The four macromeres then divide usually unequally, along an obliquely equatorial plane, giving rise to four, usually smaller, cells called micromeres. Despite having a very similar cleavage, these phyla have very different adult morphologies.Īs in other types of cleavage, typical spiralian cleavage begins with two successive nearly meridional cell divisions that give rise to four large cells (termed A, B, C and D macromeres) that lie in a plane perpendicular to the animal-vegetal (A-V) axis of the egg. The ensemble of phyla with spiralian cleavage has been suggested to form a monophyletic group ( Nielsen, 1994 Laumer et al., 2015): the Spiralia. Other lophotrochozoan phyla (platyhelminthes, rotifers, brachiopods, phoronids, gastrotrichs, and bryozoans) also exhibit spiral cleavage in at least some of their species ( Hejnol, 2010). It is found in mollusks, annelids and nemerteans. Spiral cleavage is the most abundant cleavage type at the phylum level. However, there are several phyla that share a common cleavage pattern. Usually different animal taxa exhibit different cleavage patterns. There are several types of cleavage patterns in metazoa ( Gilbert and Raunio, 1997). During this cleavage process, a specific spatial cell arrangement, referred to here as the ‘cleavage pattern’, arises in each species. Most metazoans start their development via a series of fast cell divisions that partition the zygote into a set of blastomeres. By varying the relative strength of these processes, we reproduce the spatial arrangement of cells in the blastulae of seven different invertebrate species. Specifically, cortical rotation is necessary at the 8-cell stage to direct all micromeres in the same direction. Cell polarization by an animal-vegetal gradient, a bias to perpendicularity between consecutive cell divisions (Sachs' rule), cortical rotation and cell adhesion, when combined, reproduce the spiral cleavage, whereas other combinations of processes cannot. We use a computational model of cell and tissue biomechanics to account for the different existing hypotheses about how the specific spatial arrangement of cells in spiral cleavage arises during development. Cell adhesion and cortical rotation have also been proposed to be involved in spiral cleavage. These processes include the orientation of cell division according to: an animal-vegetal gradient the main axis (Hertwig's rule) of the cell and the contact areas between cells or the perpendicularity between consecutive cell divisions (Sachs' rule). During cleavage, different cellular processes cause the zygote to become partitioned into a set of cells with a specific spatial arrangement.
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