JUNCTIONAL PORES AND CELL WALL DEPRESSIONS OF HORMOSCILLA PRINGSHEIMII (CYANOPHYCEAE)1

Sassoon's isolate identified as Borzia trilocularis (but recently renamed Hormoscilla pringsheimii Anagnostidis and Komárek (1988) was studied with transmission electron microscopy because of its unusual combination of longitudinal wall features, described here in development for the first time. Junctional pores (linear rows of circumferential L-II layer pores near crosswalls) developed into multiple, parallel series, unlike the pores in many other species, which form only single rows. In dividing cells, two single pore rows appear opposite crosswalls after crosswall initiation, but an additional parallel row is usually added to each row by completion of fission. Elongating cells reveal 3–6 parallel and uniformly spaced pore rows developing on each side of crosswall pairs; these rows may end up toward the center of the cell wall after cell elongation. Pores are 18–24 nm wide with a center-to-center and row-to-row distance of 24–36 nm and occur in an especially thick L-II area. The second group of pit-like pores of longitudinal cell walls are 50–135 nm-wide depressions and have a center-to-center distance of 100–1000 nm. These depressions arise when the L-II layer fails to form and appear next to a row-pair of junctional pores soon after fission. Most depressions form single rows, but when they form several rows they may cover much of the surface of the cell. The L-III and L-IV wall layers line these L-II layer cavities; the outer surface of the L-IV layer around and within the depressions is covered with fibrous mucilage. Given their diversity, pores and depressions of longitudinal walls deserve further attention from functional and taxonomic points of view.     

Cell division in baeocyte producing cyanobacteria

Summary An ultrastructural examination of cell division in two baeocyte producing cyanobacteria,Pleurocapsa minor andDermocarpa violaceae, reveals two distinct patterns of binary (transverse) fission. Septate binary fission, inPleurocapsa minor, involves centripetal synthesis and deposition of the mucopolymer cell wall layer (L 2). The ingrowth of the cytoplasmic membrane and L 1 cell wall layer, along with the synthesis of the L 2 cell wall layer, results in the formation of a prominent septum. Partitioning of the cell occurs by the constriction of the outer cell wall layers (L 3 and L 4) through the septum. InDermocarpa violaceae, constrictive binary fission occurs by the simultaneous ingrowth or constriction of the cytoplasmic membrane and all cell wall layers (L1, L2, L3, L4). Septate and constrictive binary fission may proceed symmetrically (medially) or asymmetrically (nonmedially). Multiple fission occurs regularly inDermocarpa violaceae and provides for a rapid means of reproduction when compared to binary fission. Successive radial and tangential divisions of the protoplast result in formation of many small daughter cells (baeocytes). The process of multiple fission is similar to septate binary fission with reduced septa being formed. However, constriction of the outer cell wall layers, through the septa, proceeds concurrently with septum formation.     

Architecture of the cell wall of a green alga,Oocystis apiculata

Summary The cell wall of a green alga,Oocystis apiculata, was visualized by electron microscopy after preparation of samples by rapid-freezing and deep-etching techniques. The extracellular spaces clearly showed a random network of dense fibrils of approximately 6.4 nm in diameter. The cell wall was composed of three distinct layers: an outer layer with a smooth appearance and many protuberances on its outermost surface; a middle layer with criss-crossed cellulose microfibrils of approximately 15–17 nm in diameter; and an inner layer with many pores between anastomosing fibers of 8–10 nm in diameter. Both the outer and the inner layer seemed to be composed of amorphous material. Cross-bridges of approximately 4.2 nm in diameter were visualized between adjacent microfibrils by the same techniques. The cross-bridges were easily distinguished from cellulose microfibrils by differences in their dimensions.     

