Electron microscopy of ultrathin sections of Schizosaccharomyces octosporus. I. Cell division

Second thoughts on septation by the fission yeast, Schizosaccharomyces pombe: pull vs. push mechanisms with an appendix--dimensional modelling of the flat and variable septa

The correlation of contraction by an actomyosin band with the closing of the septum of dividing cells of the fission yeast, Schizosaccharomyces pombe, cannot suggest cause-and-effect because contraction would be apparent whether the membrane enveloping the centripetally closing septum were pulled or were pushed. Thus the common observation of contraction is not critical. Diagrams of published electron micrographs of dividing wild-type fission yeasts illustrate variable (tilted) septal images that are counterintuitive to a pull model. Circumference calculations based on those images suggest that some variable forms might be only 6% closed even though their two-dimensional profiles would be 50% closed, if they were not tilted. Development of multiseptate forms of cdc4-8 and cdc4-377 temperature sensitive mutants incubated at their restrictive temperature was followed. These multiseptate forms are shown to have functional (functional in terms of generating divided uninucleate cytoplasts) but grotesque septa which are formed in the absence of actomyosin bands. By contrast, the myosin of the plant phragmoplast is not properly oriented for contractility, and Dictyostelium (attached cells) and Saccharomyces (mutants) have been shown to divide in the absence of myosin II, just as S. pombe does (above). Hence contractility, the essence of a pull model for septum closure, would seem to be non-essential. Other, non-contractile mechanisms of myosin are emphasized, and a push model becomes a rational default hypothesis. The essence of push models is that their synthesis/assembly mechanisms are driving force sufficient for septum closure.

Cytology of Rhodotorula glutinis

Thyagarajan, T. R. (Cornell University, Ithaca, N. Y.) and H. B. Naylor. Cytology of Rhodotorula glutinis. J. Bacteriol. 83:127-136. 1962.-The structure and manner of division of nuclei in actively dividing cells of Rhodotorula glutinis were studied with the phase contrast microscope. The nucleus consists of a dense central body, surrounded by a shell of optically uniform material of low density. The entire structure is enclosed within a nuclear membrane. Various fixation and staining techniques were employed to confirm the observations made from living cells. Since the dense central body is Feulgen-negative and is readily stained by iron alum hematoxylin, it is identified as the nucleolus. The material surrounding the nucleolus has no marked affinity for hematoxylin but is Feulgen-positive and stains intensely with Giemsa and basic fuchsin. The nucleus appears to divide by a process of elongation and constriction during which roughly half of the nucleolus, along with the surrounding chromatin, passes into the bud. The nuclear membrane was found to persist during all stages of division. Vacuoles were seldom observed in actively dividing cells. The nucleus of R. glutinis is similar in structure to the nuclei of higher organisms, but its behavior during division is quite different.

Electron microscopy of Rhodotorula glutinis

Thyagarajan, T. R. (Dartmouth Medical School, Hanover, N. H.), S. F. Conti, and H. B. Naylor. Electron microscopy of Rhodotorula glutinis. J. Bacteriol. 83:381-394. 1962.-The structure and manner of nuclear division in Rhodotorula glutinis was studied by electron microscopy of ultrathin sections. Parallel studies with the light microscope, employing conventional staining techniques and phase-contrast microscope observations on nuclei in living cells, were carried out. The nucleus is spherical to oval and is bounded by a nuclear membrane. Intranuclear structures, identified as nucleoli, and electron-transparent areas were observed. The nuclear membrane persists throughout the various stages of cell division. Observations of the nucleus with the electron microscope revealed that nuclear division occurs by a process of elongation and constriction similar to that seen in both living and stained cells. The fine structure of mitochondria and other components of the yeast cell and their behavior during cell division are described. The absence of vacuoles in actively dividing cells of Rhodotorula glutinis lends further support to the view that the vacuole is not an integral part of the nucleus. The results with the electron microscope generally support and considerably extend those obtained with living and stained cells.

MULTIPLE SCARS, A NEW TYPE OF YEAST SCAR IN APICULATE YEASTS

Streiblová, Eva (Czechoslovak Academy of Sciences, Prague, Czechoslovakia), K. Beran, and V. Pokorný. Multiple scars, a new type of yeast scar in apiculate yeasts. J. Bacteriol. 88:1104-1111. 1964.-A new type of yeast scar is described in apiculate yeasts: Saccharomycodes, Nadsonia, Hanseniaspora, and Kloeckera. These scars are formed on the distal poles of the cell walls in the course of vegetative reproduction, and are the cause of the formation of the apiculate form of the cells. The structure of multiple scars was studied by fluorescence microscopy and by electron microscopy on carbon replicas and isolated cell walls. The discussion deals with the importance of described cytological structures for the morphogenesis of cells and for determining individual reproductive capacity of cells, and considers some questions related to the interpretation of the development of multiple scars.

