Unequal Cleavage of the Posterior Blastomere and Formation of the Germ Cells
Ascidian embryos show unequal cleavage as early as the 8-cell stage (Fig.
Unequal cleavages take place at the posterior blastomeres during the early cleavage stages of ascidians, and the structure named the centrosome-attracting body restricts the position of the nuclei near the posterior pole to achieve the unequal cleavages.
For example, the difference in the cell volume between sister cells suggests the occurrence of the unequal cleavages (Tassy et al.
At the four cell stage, unequal cleavage
is seen in CD resulting in A, B, and C being the equivalent sizes while D is the largest of the four.
In non-opisthobranch mollusks with unequal cleavage, the D quadrant is always larger.
Once we have arranged the main gastropod taxa in a sequence of patterns according to equal versus unequal cleavage, heterochronic differences in their early cleavage patterns and in mesentoblast formation as shown in Figures 18, 19, these arrangements have to be compared with phyletic patterns of the main gastropod taxa derived from other criteria.
For example, van den Biggelaar (1996) found a trend in cleavage type and D-quadrant specification from equal cleavage and late specification in "archaeogastropods" to unequal cleavage and early specification in opisthobranchs and pulmonates.
Among gastropods there is a trend in both caenogastropods and opisthobranchs toward unequal cleavage and early D-quadrant specification (Freeman and Lundelius, 1992; van den Biggelaar, 1996; van den Biggelaar and Haszprunar, 1996).
Unequal cleavage is characteristic of pyramidellids and some basal opisthobranchs, whereas caenogastropods have a polar lobe; thus unequal cleavage may be a heterobranch synapomorphy.
A vegetal-specific repressive chromatin structure might be established in micromeres which, as a result of unequal cleavage
at 16-cell stage, would inherit a lower quantity of any uniformly distributed positively acting factors that were stockpiled in the egg cytoplasm.
We plan to use this technique, in combination with 4-D polarized light microscopy, to observe the behavior of spindles in compressed blastomeres in order to study the precise behavior of the spindle preceding a critical unequal cleavage
and to explore the localized distribution of the vegetal pole determinant.