Ultrastructural studies on the oil bodies of Marchantia paleacea Bert. I. Early stages of oil-body cell differentiation: origination of the oil body

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Abstract
The differentiation of the idioblastic oil-body cells (OBC's) of Marchantia paleacea begins with the formation of protoplasmic and organelle complement in some thallus cells, which are meristematic in appearance. An increase of cytoplasm quantity and density, a proliferation of rough endoplasmic reticulum (ER) membranes, free ribosomes, dictyosomes, plastids, and mitochondria, as well as the appearance of cytoplasmic microtubules and the establishment of ER–plastid relationships were observed especially. These associations, together with the increased cytoplasm quantity, constitute accurate criteria for the identification of very young OBC's.The above protoplasmic changes are accompanied by the cell polarization; the nucleus is displaced at the one end of the cell and the vacuoles at the other end or peripherally. At these stages, the dictyosomes appear active and produce numerous smooth and coated vesicles.Gradually, at a more or less central area free of vacuoles, a number of ER membranes, dictyosomes, and dictyosome vesicles are preferentially localized. Microtubules fan out from this area toward the cell walls. The oil body (OB) appears in the form of a rudimentary 'vacuole,' at the centre of this area. Its bounding membrane is identical with the one of dictyosome vesicles and quite different from the ones of ER and typical vacuoles. Microtubules are associated with its contour, while rough ER membranes may surround it partly. The nascent OB grows further by the fusion of dictyosome vesicles, a phenomenon demonstrated for the coated vesicles and suggested for the smooth ones. The microtubules form a dense framework around the growing OB, while some of them are detected bridged with its limiting membrane. Actually, these microtubules appear to radiate out from the OB and persist until the first stages of lipophilic material elaboration.From the presented observations it is suggested that the OB's originate by the fusion of dictyosome vesicles through a mechanism in which microtubules play a key role. The ER membranes also seem to participate in the development of the newborn OB.