Figure 2. Electron micrograph of the plasma membrane of control and perfused oocytes.

A, cortical region of one representative oocyte expressing ENaCs, which was voltage clamped at -100 mV for 20 min using the two-electrode voltage-clamp technique. The microvilli of the plasma membrane, just below the vitelline membrane (VM) surrounding the oocyte, can be easily recognized. Yolk platelets (Y) and cortical (C) and pigment (P) granules can be seen below the membrane. Dense ferritine patches are attached to the vitelline membrane. B, electron micrograph of the membrane of an oocyte which had been perfused using the cut-open oocyte set-up. The general architecture of the microvilli was not modified. However, the density of the cytosolic granulations below the membrane and within the microvilli was clearly decreased. Despite the perfusion, a layer of cytosolic structures (yolk platelets, cortical and pigment granules) remained attached to the membrane. This micrograph illustrates the three postulated zones within which the intracellular and extracellular perfusions do not cause convectional flux and where the Na⁺ concentration is influenced only by diffusion: (i) the space between the vitelline and plasma membranes, (ii) the compartment within the microvilli, and (iii) the layer of remaining cytosolic structures. Scale bars, 10 μm.