A blastomere (plural: blastomere) is a cell that has been created by cleavage ( cell division by constriction). Depending on the yolk distribution in the zygote , the blastomeres can be of different sizes.
These cells do not grow at first, but get smaller with each further division. The zygote from which the furrow originates is relatively large compared to other cells.
When an isolecithalic egg (e.g. a sea urchin egg) is cleaved and a radial cleavage occurs, blastomeres of different sizes are created. An isolecithal egg contains little evenly distributed yolk, so this does not constitute an obstacle to the furrowing. The blastomer cells are divided into different groups according to their size:
- The mesomers are medium-sized blastomeres.
- The micromers are small blastomeres. They are important for axial development in the embryo and induce cell fate in neighboring blastomeres. The transcription factor β-catenin is important here. It is located in the nucleus in micromers and neighboring cells. Micromers are defined by the high concentration of this substance. He gives them their inductive capacity. In the core, depending on the concentration, certain genes are transcribed by the β-catenin , the products of which induce further developments.
- The macromers are the largest blastomeres.