Ion permeability of the egg of Limnaea stagnalis L. on fixation for electron microscopy

Abstract

1. 1. Changes in the permeability, especially to ions, of the cell membrane during fixation for electron microscopy are likely to have consequences for the ultrastructure of cells to be investigated.

2. 2. Ion permeability changes of single egg cells of Limnaea stagnalis L. were studied by a microconductomeric method.

3. 3. To this end a conductometric cell was developed with electrode surfaces of 0.14 mm2 and a cell constant of 0.05. Its volume is about 26 times that of an egg cell. Conductivity changes, depending on salt concentration of the medium, were recorded as variations of a 1000 cycles/sec current through the conductivity cell.

4. 4. By means of electrical cytolysis of the egg cell in distilled water its salt content could be determined as to correspond to a solution of about 0.04 M NaCl.

5. 5. On fixation with OsO4 the cell membrane becomes completely permeable to ions in a few seconds. On fixation with glutaraldehyde, ions leave the egg cell only very slowly, as they do in distilled water.

6. 6. On the basis of these observations the optimal ionic composition of an OsO4 fixing solution is discussed, taking into account the establishment of a Donnan equilibrium between egg content and fixative medium.