Mosaic cleavage
In embryology, determinate cleavage, also known as mosaic cleavage, is a pattern of cell division in the early embryo where the developmental fate of each cell (blastomere) is established very early on. From a very early stage, each blastomere is destined to contribute to a specific part of the overall organism, and it cannot change or substitute for another cell.
The term "mosaic cleavage" is used because the embryo is constructed like a tile mosaic: each cell is a distinct piece with a fixed position and role in the final picture. If one of these early cells is removed or destroyed, the resulting organism will be missing the specific organ or tissue that cell was programmed to form, as the other cells cannot regulate their development to compensate for the loss.
This developmental pattern is a hallmark of the animal lineage known as Protostomia, which includes phyla such as annelids (earthworms), mollusks (snails, clams), and arthropods (insects, crustaceans).
Characteristics
The core principle of determinate cleavage is the early loss of totipotency. While the very first one or two blastomeres may be totipotent (capable of forming a complete organism), this potential is quickly lost. The fate of the cells is sealed due to the asymmetric distribution of cytoplasmic determinants—specialized molecules like proteins and messenger RNAs—within the egg's cytoplasm. As the zygote divides, these molecules are partitioned into specific blastomeres, locking them into their developmental pathways.
Comparison with Indeterminate Cleavage
Determinate cleavage is best understood in contrast to its opposite, indeterminate cleavage (or regulative cleavage), which is characteristic of deuterostomes (including vertebrates like humans).
| Feature | Determinate (Mosaic) Cleavage | Indeterminate (Regulative) Cleavage |
| Cell Fate | Determined very early. | Remains flexible for a longer period. |
| Potency | Blastomeres quickly lose totipotency. | Blastomeres remain totipotent for several divisions. |
| Result of Cell Loss | A specific part of the embryo fails to develop. | Other cells compensate; a complete organism can still form. |
| Twinning | Identical twins are not naturally possible. | Allows for the formation of identical twins. |
| Associated Animals | Protostomes (e.g., mollusks, annelids). | Deuterostomes (e.g., echinoderms, vertebrates). |
Association with Spiral Cleavage
Determinate (mosaic) development is often, but not exclusively, associated with spiral cleavage. This is a specific pattern of cell division where the mitotic spindles are oriented at oblique angles to the embryo's main axis, resulting in a new layer of cells that is rotated and sits in the furrows between the cells of the layer below, creating a spiral arrangement. This precise, geometric division pattern is thought to aid in the exact partitioning of cytoplasmic determinants to the correct cells.