Microscopic observations of germination and septum formation in pycnidiospores of Botryodiplodia theobromae

A population of aseptate pycnidiospores of the fungus Botryodiplodia theobromae can be induced to germinate or to form septa delimiting two cells; this developmental process is dependent upon nutritional and environmental factors. Transmission electron microscope investigations indicate that during germination of the aseptate spore, a new inner wall layer is synthesized de novo at the site of germ tube emergence. Formation of the septum also involves the de novo synthesis of an inner wall layer which comprises the majority of the septum and completely surrounds the spore. The wall of the germ tube emerging from the septate spore is a direct extension of this inner layer deposited during the formation of the septum. Although the early stages of spore germination may involve localized enzymatic degradation of the internal layers of the spore wall, transmission and scanning electron micrographs of germinating spores show that the outer wall layers are physically fractured by the emerging germ tube. It is suggested that spore germination and septum formation are initially similar processes regarding cell wall genesis but that some mechanism responsive to environmental and nutritional conditions determines the course of development.     

CELL DIVISION AND FILAMENT FORMATION IN THE DESMID BAMBUSINA BREBISSONII (CHLOROPHYTA)1

Cell division and semicell expansion in the filamentous desmid Bambusina brebissonii Kütz. were investigated using transmission and scanning electron microscopy. Interphase cells are typical of desmids, containing a full complement of organelles and a cell wall penetrated by complex pores, but the cells lack a well-defined median constriction. Cell division involves an open spindle and the centripetal growth of a primary septum formed by the fusion of small, dark-staining vesicles probably derived from dictyosomes. Telophase nuclei are separated by a system of interzonal microtubules and numerous large, lighter-staining vesicles also derived from the dictyosomes. Following cell division, an elaborate replicate cross wall is formed which consists of both primary and secondary wall layers. During semicell expansion, a portion of the primary wall splits apart as the new semicells evaginate and expand to their full size. The primary wall stops splitting at a thick ring of secondary wall material leaving the cells united by the remaining common layer of primary wall. When semicell expansion is completed, the primary wall is not shed from the lateral walls of the new semicells, and pores through both primary and secondary wall layers begin to produce sheath material. However, pores in the end walls of cells do not function unless the filament is broken. The intact primary wall between cells and the absence of sheath production between cells comprise the mechanism serving to hold the cells of Bambusina brebissonii together in long filaments.     

ONTOGENETIC STUDY OF FLORAL ORGANS OF PEACH (PRUNUS PERSICA) UTILIZING CYTOCHIMERAL PLANTS

Cell lineages were followed throughout floral ontogeny in cytochimeral peaches [Prunus persica (L.) Batsch] by observations of chromosome number and nuclear size. The contribution of the three apical cell layers to the organs of the flowers was determined. In addition to the epidermal tissue, L-I produced several layers of cells at the suture of the ovary wall, seven or eight cell layers of the nucellus at the micropylar end of the ovule, and almost all of the integuments. L-II gave rise to extensive internal tissue in the calyx and corolla tubes and to all internal tissue of the petal, anther, and ovule except for a small region at the base of the latter two organs. L-III contributed significantly only to the central region of the calyx and corolla tubes and the ovary wall. A single apical layer gave rise to several different tissues, and at times a single tissue was made up of cells from 1–3 different apical layers. Within the limits imposed by their genotype the final form of differentiated cells was determined by their position in the mature organ and not by the apical layer from which they were derived. The corolla tube was shown to be a single structure, congenitally fused, and the ovary to be ontogenetically fused at the suture.     

PIT CONNECTION FORMATION IN THE RED ALGA PSEUDOGLOIOPHLOEA

Pit connection formation in the marine red alga Pseudogloiophloea confusa was studied with the electron microscope. The process of formation occurs in 2 stages. First, a septum forms as an annular ingrowth from the lateral walls. Lomasomes are associated with the centripetal accretion of wall material. The completed septum contains a large rimmed aperture, bounded by the continuous plasmalemma, and through which the cytoplasm is continuous from cell to cell. In the second stage, a highly structured plug is formed which completely blocks the aperture. The plug is condensed on flattened vesicles which lie parallel to one another and which traverse the aperture. The mature plug is composed of granules 50–100 A in diameter and surrounded by several dense layers which appear to enclose an 80 A limiting membrane. Once the pit connection is formed, no material is seen to traverse it.     

