Techniques to preserve soluble surface components in birch pollen wall: a scanning and transmission electron microscopic study

The exine of birch pollen was examined by scanning and transmission electron microscopy in the native state and after fixation in different aqueous fixatives: glutaraldehyde + OsO4; glutaraldehyde + cetylpyridinium chloride (CPC) + OsO4; glutaraldehyde + cuprolinic blue (CB); and periodate + lysine + paraformaldehyde (PLP). The native pollen exine showed a thin (3-5-nm) border of electron-dense material lining the tectum and electron-dense material within microchannels and bacula cavities. Fixation with the addition of CPC resulted in a voluminous surface coat surrounding the pollen grain, but empty microchannels and bacula cavities. After fixation with the addition of CB, there was a thin surface coat, whereas microchannels and bacula cavities were partially filled with electron-dense material. The other fixatives led to empty microchannels and bacula cavities. There was no surface coat on the pollen grain. However, after all fixation procedures, a thin electron-dense border of the tectum remained visible. Concerning the electron-dense material filling microchannels and bacula cavities in the native pollen grain, the results obtained in the present study suggest that it is either completely lost (after conventional and PLP fixation) or, after fixation with a precipitating additive, partially (CB) or completely (CPC) solubilized and precipitated on the surface of the pollen grain as a surface coat.