In humans, blastomere formation begins immediately following fertilization and continues through the first week of embryonic development. About 30 hours after fertilization, the egg divides into two cells. These mitotic divisions continue and result in a grouping of cells called blastomeres. During this process, the total size of the embryo (also called a "zygote") does not increase, so each division results in smaller and smaller cells. When the zygote contains 12 to 32 blastomeres it is referred to as a "morula."
It is possible for errors to occur during this process of repetitive cell division. Common among these errors is for the genetic material to not be divided evenly. Normally, when a cell divides each daughter cell has the same genetic material as the parent cell. If the genetic material does not split evenly between the two daughter cells, an event called "nondisjunction" occurs. Since this event occurs in only one of the several cells that exist at this point, the embryo will continue to develop but will have some normal cells and some abnormal cells. The people are "mosaics" of normal and abnormal cells, so this disorder is called "numerical mosaicism".
- "Blastomere." Stedman's Medical Dictionary, 27th ed. (2000). ISBN 0-683-40007-X
- Moore, Keith L. and T.V.N. Persaud. The Developing Human: Clinically Oriented Embryology, 7th ed. (2003). ISBN 0-7216-9412-8