Freeze-fracture electron microscopy of preexisting and nascent cell membrane in cleaving eggs of Xenopus laevis.

During cell division in the Xenopus egg (diameter 1.25 mm) new cell membrane is formed in the furrow region (rate of growth approx 4-10(4) mum2/min). Freeze-fracture electron microscopy has produced the following data. Preexisting plasma membrane faces show a reversed polarity with respect to particle distribution, i.e. more particles are attached to the E-face (density 1600-2200 particles/mum2) than to the P-face (300 particles/mum2). A frequency histogram of 2331 measured intramembranous particles does not show a continuous range of sizes. The following sizes were very obvious: 95 A (12%), 125 A (30%) and 180 A (6%). At the tips of surface protrusions both the E- and the P- face are particle-free. Nascent cell membrane fracture faces are more difficult to obtain. The particle density is low (E-face 300-500 particles/mum2). Lowering the ambient temperature to 5 degrees C for approx. 5 mins does not change the normal particle pattern, but it improves the output in nascent membrane fracture faces. The fact that in the Xenopus egg preexisting and nascent membrane regions are continuous but nevertheless maintain their highly different particle densities is noteworthy. The freeze-fracture data are discussed in relation to, among other things, the known values of the specific resistances of these membrane regions.