Ultrastructure of the wood cell wall

Abstract: The ultrastructure of wood fibres has been studied incambial cells, differentiating fibres and mature xylem fibres,using the Rapid-Freeze Deep-Etching (RFDE) technique combinedwith transmission electron- and epifluorescence microscopy.Pectin distribution in mature wood fibres, and in fibres duringdevelopment, have been studied by immunolabelling. Furthermore,lignin polymerisation has been studied by13C-CP/MAS NMR. With these modern techniques newfindings regarding the ultrastructure of wood fibres have beenfound. Lignin is one of the major components in mature wood andis an obstacle in ultrastructural studies of the other woodpolymers (cellulose, hemicellulose and pectin). Therefore,fibres have been studied before, during and afterlignification, and after delignification, in order to obtaininformation and draw conclusions on the structure of fullylignified fibres. The lignification process has also beenstudied. The RFDE-technique revealed an extensive fibrillarnetwork in both the primary and secondary cell wall. Thefibrillar structure was visible both in unlignified andlignified cell walls indicating that lignin is deposited ontothe cellulose/hemicellulose fibrils in the cell wall, and notas a continuous matrix. The network in the primary wall showedextensive "crosslinking". The crosslinks in the primary wallsuggestively consists of pectin and hemicelluloses. Crosslinkswere also found in secondary walls, and they are probably ofhemicellulose origin. An extensive network structure was alsopresent in the unlignifiedmiddle lamella. This network wasclearly different from the cell wall polymer network andprobably consists mainly of pectin. The fully lignified middlelamella is compact with no visible pores. After chloritedelignification an underlying network was revealed in themiddle lamella. This network was thin, irregular and differentfrom any structures found in the unlignified middle lamella orcell wall, and it is suggested to be composed ofhemicellulose/cellulose.Pectin is a polymer whose chemistry and distribution isdifficult to study in native wood. However, byimmunolocalisation using monoclonal antibodies (JIM5 and JIM7),pectin, acidic and methyl esterifed polygalacturonan,distribution has been revealed. Pectin was found in thecompound middle lamella, ray cell wall and pit membrane inpine, spruce, birch and aspen. The distribution pattern indifferent wood species seemed to vary. Birch showed only smallamounts of acidic pectin, and in aspen both methylated andacidic pectin were concentrated to the middle lamella cellcorners. Spruce showed a very similar distribution pattern topine.A method to produce specifically13C-labelled polylignols in natural wood tissue(cell wall-DHP) has been developed, with the intention to usethe cell wall-DHP complex in structural studies of thelignification process, as well as in studies of ligninreactions in technical applications. The13C-enriched cell wall-DHP complex has shown to be agood system to mimic the natural lignification process, and bymaking difference spectra of unenriched and13C enriched samples, the bond frequencies involvingthe specifically13C-labelled atom can be determined quantitativelyby13C- NMR.Keywords: cambium, differentiating xylem, xylem,rapid-freeze deep-etching technique, immunolocalisation, JIM5,JIM7,13C-CP/MAS NMR, DHP, ultrastructure, microfibrils,crosslinks, middle lamella, cell wall,Pinus sylvestris,Picea abies,Betula pubescens,Populus tremula,Pinus thunbergii, lignin, pectin, hemicellulose,cellulose, lignification