Spores of microorganisms

Spores ofBacillus cereus (strain NCIB 8122) were germinated in a synthetic germination limited medium (GL-medium), which permitted germination but did not make the termination of post-germinative development possible. Incorporation of¹⁴C-diaminopimelic acid into the newly formed cell wall was followed in this culture. Morphological changes were studied by optical and electron microscopy. Germination was associated with the usual germination changes,i.e. depolymerization of the “bulky” cortex, differentiation of nuclear structure and mesosomes and ribosomes in the cytoplasm. At this stage the spore protoplast is surrounded by several layers: exosporium, laminated coat with four layers, residual spore wall and the protoplast membrane. During incubation in this limited medium the residual wall layer thickens and the nuclear structure, mesosomes and ribosomes were not more detectable. After enrichment of the GL medium (shift up) the thick-walled cells can form additional cell wall material, elongate and an atypical septum formation can occur. The cell wall material forms local thickenings. On long-term cultivation in the GL medium some of the cells in the GL medium lyze. If, in the course of 3–6 h the cells are transferred from the GL-medium to a solid complex medium (Difco Nutrient Agar) the thickwalled cells are transformed into dividing cells. When the cells are transferred later, their colony-forming ability rapidly decreases. The decrease of viability of the thick-walled cells derived directly from spores after their germination in the limited medium indicates that these cellular forms probably do not represent more stable cellular types that would be of considerable importance for survival of the populat ion of bacilli.     

Bipolar budding in yeasts — an electron microscope study

Bud formation in yeasts with bipolar budding was studied by electron microscopy of thin sections.Budding in yeasts of the speciesSaccharomycodes ludwigii, Hanseniaspora valbyensis andWickerhamia fluorescens resulted in concentric rings of scar ridges on the wall of the mother cell. The wall between the ridges consisted of the scar plug left by the former budding and opened up in the formation of the next bud. The wall of the bud arose from under the wall of the mother cell.In the yeasts of the speciesNadsonia elongata more than one bud might be formed from the same plug.InSchizoblastosporion starkeyi-henricii the scar ridges were close together and apparently not separated by the entire plug.In all species a cross wall was formed between mother cell and bud which consisted of an electron-light layer between two layers of more electron-dense material. The cells separated along the light layer.The authors wish to thank Dr J. A. Barnett for corrections of the English text, and Mr J. Cappon for drawing Fig. 1.     

Structure and Elemental Composition of the Cyst Wall of Echinosphaerium nucleofilum Barrett (Heliozoea,Actinophryida)1

The structure of the cyst wall of the heliozoon Echinosphaerium nucleofilum has been investigated using light microscopy, scanning and transmission electron microscopy, and X-ray microanalysis. The cyst wall is a composite structure of seven or eight layers. These are: an enveloping gelatinous layer; a layer of siliceous spheroidal bodies; an electron-dense supporting membrane; a broad electron-lucent zone; an electron-dense layer; a layer of helicoidally packed material; and one or two layers with a granular appearance lying next to the plasma membrane of the encysted organism. The structure of the cyst wall closely resembles that of Actinophrys sol, confirming the close relationship of these actinophryid heliozoa while emphasizing their distinctiveness from other amoeboid protista.     

Ultrastructure of the septum inCandida albicans

The ultrastructure of the septum in the mycelial phase of the dimorphic fungusCandida albicans has been studied both in thin sections of fixed material and in shadow casts of the chemically purified chitinous wall layer. The septum has a 25-nm central micropore which would not allow the passage of nuclei or mitochondria, thus delimiting these organelles within the mycelial compartments without preventing cytoplasmic continuity.      

