A simple plasmodesma comprises a single pore, lined by the plasma membrane and traversed in its centre by a structure known as the desmotubule. The desmotubule is formed by the cortical ER which remains continuous between cells via the desmotubule. The space between the plasma membrane and the desmotubule is called the cytoplasmic sleeve and is believed to be the principal route for the movement of small molecules between cells. Each plas-modesma is surrounded by an electron-lucent sleeve and at each end are neck regions, one or both of which may be constricted (Fig. 1 and Fig. 2a).
Structural models of plasmodesmata are based predominantly on data from transmission electron microscopy. On the basis of ultrastructure alone, plasmodesmata are classified into two basic types: simple and branched. The majority of models describe the structure of simple plasmodesmata, and interpretations have been influenced by the effects of the various fixation and staining procedures on plasmodesmata (Gunning and Robards 1976; Robards and Lucas 1990; Beebe and Turgeon 1991; Tilney et al. 1991; Ding et al. 1992b; Botha et al. 1993; Turner et al. 1994; White et al. 1994; Overall and Black-man 1996; Ding 1997, 1999; Waigmann et al. 1997; Radford et al. 1998; Overall 1999). Although there is consensus that all plasmodesmata have the same basic configuration, there is still much debate about their fine structure.
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