cleavage
Summary
Cleavage is the series of rapid mitotic divisions of the zygote after fertilization, with little or no growth between divisions. Cell number rises while individual cells shrink, so the embryo's overall size stays roughly constant as it becomes a morula and then a blastula.
Detail
Meaning: Cleavage is the set of rapid mitotic divisions the zygote undergoes after fertilization. The daughter cells, called blastomeres, divide again without growing in between, so cells become smaller and more numerous while the embryo's total size stays roughly constant. The embryo passes from zygote to morula (a solid ball of cells) to blastula (a hollow ball around a fluid-filled blastocoel). In humans the blastula stage is the blastocyst, with an inner cell mass and a trophoblast. Gastrulation, which forms the germ layers, follows cleavage.
On the MCAT: Questions ask you to order the stages (zygote → morula → blastula → gastrula) or to predict what changes during cleavage: cell number rises, cell size falls, each cell stays diploid, and the ratio of nucleus to cytoplasm increases.
Common confusion: Cleavage is not growth and not meiosis. Because the divisions are mitotic, each blastomere keeps a diploid chromosome set, and because little new cytoplasm is made, the embryo does not enlarge much. In biochemistry, cleavage means breaking a bond, such as a protease cutting a peptide bond.
Sources
- Campbell Biology
- Developmental Biology
- Molecular Biology of the Cell
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.