Spiral cleavage is a pattern of early embryonic development that establishes the body plan in many animal groups. Following fertilization, the single-celled zygote begins a period of rapid cell division known as cleavage. During cleavage, the zygote is partitioned into numerous smaller cells called blastomeres. This process transforms the zygote into a multicellular embryo, preparing the organism for the complex cellular movements of gastrulation. This particular arrangement of cells is dictated by the angle of the mitotic spindle during division, which ultimately determines the precise placement and future role of every cell in the developing organism.
The Unique Mechanics of Cell Division
The defining feature of spiral cleavage is the oblique angle at which the cleavage planes cut through the embryo. This results in the upper layer of cells being offset and nestled into the grooves between the cells of the lower layer, creating a staggered, non-radial arrangement. This shift is responsible for the characteristic spiral appearance when the embryo is viewed from the animal pole. The newly formed cells are displaced to the right or left of their parent cells.
The direction of this shift alternates in subsequent divisions, switching between clockwise and counterclockwise rotation at each successive cell generation. During this process, the larger, lower cells in the embryo are known as macromeres. The smaller cells that “bud off” toward the animal pole are called micromeres. Each quartet of micromeres is displaced relative to the four underlying macromeres, generating the nested, spiraling tiers of cells.
This arrangement can occur in two distinct forms, depending on the initial direction of the oblique cleavage: dextral (right-handed, or clockwise) or sinistral (left-handed, or counterclockwise) coiling. The difference between dextral and sinistral coiling is often determined by the mother’s genotype, a concept known as maternal effect. This maternal effect dictates the initial orientation of the mitotic spindle in the zygote.
Protostomes: Animals That Use Spiral Cleavage
Spiral cleavage is a signature trait of a major evolutionary grouping within the animal kingdom known as Spiralia. Spiralia is a large clade within the Protostomes. The presence of spiral cleavage has long been used by embryologists as evidence that seemingly disparate animal groups share a common evolutionary history.
The Spiralia clade, often referred to as Lophotrochozoa, includes numerous phyla that display this specific cleavage pattern:
- Annelids, which include segmented worms like earthworms.
- Molluscs, which encompass snails, clams, and chitons.
- Nemerteans (ribbon worms).
- Certain groups of Platyhelminthes (flatworms).
While the adult forms of a mollusk and an annelid are vastly different, their shared early developmental process highlights a deep, conserved evolutionary link. Other phyla within Spiralia, such as the Bryozoans, have secondarily lost the spiral pattern, adopting other cleavage modes. However, the ancestral presence of spiral cleavage remains a defining feature of the larger group.
The Outcome: Determinate Cleavage and Cell Fate
Spiral cleavage dictates a major functional consequence for the developing embryo: determinate cleavage. In this developmental mode, the future developmental fate of each individual blastomere is fixed very early in the process, often by the four-cell or eight-cell stage. The precise, stereotypic placement of cells ensures that specific cytoplasmic components, which act as developmental signals, are partitioned into particular cells during division.
Because cell fate is determined early, these embryos are often described as exhibiting “mosaic development.” If one of the early blastomeres is experimentally removed or destroyed, the resulting larva will lack the specific structures that the missing cell was destined to form, resulting in a defective embryo. This is in contrast to indeterminate cleavage, found in many other animals, where the cells remain pluripotent for a longer period. In those cases, if a cell is removed, the remaining cells can “regulate” their development and compensate for the loss, still forming a complete organism.
Determinate cleavage relies on the strict segregation of morphogenetic determinants, or fate-determining molecules, into specific blastomeres during the oblique cell divisions. A classic example of this determinism is the 4d blastomere, which is a specific micromere that forms in the fourth quartet of cells. This cell, known as the mesentoblast, is solely responsible for generating the entire mesoderm, which forms the muscle and other internal structures in the adult animal.