Jean-Pierre Mignot

The centrosomal big bang: From a unique central organelle towards a constellation of MTOCs

Summry— This article is the fruit of reflections on the comparative study of centrosomal structures of various protists, and on recent data on the organization and composition of the centrosome of multicellular organisms. On the basis of a few significant situations encountered in protists, a model is proposed in which the metazoan centrosome represents a complex of three types of specialized microtubule‐organizing centers (MTOCs) of different origin and function, designated MTOC1, MTOC2, MTOC3. MTOCs1, presumably the most primitive, drive the mitosis. Associated with the chromosomes, they would be primitively included in the nuclear matrix acting as polar bodies in closed mitosis which characterizes many protists. MTOCs2 correspond to the centrioles, relics of basal bodies, whose primitive function was to engender a motor organelle. They differ from other cytoskeletal organizing centers only by their ubiquity and their 9+0 organization. MTOCs3, which may form stable structures of specific shape in some protists, control cellular morphology and intracellular traffic. The relationships between the various components and the nucleus are considered. Using a speculative scheme, we attempt to understand how this ensemble has diversified over the evolution of protists.

Nuclear fine structure at interphase and during encystment in two forms of the testacean Arcella vulgaris.

Arcella vulgaris and A. vulgaris var. multinucleata have two and seven vesicular nuclei, respectively. In early interphase, the nuclei are spherical, with a main central nucleolus and several small peripheral nucleoli. The main nucleolus has mixed fibrillar and granular components and no apparent chromatin bodies in the nucleolus-organizing regions (NORs). The nuclear chromatin is dispersed except for small heterochromatin lumps. Nuclear bodies of fibrous structure occur outside the main nucleolus. The nuclear envelope shows an internal lamina and an external layer of tangential fibres. In middle interphase, the nuclei become irregular in shape and adjacent to the plasma membrane at the dorsolateral cell surface. The condensation of the chromatin increases. The nucleolus is often eccentric and its NORs show conspicuous bodies of condensed chromatin surrounded by a halo and a fibrous transcription zone. By the end of interphase, the main nucleolus becomes polymorphic and segregated into a fibrillar basal part which contains a vacuolar area with dense inclusions, and a fibro-granular cap which contains many fibrous electron dense bodies. These are likely to be the nuclear chromatin elements. In early resting cysts, the nuclei detach from the cell membrane and approach one another. The main nucleolus segregates into large, mainly peripheral fibrillar blocks inside a granular mass. The NORs consist of condensed chromatin bodies without a transcription zone around them. The nuclear chromatin is condensed. Conspicuous intranuclear annulate lamellae bearing pore complexes occur only in cysts.

Patterning in opalinids: II. Endocytofibrillar complex in Protoopalina pseudonutti Sandon, 1976

Summary At certain phases of the life cycle of Protoopalina pseudonutti silver impregnation according to the technique of Fernandez-Galiano reveals a large axial fibrillar complex which divides into numerous lateral branches linked to the falcular zone and the somatic kineties. An electron microscopy study, using a new method of fixation, confirmed the existence of a well-developed fibrillar network composed of microfibrillar bundles with irregularly spaced dense zones. It continues in the falcular matrix and branches extensively, producing lateral expansions, some of which extend along the somatic kineties. Others surround the nuclei which, during interphase and division, also carry a thin microfibrillar layer on the cytoplasmic face of the nuclear envelope. This fibrillar network extends caudally and forms the framework of the tip. In its morphology and ultrastructural organization, this network is similar to certain skeletal constituents observed in ciliates, such as the ecto-endoplasmic boundary of the rumen ciliates. It could thus also be composed of centrin-like proteins and has resemblances to the cortical network which characterizes the species Opalina ranarum. Consequently, it may play a role in cell morphogenesis. Its development, which in some species is more pronounced, and its location varies according to the morphology of the cell and the number of nuclei or both. In species with few nuclei cytoskeletal differentiation is required to control the positioning of the nuclei to ensure the transmission of genetic information.

Fine-structural study of mitosis in the testacean Arcella vulgaris Ehrbg

The two nuclei of Arcella vulgaris divide synchronously by closed intranuclear orthomitosis during the formation and deployment of the thecagenous cytoplasmic bud. In prophase, the two nuclei are stacked one behind the other in front of the pseudostome, and markedly flattened, which brings the chromosomes near the equatorial plane. The nucleolus disperses, the nuclear lamina gradually disappears, and non-oriented microtubule bundles grow from intranuclear MTOCs. The condensing chromosomes already show attachment sites for one or more microtubules. In prometaphase, the nuclei are still stacked; the chromosomes further condense, the microtubules are oriented but short, leaving large polar regions of the nucleus filled with diffuse nucleolar material. At metaphase, the nucleus elongates to become spherical to barrelshaped; the spindle microtubules are longer but do not reach the nuclear envelope at the poles. In the polar regions, the nucleolar material aggregates into globular masses. The nuclear envelope remains continuous and devoid of lamina, but becomes sinuous. The kinetochores are improminent, associated with up to 6 microtubules. There are no nuclear pole bodies or extranuclear microtubules. At telophase, the nuclear envelope becomes very convoluted, suggesting that portions of the daughter nuclei bud off into the cytoplasm. Annulate lamellae occur inside the nuclei. The chromosomes decondense and the nuclear lamina is reformed, simultaneously with the appearance of new nucleoli which then fuse into one. Post-telophase nuclei undergo shuttle migrations with the cytoplasm between daughter cells.

Patterning in Opalinids.III.The Cytoskeleton of Cepedea sudafricana (FANTHAM, 1923) AFFA'A & LYNN 1994, an Intermediate Type between Opalina ranarum and Protoopalina pseudonutti

Summary The cytoskeleton of Cepedea sudafricana was studied by silver staining according to the method of FERNANDEZ-GALIANO 1976) and by electron microscopy after fixation, which preserves the microfibrils better.The fibrillar cytoskeleton of this species, which has many nuclei, seems to be an intermediate model between the microfibrillar skeleton of Opalina ranarum, which consists of a bi-dimensional network located solely in the cortex of the cell and the endoplasmic skeleton of Protoopalina pseudonutti, which forms a large axial skein surrounding the two nuclei.Cepedea sudafricana also possesses an endoplasmic fibrillar network, but it is more scattered and does not make contact with the nuclear envelope.