Fig. 9

ACD transcripts in the spiralian egg and embryo. a-d Schematic illustrations of early embryonic stages, the mature oocyte (a), the zygote after ooplasmic segregation (b), prophase of the first mitotic cell division (c), and 2-cell stage (d). Compare to Figs. 1 and 2 for details. a The oocyte is loaded with centrally located mRNA transcripts (red) encoding ACD components. b Upon fertilization ACD transcripts within the clear cytoplasm relocate towards the animal pole in the wake of the cytoplasmic rearrangement within the zygote. c, d Hypothetical mechanism for ACDs in spiralians. Maternal ACD components form cortical domains, set up polarity cues, and generate asymmetries in orientation and architecture of the first mitotic spindle prior to the first cell division. Maternally provided ACD components provide the cellular machinery to perform the ACDs during early development generating the precise cell size asymmetries and spiral arrangements of embryonic cells in spiral cleaving embryos