cell stage, blastomeres undergo compaction, increasing cell–cell contacts and forming the morula, typically consisting of 16–32 cells. Further cleavage and fluid accumulation within the embryo lead to the blastocyst, with a fluid-filled blastocoel, an outer epithelial layer called the trophectoderm, and an inner cell mass.

Patterns of cleavage depend on yolk content: holoblastic cleavage (entire cell divides) in species with little yolk, including mammals, and meroblastic cleavage (partial division) in birds, reptiles, and many fish with substantial yolk.

Blastomere potency is highest early; mammalian blastomeres are largely totipotent up to the 4- to 8-cell stage, capable of giving rise to all embryonic and extraembryonic tissues. After compaction and lineage specification, developmental potential becomes restricted to specific lineages (inner cell mass vs trophectoderm).

In clinical embryology, blastomere biopsies from cleavage-stage embryos or trophectoderm biopsies from blastocysts are used for preimplantation genetic testing, to assess aneuploidy or genetic disorders before implantation. Such procedures carry risks to embryo viability and are subject to ethical and regulatory considerations.