Cytoplasmic and cortical determinants interact to specify ectoderm and mesoderm in the leech embryo

In leech embryos, segmental ectoderm and mesoderm are produced by a pair of sister cells located near the animal and vegetal poles, respectively. We have investigated the mechanism that localizes ectodermal and mesodermal fates along the animal-vegetal axis. The results of cleavage arrest and cell ablation experiments suggest that the full range of normal cell interactions are not required for this process. However, when the animal and vegetal hemispheres are separated by re-orientation of the first cleavage plane from meridional to equatorial, the ectodermal fate co-segregates with the animal hemisphere and the mesodermal fate with the vegetal hemisphere. Two pools of yolk-deficient cytoplasm, called teloplasm, are located at the animal and vegetal poles of the zygote, but separation of the animal and vegetal teloplasms is not sufficient for the segregation of ectodermal and mesodermal fates. Rather, complete segregation of fates requires an equatorial cleavage orientation that separates not only the two teloplasms, but also the animal and vegetal cortical regions. This, in conjunction with previous findings, indicates that ectodermal determinants are localized to the cell cortex in the animal hemisphere of the zygote. We propose that these determinants segregate to the ectodermal precursor and interact with factors in teloplasm to transform the fate of this cell from a mesodermal ground state to ectoderm.