RESUMEN
The mechanism of electric field-induced fusion has been studied in detail (Zimmermann, Biochim. Biophys. Acta. 694 (1982) 227), but little is known about the process by which the two fused cells become a single entity, a process we term spherulation. We observe a clear difference between activated and unactivated Xenopus eggs in the time after electic field (EF) application when spherulation starts, and in the time required for spherulation to be completed. In unactivated eggs, spherulation started 7 min after EF application and was completed within 5 1/2 min. In activated eggs, the lag between EF application and the start of spherulation increased with the cell cycle. At the end of the first cell cycle spherulation started 78 min after EF application and was competed 30 min later. The lag period is not due to delayed fusion, for electric coupling between activated eggs can be recorded before the start of spherulation. The morphology of the contact zone between paired eggs, as observed by light and electron microscopy, is also described. We suggest that the difference in the timing of spherulation reflects a difference in the lability of the cytoskeleton through the cell cycle.
RESUMEN
The organization of the cortex of Xenopus laevis eggs was investigated by freeze-fracture electron microscopy. The cortical endoplasmic reticulum (CER) formed a network surrounding and interconnecting the cortical granules. It formed junctions with the plasma membrane and was confluent with the ER in subcortical regions. Intramembranous particles (IMP1 ) were only present in the P face of the CER, the E face being apparently devoid of pits and particles. Arrays of densely packed IMP1 , having a mean diameter of 17 nm, were restricted to the microvillar region of the plasma membrane. The cortical granule membrane also contained IMP1 (mean diameter, 21 nm) that were sparsely and randomly distributed. Several types of cortical granule seemed to exist based on an analysis of the distribution of the different IMP sizes.
RESUMEN
The cortical contraction begins 4 min after insemination and one minute after prick activation. During the next 4 min, the pigment margin moves 15 degrees toward the animal pole. The cortex then relaxes to the prefertilization level over the next 10 min. Contrary to earlier estimations, the cortical contraction occurs during the same time span as the wave of cortical granule exocytosis. We suggest that the two events may result from a common stimulus. The sperm trail (ST) forms during the relaxation of the cortex. The ST first appears as a conically-shaped trail of pigment in the cytoplasm; it then elongates into a funnel-shaped trail as the male pronucleus migrates into the egg. The base of the cytoplasmic ST can be seen on the surface of the egg as a circular condensation of pigment. The male and female pronuclei migrate at a constant rate of 12 µm per minute. The male pronucleus migrates by the enlargement of its aster, whereas, it appears that the female pronucleus is dependent on the male aster for its motion.