Ultrastructural characteristics of Betula pendula tapetum and pollen grains after rapid freezing and freeze-substitution

Using rapid fixation and freeze substitution method, the ultrastructure of the cytoplasm, including the membrane system, of Betula pendula pollen is w ell preserved. The cytoplasm of Betula pollen appears compact and organelle s such as Golgi and RER are not yet demonstrating any secretory activities. The cytoplasm streaming is not established because the pollen grains are n ot completely hydrated and the microfilaments are still polymerized in bund less. The cytoplasm of the generative cell is well preserved including the cell wall like structure and cytoskeletal complexes present in the cell. Mo reover the application of rapid freeze fixation of microspores and pollen g rains of Betula showed clear details of ultrastructure of the Zwischenkorpe r (Z) at the aperture sites and the Z-layer (ZL) between the apertures. The y are mainly composed of dense helical units that extend from the plasmalem ma. Rapid freezing improves to a great extent the fixation of the delicate cell structure of the tapetum. After freeze fixation and substitution it is possible to observe radial helical units similar to those of the Zwischenk orper, throughout the thick coat of tapetal cells.