Review - The orientation of cellulose microfibrils in the cell walls of tracheids in conifers

We examined the orientation of cellulose microfibrils (Mfs) in the cell walls of tracheids in some conifer species by field emission-scanning electron microscopy (FE-SEM) and developed a model on the basis of our observations. Mfs depositing on the primary walls in differentiating tracheids were not well-ordered. The predominant orientation of the Mfs changed from longitudinal to transverse, as the differentiation of tracheids proceeded. The first Mfs to be deposited in the outer layer of the secondary wall (S I layer) were arranged as an S-helix. Then the orientation of Mfs changed gradually, with rotation in the clockwise direction as viewed from the lumen side of tracheids, from the outermost to the innermost S, layer. Mfs in the middle layer of the secondary wall (S(2) layer) were oriented in a steep Z-helix with a deviation of less than 15 degrees within the layer. The orientation of Mfs in the inner layer of the secondary wall (S(3) layer) changed, with rotation in a counterclockwise direction as viewed from the lumen side, from the outermost to the innermost S(3) layer. The angle of orientation of Mfs that were deposited on the innermost S(3) layer varied among tracheids from 40 degrees in a Z-helix to 20 degrees in an S-helix.