Asymmetric Cell Division - Asymmetric Cell Division in Spiralian Development

Asymmetric Cell Division in Spiralian Development

Spiralia (commonly synonymous with lophotrochozoa) represent a diverse clade of metazoan organisms whose species comprise the bulk of the bilaterian animals present today. Examples include mollusks, annelid worms, and the entoprocta. Although much is known at the cellular and molecular level about the other bilateralian clades (ecdysozoa and deuterostomia), research into the processes that govern spiralian development is comparatively lacking. However, one unifying feature shared among spiralia is the pattern of cleavage in the early embryo known as spiral cleavage. This pattern, which contributed to the initial phylogenetic placement of this group of organisms, depends heavily on asymmetric cell division.

The general cleavage pattern of a spiralian embryo follows a classic and predictable set of cellular divisions. The first two divisions yield four progenitor macromeres which specify four geometric quadrants of the developing organism. Following this, the individual macromeres undergo a series of divisions which generate a string of micromeres at the animal pole of the embryo. Micromere production occurs in an alternating clockwise and counterclockwise fashion and is accomplished via asymmetric division. For more information on this pattern of cellular division see the cleavage (embryo) page.

Although the conserved cleavage of spiralian macromeres to generate micromere quartets is well established, more has been discovered about the first two cleavages at the molecular level. The initial cleavage can occur with several, sometimes overlapping, outcomes (See Figure, left panel). For example, in symmetrically cleaving embryos, the zygote can cleave twice to yield four cells of equal size and equipotent fate. In contrast, asymmetrically cleaving spiralians develop an embryonic polarity beginning with the first cellular division. The zygote cleaves once to generate a smaller AB cell and a larger CD cell. The second division usually occurs asynchronously, and generates three similarly sized macromeres (A, B, and C) as well as a larger D macromere. Known as D quadrant specification, this example of asymmetric cell division sets up the D quadrant of the embryo such that it has a specific and independent fate, often for mesoderm production.

Mechanisms of asymmetric division (See Figure, right panel):

• Tubifex tubifex: The sludge worm Tubifex tubifex has been shown to demonstrate an interesting asymmetric cell division at the point of first embryonic cleavage. Unlike the classic idea of cortical differences at the zygotic membrane that determine spindle asymmetry in the C. elegans embryo, the first cleavage in tubifex relies on the number of centrosomes. Embryos inherit a single centrosome which localizes in the prospective larger CD cell cytoplasm and emits radial microtubules during anaphase that contribute to both the mitotic spindle as well as cortical asters. However, the microtubule organizing center of the prospective smaller AB cell emits only microtubules that commit to the mitotic spindle and not cortical bound asters. When embryos are compressed or deformed, asymmetric spindles still form, and staining for gamma tubulin reveals that the second microtubule organizing center lacks the molecular signature of a centrosome. Furthermore, when centrosome number is doubled, tubifex embryos cleave symmetrically, suggesting this monoastral mechanism of asymmetric cell division is centrosome dependent.

• Helobdella robusta: The leech Helobdella robusta exhibits a similar asymmetry in the first embryonic division as C. elegans and tubifex, but relies on a modified mechanism. Compression experiments on the robusta embryo do not affect asymmetric division, suggesting the mechanism, like tubifex, uses a cortical independent molecular pathway. In robusta, antibody staining reveals that the mitotic spindle forms symmetrically until metaphase and stems from two biastral centrosomes. At the onset of metaphase, asymmetry becomes apparent as the centrosome of the prospective larger CD cell lengthens cortical asters while the asters of the prospective smaller AB cell become downregulated. Experiments using nocodazole and taxol support this observation. Taxol, which stabilized microtubules, forced a significant number of embryos to cleave symmetrically when used at a moderate concentration. Moreover, embryos treated with nocodazole, which sequesters tubulin dimers and promotes microtubule depolymerization, similarly forced symmetric division in a significant number of embryos. Treatment with either drug at these concentrations fails to disrupt normal centrosome dynamics, suggesting that a balance of microtubule polymerization and depolymerization represents another mechanism for establishing asymmetric cell division in spilarian development.

• Ilyanasa obsoleta: A third, less traditional mechanism contributing to asymmetric cell division in spiralian development has been discovered in the mollusk Ilyanasa obsoleta. In situ hybridization and immunofluorescence experiments show that mRNA transcripts co-localize with centrosomes during early cleavage. Consequently, these transcripts are inherited in a stereotypical fashion to distinct cells. All mRNA transcripts followed have been implicated in body axis patterning, and in situ hybridization for transcripts associated with other functions fail to exhibit such a localization. Moreover, disruption of microtubule polymerization with nocodazole, and of actin polymerization with cytochalisin B, shows the cytoskeleton is also important in this asymmetry. It appears that microtubules are required to recruit the mRNA to the centrosome, and that actin is required to attach the centrosome to the cortex. Finally, introducing multiple centrosomes into one cell by inhibiting cytokinesis shows that mRNA dependably localizes on the correct centrosome, suggesting intrinsic differences between each centrosomal composition. It is important to note that these results reflect experiments performed after the first two divisions, yet still demonstrate a different molecular means of establishing asymmetry in a dividing cell.

These examples of mechanisms that establish asymmetric cell division in early spiralian development showcase the plasticity of evolution. A variety of methods can contribute to differences in daughter cells in order to found independent cell fates.