Ultrastructural studies on the oil bodies of Marchantia paleacea Bert. II. Advanced stages of oil-body cell differentiation: synthesis of lipophilic material

Abstract

At the onset of the active synthesis of oil-body (OB) lipophilic material, the oil-body cells (OBC's) appear to have gained a high degree of specialization towards a specific type of secretory cell. They possess a dense ribosomal cytoplasm, an active Golgi apparatus, abundant rough endoplasmic reticulum (ER) membranes, ER-ensheathed chloroplasts, a peculiar compartment encompassed by a membrane resembling the plasmalemma, small vacuoles, and OB-associated microtubules as well as some subplasmalemmal ones. At these stages intimate associations between ER membranes and OB's are established, while another class of cytoplasmic tubules, some of which are associated with microbodies, is formed in the cytoplasm.Initially, some material (matrix) starts being accumulated in the OB compartment as well as on the inner face of its limiting membrane, where it forms a distinct layer. This accumulation is followed by the appearance of minute opaque lipophilic globules. The number and the size of the globules increase considerably, their structure is gradually differentiated, and matrix fills the globule-free space in scale and epidermal OB's. Each of them is not surrounded by a true membrane but is delimited by a thin layer of dense material. Some of the epidermal and a few scale OB's of the young thalli follow a diverse differentiation process and form one or a few large globules surrounded by a very thin layer of matrix. Ultimately, these OB's are destroyed and force the OBC's to break down. The mature inner OB's generally contain small globules, the membrane-associated material, and traces of matrix. A structural diversity exists between the OB's of young and mature thalli. In the latter all the OB's exhibit small globules only.The globules are exclusively elaborated in the OB which is an active cell element; cytoplasmic or plastidic lipophilic material has never been observed entering the OB. After the completion of their development, the OB's occupy most of the cell space; the protoplasm diminishes while the dictyosome and ER activity gradually ceases.The OB's are well preserved with osmium tetroxide fixation or with the double one with glutaraldehyde, performed at 0 °C, 4 °C, or at room temperature for 20 min, followed by osmium tetroxide. On the contrary, a prolonged glutaraldehyde prefixation at room temperature causes a destruction of the OB containing large globules, a partial degradation of the ones containing smaller globules, and removes the matrix almost totally.