A set of simple cell processes are sufficient to model spiral cleavage

Abstract

During cleavage the zygote becomes partitioned into a set of cells with a particular spatial arrangement. Spiral cleavage is the most abundant cleavage type at the phylum level. Different cellular processes have been hypothesized to be responsible for the development of the specific spatial arrangement of blastomeres in the spiral blastula. These include the orientation of cell division according to an animal-vegetal gradient, according to cells' main axis (Hertwig's rule), according to the contact areas between cells or orthogonally to previous divisions (Sach's rule). Cell adhesion and cortical rotation have also been proposed to be involved in spiral cleavage. We use a computational model of cell and tissue bio-mechanics to implement the different existing hypotheses about how the specific spatial arrangement of cells in spiral cleavage arises during development. We found that 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 while other combinations of processes can not. Specifically, cortical rotation is necessary in the 8-cell stage to displace all micromeres into the same direction, being this displacement random in direction if only cell adhesion is included. By varying the relative strength of these processes we reproduce the spatial arrangement of cells in the blastulae of seven different species (four snails, two polychaetes and a nemertean).