Ultrastructural changes in the compound middle lamella of Pinus thunbergiiduring lignification and lignin removal

The structure of the middle lamella in Pinus thunbergii has been studied by the rapid-freeze deep-etching (RFDE) technique in combination with transmi ssion electron microscopy (TEM). The ultrastructure of the compound middle lamella was studied in the early phases of the development of woody tissue in the cambial and differentiating xylem, before the heavy incrustation wit h lignin had occurred. Lignified middle lamella in the xylem was studied bo th directly and after delignification. It was found that the structure of t he unlignified middle lamella in the cambium/developing xylem consists of a fine irregular network probably containing pectin and hemicellulose. As a result of lignin incrustation, the middle lamella becomes increasingly dens e and the surface structure of the fully lignified middle lamella appeared to be compact and partly covered with globular structures. After delignific ation of the lignified middle lamella a thin network with a different struc ture was revealed. This network probably mainly consists of hemicellulose. No microfibrils of the type that occurs in the primary and secondary walls were Found in the middle lamella.