Formation of the male pronucleus, organization of the first interphase monaster, and establishment of a perinuclear plasm domain in the egg of the glossiphoniid leech Theromyzon rude.

Whole-mounted or sectioned eggs of the glossiphoniid leech Theromyzon rude were studied under the dissecting, fluorescence, light, and electron microscope. The egg is often penetrated by a single sperm that enters the animal hemisphere and becomes subjected to migration block. The latter is released shortly before or after discharge of the first pole cell, when the sperm centrosome initiates aster formation, the nucleus begins to be untwisted, and its chromatin decondensed. Sperm centration occurs along one side of the egg and appears to follow an arc-like trajectory as a result of vegetal and inward movements affected by colchicine and cytochalasin B but not by taxol. Results indicate that growing microtubules are needed for both movements, whereas actin filaments are essential for the vegetalward movement only. The sperm centrosome becomes the main microtubule organizing center (MTOC) of the egg and concomitantly originates the elaborate first interphase monaster. Additional peripherally situated MTOC form cytaster-like bodies whose visualization is improved by taxol treatment. A voluminous centrosphere, formed around the sperm centrosome, becomes a center of organelle accumulation, giving rise to a perinuclear plasm domain. This process seems to involve both import and replication of organelles.