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Embryonic mouse cells cultivated in vitro and infected with the EMC virus have been examined with the electron microscope at various time intervals after infection. Nuclear alterations appear prior to significant cytoplasmic changes and before virus particles are visible. These changes consist of chromatin margination, appearance of nucleolar condensations and a dense granular substance in the interchromatinic zone, followed by nuclear shrinkage. In the cytoplasm crystal-like virus aggregates develop which consist of many virus particles of about 20 mμ in diameter. A possible relationship between these viruses and the ribosomes is discussed. These findings are compared with those reported on various enteric viruses.

A plant agglutinin is used to visualize with the electron microscope carbohydrate-carrying cellular components, in particular, the cell coat. Concanavalin A has two reactive groups. One binds specifically to certain sugars of the cell, the other acts as receptor for the carbohydrate-containing peroxidase, the presence of which can be revealed with the diaminobenzidine reaction.

Localization of radioactive labelling over the nuclei of BSC 1 cells is visualized after long periods of 3 H-5-uridine incubation followed or not followed by periods of postincubation in nonradioactive medium for up to several days, using high resolution autoradiography combined with a preferential staining method for ribonucleoproteins. It is shown that when cells are labelled for 1 or 6 h with 3 H-uridine and postincubated with a non-radioactive medium up to several days, there is always some radioactivity present in the nucleolus and nucleoplasm. When sections of cells fixed after 1 h of labelling followed by 24 h of postincubation are treated with RNase, part of the radioactivity found in the nucleus disappears almost completely only after a succeeding DNase digestion. The majority of interchromatin granules are weakly labelled after most incubation times, with the label localized rather at the periphery of clusters of granules, or are unlabelled. The results are discussed in the context of recent biochemical findings. It is proposed that interchromatin granules might represent a structure containing a limited quantity of slowly labelled nuclear RNA.

Resume L'etude au microscope electronique du nucleole de cellules normales et de cellules cancereuses permet de preciser l'ultrastructure de cet organite. L'existence d'un element reticule ou filamenteux (⪡ nucleolonema ⪢) et de zones intra et perinucleaires plus diffuses (⪡ pars amorpha ⪢ selon Clemente Estable) est confirmee. Grâce au developpement recent de la technique des coupes ultrafines, l'ultrastructure de ces deux composants se revele comme granulaire. Les dimensions des grains sont de l'ordre macromoleculaire. Leur identification aux constituants chimiques du nucleole est discutee. Il n'a pas ete constate de differences ultrastructurales constantes entre les nucleoles des cellules normales et ceux des cellules cancereuses.

Resume Nous avons etudie au microscope electronique l'action des fixateurs sur l'ultrastructure du cytoplasme de leucocytes et de thrombocytes du sang humain normal. 1. 1. La plupart des images obtenues sous l'influence des differents fixateurs sont comparables, quel que soit le mode d'action de ceux-ci. Elles mettent en evidence une trame hyaloplasmique reticulee, ou fibrillaire, plus rarement granulaire, de dimensions submicroscopiques mais supramoleculaires. 2. 2. Les fixateurs peuvent etre classes en deux groupes: ceux qui affinent la trame ou meme l'effacent completement, et ceux qui la grossissent a des degres divers allant jusqu'a la rupture totale de l'architecture hyaloplasmique. Les melanges fixateurs donnent des images intermediaires suivant que le liquide predominant appartient a l'un ou l'autre groupe. 3. 3. La plupart des fixateurs ⪡creent⪢ ainsi une image qui leur est particuliere, mais ils respectent en general l'orientation des fibrilles ou du reseau qui est propre a la cellule. 4. 4. Cette orientation peut etre conditionnee par des forces purement mecaniques (tension de surface, courants cytoplasmiques lors de l'etalement de la cellule), mais d'autres facteurs semblent intervenir egalement. 5. 5. La specificite de l'ultrastructure hyaloplasmique des elements sanguins ne fait pas de doute. On peut supposer qu'un grand nombre d'autres cellules ont, elles aussi, leur architecture propre. 6. 6. Les travaux de microscopie electronique concernant l'ultramorphologie du cytoplasme ne nous paraissent pas confirmer d'une facon absolue la theorie des points de jonction de Frey-Wyssling, mais ne l'excluent nullement. Elle reste une bonne hypothese de travail, si l'on se garde de l'appliquer trop schematiquement, car l'ultrastructure du hyaloplasme montre une tres grande variete de dimensions et d'orientation, specifique pour certaines souches cellulaires.

Adenovirus 12-induced hamster tumor cells are characterized by the presence of large, atypical mitochondria of variable number which contain relatively few cristae, an electron-lucid matrix and distinctive fibers. On the basis of various methods of fixation, embedding and staining for electron microscopy and by treatment of ultrathin sections with the nucleases, it has been demonstrated that the fibers contain DNA and possibly some RNA. These atypical mitochondria co-exist in cells with smaller, elongate, normalappearing mitochondria which contain a normally dense matrix and apparently less or no visiable DNA fibers. The atypical mitochondria are present mainly during rapid cell proliferation. They are abundant in young cultures but they then disappear and only the typical form remains. In the subsequent cell passage the atypical mitochondria reappear, hence they originate from the typical ones. The timing of this cyclic appearance and disappearance varies with environmental conditions. Inasmuch as their disappearance cannot be accounted for entirely by degeneration, it is postulated that they can revert to the typical form.

Autoradiography combined with electron microscopy is used to visualize the incorporation of 3H-uridine into the nuclei of cultured BSC1 monkey kidney cells. A preferential RNP stain is applied to differentiate between chromatin and nuclear ribonucleoproteins. It is demonstrated that the incorporation of the radioactive precursor can be localized after pulses of only 2 min., on the one hand at the limit of the chromatin and of fibrillar areas of the nucleolus, and on the other hand in the proximity of condensed chromatin throughout the nucleoplasm where perichromatin fibrils are present. Pulses of 5 to 15 min show a considerable increase of labelling. The number of silver grains over the fibrillar areas of the nucleolus increases. The pattern of extranucleolar incorporation is not changed. If these pulses are followed by a chase varying from 15 min to 3 h, some radioactivity can always be demonstrated throughout the nucleolus, with a considerable amount in the RNP containing interchromatin area as well. However, interchromatin granules are not, or only weakly, labelled even after a labelling of 1 h followed by a chase of up to 3 h. The results are discussed in view of recent biochemical findings of a rapidly labelled and locally metabolized nuclear RNA. It is suggested that the early-labelled RNA visualized in the vicinity of the condensed chromatin may at least partially correspond to the DNA-like RNA or HnRNA fraction.