Ooplasmic segregation of the myoplasmic actin network in stratified ascidian eggs

Ooplasmic segregation in ascidians includes the movement of the myoplasm, a pigmented cytoplasmic region thought to be involved in the determination of the embryonic muscle and mesenchyme cell lineages, into the vegetal hemisphere of the egg. A myoplasmic cytoskeletal domain (MCD), composed of a cortical actin network (the PML) and an underlying filamentous lattice extending deep into the cytoplasm, is present in this region. The MCD gradually recedes into the vegetal hemisphere during ooplasmic segregation. It has been proposed that the segregation of the myoplasm is mediated by the contraction of the PML. To test this possibility we have examined ooplasmic segregation in eggs in which the internal parts of the MCD were separated from the PML by centrifugal force. Transmission and scanning electron microscopy of eggs extracted with Triton X-100 showed that the PML remained intact when the internal portions of the MCD were displaced and stratified by centrifugation. When stratified eggs were fertilized there were no rearrangements of the visible cytoplasmic inclusions, but the cellular deformations and the recession of the PML characteristic of ooplasmic segregation occurred as usual. The results indicate that the recession of the PML occurs independently of the internal constituents of the MCD and suggest that PML contraction is the motive force for ooplasmic segregation.