Cellulose microfibril aggregates and their size variation with cell wall type

The organization of wood cell wall components involves aggregates of cellulose microfibrils and matrix known as macrofibrils. A combination of field emission electron microscopy and environmental scanning electron microscopy was used to visualise the organization of macrofibrils in different cell wall types comparing normal and reaction wood of radiata pine and poplar as examples of a typical softwood and hardwood. The size of macrofibrils is shown to vary among cell wall types with the smallest structures occurring in the gelatinous layer of tension wood (14 nm) and the largest structures in the S2_L layer of compression wood (23 nm). A positive correlation between macrofibril size and degree of lignification is observed, with macrofibrils apparently increasing in size in more highly lignified cell wall types. The fibrillar structure of the secondary wall varies from microfibril-sized structures of 3–4 nm up to large aggregates of 60 nm diameter. The size of macrofibrils also varies slightly among adjacent cells of the same cell wall type. Macrofibrils occur predominantly in a random arrangement, although radial and tangential lamellae may sometimes be seen in individual cells.