Formation of the female pronucleus and reorganization and disassembly of the first interphase cytoskeleton in the egg of the glossiphoniid leech Theromyzon rude.

Eggs of the leech Theromyzon rude were studied under dissecting, fluorescence, light, and electron microscopes. The sperm-derived centrosome divides and separates at early first interphase. The female pronucleus forms beside the male pronucleus at the centrally located perinuclear plasm domain, by fusion and remodelling of karyomeres that have descended from the animal pole at the tip of an ooplasmic process (centripetal ooplasmic flow). Retraction of this process (centrifugal ooplasmic flow) appears to return ooplasm to the animal pole. The reversible flow of ooplasm occurs along a subset of monaster microtubules that exhibit delayed depolymerization, and organelles thus move in both directions. Depolymerization of the first interphase monaster fibers seems to occur as a regular wave, moving from the internally located perinuclear plasm to the peripherally situated egg poles. However, disassembly of the ectoplasmic monaster fibers is preceded by furrowing and reorganization of both microtubules and microfilaments along polar rings and meridional bands. Bipolar contraction of microfilaments and shortening of microtubules lead to organelle concentration at the egg poles.