Changes in cell wall architecture of differentiating tracheids of Pinus thunbergii during lignification

The cell wall architecture, before and after lignification, of differentiating tracheids in Pinus thunbergii has been examined using a rapid-freeze deep-etching technique combined with transmission electron microscopy. Replicas of cells from the cambial zone showed that the unlignified primary cell wall was highly porous with microfibrils extensively interconnected by crosslinks. The unlignified secondary cell wall has unidirectional microfibrils, more or less associated in bundles, forming a wavy pattern around pores of characteristic slit-like shape with narrowing ends. As the lignification progresses, the cell wall structure becomes dense, with no detectable pores. Delignification of wood samples leads to the reappearance of crosslinks, individual microfibrils and pores in the secondary cell wall, although in a somewhat altered shape. In addition, cellulose-synthesizing enzyme complexes (rosettes) have for the first time been detected on the plasma membrane of differentiating xylem cells of softwood.