Cell division in yeasts: movement of organelles associated with cell plate growth of Schizosaccharomyces pombe

Electron microscopy of dividing fission yeast cells shows establishment of an annular rudiment (AR) of electron-transparent material under the old cell wall as the first sign of elaboration of the cell plate. The AR grows centripetally, finally closing at the mid-point of the cell. During the inward growth of the AR it is thickened by addition of denser material which becomes the scar plug after fission; the electron-transparent material is lost at fission. Lying always between the cytoplasmic membrane and the cell wall is a dark layer of variable thickness. This layer becomes markedly thickened into a fillet at the base of the centripetally growing cell plate. The fission process begins after the cell plate is completely elaborated. One striking feature of fission is the migration of dense material from the fillet at the base of the cell plate outwardly through the matrix of the cell wall to its final resting place as a dark ring, a "fuscannel," adjacent to the fission scar. The inclusion of Golgi bodies in many sections suggests their involvement in cell plate elaboration, presumably through production of the dense bodies which are seen to fuse with the dark layer proximal to the growing cell plate.

Micromorphology of Cryptococcus neoformans

Fine details of the internal and external morphology of Cryptococcus neoformans as seen in ultrathin sections are described and illustrated with electron micrographs. The capsule characteristic of this species contained microfibrils (30 to 40 A in diameter) that appeared to radiate from the cell wall and to coil and intertwine in various directions. These thin, uniformly structured, electron-dense filaments are believed to represent complex polysaccharide molecules. The internal morphology of C. neoformans was in many ways similar to that of yeasts studied by other authors. The cell was uninucleate with a single nucleolus. The nuclear envelope, a pair of unit membranes interrupted by pores, was typical of that found in eucaryotic organisms. Smooth endoplasmic reticulum, mitochondria, vacuoles, storage granules, and ribosomes were consistent features of the cytoplasm. In addition, C. neoformans presented membranous organelles derived from the plasma membrane and comparable to bacterial mesosomes and mitochondria of an annulate type.

Cell wall changes during the budding process of Paracoccidioides brasiliensis and Blastomyces dermatitidis

The difference between the budding process of Paracoccidioides brasiliensis and Blastomyces dermatitidis is reported herein. A characteristic feature in P. brasiliensis is that the optical density of the cell wall increases at the site where budding begins and at the neck of the dividing cell, whereas B. dermatitidis does not undergo this alteration. The neck which is formed between the mother and daughter cell at the site of division is much wider in B. dermatitidis than in P. brasiliensis. The bud scar in P. brasiliensis appears as a truncated cone, the top of which is covered only by the inner layer of the cell wall; in comparison, in B. dermatitidis the bud scar exhibits a flattened surface covered by the cell wall. Both fungi show an increase in the number of mitochondria and infoldings of the cytoplasmic membrane at the site of separation, which indicates that at this site there is an increase of metabolic activity.

Structural changes in the cell wall of Schizosaccharomyces pombe during cell division

Streiblová, Eva (Czechoslovak Academy of Sciences, Prague, Czechoslovakia), I. Málek, and K. Beran. Structural changes in the cell wall of Schizosaccharomyces pombe during cell division. J. Bacteriol. 91:428-435. 1966.-Individual stages of growing and dividing cells of Schizosaccharomyces pombe were studied by means of fluorescence and electron microscopy with the use of metal-shadowed isolated walls, replicas, and ultrathin sections. Vegetative cells were found to contain division scars (six at the most); their formation and structure are described. More data on the growth of arthrospores were obtained. New structural observations were made on the architecture of the cell wall (original wall ring, polar cell wall, plug wall band, additional wall ring). Structural changes of cell surfaces and lateral walls during fission are represented schematically to the fourth generation. The question of origin of the septum is discussed, and on this basis the entire structure of the cell wall is interpreted.

EFFECTS OF SODIUM CAPRYLATE ON CANDIDA ALBICANS I

Adams , J. N. (University of Georgia, Athens), Barbara G. Painter, and W. J. Payne . Effects of sodium caprylate on Candida albicans . I. Influence of concentration on ultrastructure. J. Bacteriol. 86: 548–557. 1963.—Morphological effects of exposure to various concentrations of sodium caprylate were studied in living cells and ultrathin sections of Candida albicans . Budding was inhibited when cells were cultured in 0.0025 m sodium caprylate. Cells relieved from treatment at this concentration initiated reproductive processes at a much-stimulated rate. Momentary or 30-min exposures to 0.1 m caprylate affected cells in much the same manner as those treated at the lower concentration. Alteration of ultrastructure was brought about by treating with 0.0025 and 0.005 m concentrations prior to embedding and sectioning. Condensation of vacuolar material, change in size and number of mitochondria, loss of mitochondrial cristae, and increased electron density of the cytoplasm were observed. Cellular integrity was progressively lost as a result of treatment at levels up to 0.1 m . The highly electron-dense cytoplasm of cells cultured in media containing 0.025 m or higher inhibitor appeared to break into irregular masses, but the nucleus and vacuole sometimes could be identified. Cells treated at the 0.1 m level contained a condensed mass of opaque and unidentifiable cytoplasmic constituents within the skeletal cell wall. Cytological observations were correlated with physiological studies.