Cell corner

In some species, onion (2), tomato, and sugar beet (13), the interface regions between cells, ie the middle lamella and the cell corners, are rich in relatively unesterified pectins which may function in cell-cell adhesion and play an important structural role in tissue integrity. Cell corners, in particular, may act as joists in the scaffolding function of the wall, bearing much of the mechanical load of the tissue (Jeronomidis, pers. comm.). In Zinnia leaves, although all of the cell-walls contain methyl-esterified pectin. 

Advanced stages of penetration by the fungus were characterized by displacement of the pectin-rich cell corner regions with concurrent stretching of the cellulose-rich primary cell walls (Fig. 2). 

A clear understanding of lignin deposition in the cell wall is not yet possible, but a number of facts are known. Lignin precursors of the phenylglucoside type are formed either in the region of the cambium (the zone of new cell synthesis) or within the lignifying cell itself. Lignification is thus initiated in the differentiated wood cells from the primary walls adjacent to the cell corners and then extends into the inter-cellular area, the lamella, and thereafter to the primary and secondary cell walls. 

Figure 5. A schematic representation of the process of deposition of cell wall components and the heterogeneous formation of protolignin macromolecule. ML, middle lamella CC, cell corner P, primary wall CML, compound middle lamella S1 S2, and S3, outer, middle, and inner layer of secondary wall H, G, and S,p-hydroxy-, guaiacyl-, and syringylpropane units.

Figure 3. Beech heartwood, decayed for 4 weeks with C. versicolor, showing thinning of the secondary wall with enlargement of the cavity at the cell corner (the protruding ends of the middle lamella can be seen). A single hypha close to the wood cell wall shows where the secondary wall has been degraded. Label for laccase is seen uniformly distributed in the secondary wall. Magnification x 12,600.

Figure 9. Beech heartwood decayed for 12 weeks with C. versicolor labelled for lignin-peroxidase. The label is distributed uniformly over the secondary wall which shows characteristic thinning and marked reduction in electron density. No label occurs in the middle lamella and cell corners. Magnification x 22,000.

The crystal structure of the 1-2-3 superconductor, YBazCusOy- is depicted in Figure 10.8. Figure 10.8(a) depicts only the positions of the metal atoms. If we discuss it in terms of the perovskite structure ABO3, where B=Cu, the central section is now an A-type perovskite unit cell and above and below it are also A-type perovskite unit cells with their bottom and top layers missing. This gives copper atoms at the unit cell corners and on the unit cell edges at fractional coordinates A and Ys. The atom at the body-centre of the cell (i.e., in the centre of the middle section) is yttrium. The atoms in the centres of the top and bottom cubes are barium... 

Fig. 11.10. The lowest isovalent contour in a-AgI that permits diffusion through the crystal. I ions (not shown) occur at the cell corners and cell centre. Reproduced with permission from Adams and Swenson (20006).

The compound two-atom unit cell of a-iron is termed body-centred . The arrangement is similar to that in ammonium chloride (Fig. 116), with the important difference that in a-iron the atoms in the centres of the cells are the same as those at the corners, whereas in ammonium chloride (which is not called body-centred ) there are weakly diffracting ammonium ions at the cell centres and more strongly diffracting chlorine ions at the cell corners. On account of this difference, all those reflections which are weak in the ammonium chloride pattern (owing to opposition of the waves from corner and centre atoms) are necessarily completely absent from the a-iron pattern (see Plate XII). Thus, for the 010 reflection (Fig. 124), waves from alhcorner atoms are in phase with each other, while waves from centre atoms are exactly opposite... 

Figure 8. Xylanase-treated late-wood tracheid wall with dissolution of cell corner (C) and contrast-poor loosened texture in St. Scale = I fim.

Xylanase + Mannanase. The combined action of xylanase and mannanase leads to a degradation of wall material, which resembles the mode of attack met with in the individual enzyme treatments only the degree of dissolution is rather more intense in the combined treatment. Of special interest is the decrustation of the cell corners, which appear to be attacked on a large scale (Figure 11). The Sx and tertiary wall zones also undergo a decomposition process (Figure 11). 

Figure 11. Xylanase + mannanase treatment. Decrustation of S. layer and cell corner (C) in an early wood tracheid. Scale = 1 fim.

The current observations confirm previous studies on beechwood and sprucewood holocellulose (7,10,19). The attack of the hemicellulose proceeds from the primary wall/Si as well as from the tertiary wall into S2 the pit chambers constitute preferred paths of enzyme diffusion into the walls. Also, substances of the middle lamella, especially in the cell corners, are removed by the xylanase and the mannanase treatments. Parallel to the removal of hemicelluloses, the fibrillar structure of the cellulose and its lamellar arrangement in transections of cell walls became obvious. In samples treated with cellulases, the cellulose fibrils were often completely hydrolyzed in the Si layer, occasionally accompanied by complete dissolution of cell-wall portions. This is also in conformity with the previous conclusion that the cellulases hydrolyze highly ordered zones of cellulose and remove hemicelluloses by hydrolysis or by detachment. 

The middle lamella is located between the cells and serves the function of binding the cells together. At an early stage of the growth it is mainly composed of pectic substances, but it eventually becomes highly lignified. Its thickness, except at the cell corners, is 0.2-1.0 /tm. The primary wall is a thin layer, 0.1 -0.2 jam thick, consisting of cellulose, hemicelluloses, pectin,... 

Fig. 4-11. Transverse section of a spruce tracheid photographed in UV light (240 nm) (Fergus et a/., 1969). The densitometer tracing has been taken across the tracheid wall along the dotted line. S, secondary wall ML, compound middle lamella CC cell corner.

Fig. 7-3. UV absorbance (222 nm, 0.5 p,m section thickness) by various morphological regions of spruce fibers delignified to various lignin contents by the kraft and acid sulfite method (Wood and Goring, 1973). S, secondary wall P, primary wall CCP, primary wall at the cell corner.

Fig. 4.2.4. a-d Changes in the UV absorption spectra ol various cell wall layers of hybrid poplar during differentiation, showing the initial (/), middle (2), and later (5) stages of lignification. V-SIV Secondary wall of vessel F-SW secondary wall of fiber FF-CC cell corner of middle lamella between fibers VF-CC cell corner of middle lamella between vessel and fiber, e UV absorption spectra of mature fiber secondary wall (F-SIV) and mature cell corner of middle lamella between fibers (FF-CC) obtained from the same poplar fiber. The UV spectrum shown by (FF-CC)-(F-SW) is the difference spectrum between the cell corner middle lamella and fiber secondary wall. (Takabe et al. 1987)... 

Fig. 4.3.2A-C. A series of interference micrographs showing the maximum extinction positions of (A) the background, (B) the cell corner middle lamella of Pinus radiala carlywood, and (C) the S2 region of the secondary wall (interference photomicrograph)...

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