A Freeze-Fracture Study of the Cortex of Xenopus laevis Eggs

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.