Cellulose Microfibril Bundles Research Articles

Microtubules and epithem-cell morphogenesis in hydathodes of Pilea cadierei

When cell divisions have ceased, the epithem of the hydathodes of Pilea cadierei Gagnep. et Guill. consists of small polyhedral cells exhibiting a meristematic appearance, and completely lacks intercellular spaces. The cortical microtubules in epithem cells exhibit a unique organization: they are not scattered along the whole wall surface but form groups lying at some distance from each other. In sections, from two to eight groups of microtubules can be observed, each lining a wall region averaging between 0.5 and 1.5 μm in length. These groups represent sections of microtubule bundles girdling a major part or the whole of the cell periphery. They are connected to one another by anastomoses, forming a microtubular reticulum. The assembly of microtubule bundles is followed by the appearance of distinct local thickenings in the adjacent wall areas. The cellulose microfibrils in the thickenings are deposited in parallel to the underlying microtubules. Gradually, the vacuolating epithem cells undergo swelling, except for the areas bounded by the wall thickenings. Since the latter, and actually their constituent bundles of cellulose microfibrils, cannot extend in length the differential cell growth results in schizogenous formation of intercellular spaces between contiguous cell walls at their thickened regions. The spaces then broaden and merge to become an extensive intercellular space system. As a result of the above processes, the epithem cells become constricted and finally deeply lobed. The observations show that (i) the cortical microtubules are intimately involved in the morphogenesis of the epithem cells and (ii) the initiation and development of the epithem intercellular spaces is a phenomenon directly related to cell morphogenesis and therefore to the cortical microtubule cytoskeleton. The sites of initiation of these spaces are highly predictable.

The structure and formation of the test ofPyura stolonifera (Tunicata)

Previous X-ray diffraction studies have established the crystallographic identity of tunicin with cellulose from plant cell walls. Enzymatic degradation of the tunicin results in the isolation of microfibrils 120–130 A in width (which appear to consist of two sub-units). Using staining and freeze-etching techniques it has been demonstrated that the microfibrils occur in the test as aggregates measuring 2000–4000 A in diameter. In this respect the physical texture of the cellulose ofPyura resembles that of collagen in animal connective tissue and does not resemble the texture of cellulose in either the primary or secondary cell walls of plants. Examination of fixed and frozen etched specimens showed that ferrocytes of the test and vesicles derived from them are closely associated with the cellulose microfibril bundles. However, at the optical level, autoradiographs of animals treated with C14 glucose showed greatest radioactivity to be in the epidermal cells of the mantle and of the blood vessels, but not in the ferrocytes. These cells also showed a considerable development of rough ER and of Golgi bodies. On the evidence obtained it is considered that the sites of cellulose synthesis are the epidermal cells of the mantle and of the blood vessels. The function of the test cells is unknown. The migration of ferrocytes to areas of wounding in the test suggests that they may have some lytic function associated with wound repair.