Ultrastructure of the Cell Wall and Cell Division of Unicellular Blue-green Algae

The fine structure of the cell wall and the process of cell division were examined in thin sections of two unicellular blue-green algae grown under defined conditions. Unilateral invagination of the photosynthetic lamellae is the first sign of cell division in the rod-shaped organism, Anacystis nidulans. Symmetrical invagination of the cytoplasmic membrane and inner wall layers follows. One wall layer, which appears to be the mucopolymer layer, is then differentially synthesized to form the septum; the outer wall layers are not involved in septum formation. Centripetal splitting of the inner layer separates the two daughter cells. A second division, in a plane parallel to the first, usually occurs before the first daughter cells are separated. In the coccoid organism, Gleocapsa alpicola, the features of cell division are broadly similar; however, unilateral invagination of the lamellae is not observed and the second division takes place in a plane perpendicular to the plane of the previous division.     

An electron microscopic study of bud development in Saccharomycodes ludwigii and Saccharomyces cerevisiae

Summary Examination of sectioned cells fixed in KMnO₄ has shown that the wall of the first bud of a cell of Saccharomycodes ludwigii arises as an extension of the main wall of the parent, while in subsequent buds it develops by extension of the half-septum remaining at a previous detachment scar. Septa are formed by the deposition of wall material on each side of an electron transparent plate which develops centripetally. Structural changes occur in the marginal region of the septum prior to rupture of the main wall and the separation of cells at the surface of the septum-plate. The broken walls remain as annular rings around the scars following the successive development of buds at both apices of the cell.In Saccharomyces cerevisiae the bud wall arises as a direct extension of the parent wall or as an extension of an additional inner layer developed locally.The two types of bud origin are compared in the two yeasts and a comparison is also made with the development of buds, fission cells, conidia and germ tubes in other organisms.     

Isolation and characterization of prosthecae ofAsticcacaulis biprosthecum

The budding process of the yeast form of Mucor rouxii was examined by electron microscopy of thin sections with particular reference to wall ontogeny. In most instances the bud wall is seen as a continuation of the inner layers of the parent cell wall. As the bud emerges it ruptures the outer layers of the parent wall. The bud wall is much thinner than the parent wall and remains so while the bud grows into a sphere of about one half the diameter of the parent cell. Then a septum begins to form centripetally, at the neck, by invagination of the plasmalemma. Before the neck canal is completely occuluded, electron-dense wall material is deposited into the septum space. Two separate septum walls are deposited, one on the parent side and one on the bud side of the invaginating plasmalemma. Septum wall formation extends to the surrounding neck walls. In this manner, the parent and bud cytoplasms become fully separated and each is surrounded by a continuous wall. The two cells remain attached to each other by the original neck wall; eventually, the bud abscisses leaving a birth scar on the bud cell and a more pronounced bud scar on the parent cell. In general, the mechanism of budding in this zygomycetous fungus resembles that of an ordinary ascomycetous yeast such as Saccharomyces cerevisiae.     

Electron microscopy of sporulation inSchwanniomyces alluvius

Sporulation inSchwanniomyces alluvius appeared to be preceded by fusion of a mother and a daughter cell. Meiosis probably occurred in the mother cell and one or two spores were formed in the latter. A study of thin sections showed that the spore wall developed from a prospore wall. The mature spore wall consisted of a broad light inner layer and a thinner dark outer layer including warts. An equatorial ledge was present. During germination in the ascus, a new light inner layer was formed and the old layers of the spore wall partly broke up. Ascospores in a strain ofS. persoonii had a different wall structure in that the dark layer had changed into light areas separated by dark material which formed bulges at the surface.     

Electron microscope studies on Leucothrix mucor

Summary Electron microscopic studies of thin sections of filaments, knots, resettes, gonidia, and gonidial-forming filaments of Leucothrix mucor were carried out. The cell wall is typical of gram-negative bacteria, with a double outer layer of variable thickness, a single thin middle layer which is probably peptidoglycan, and a double inner layer which is the cell membrane. The transverse septa of these filaments show two peptidoglycan layers, and no clearly demarked outer layer. During gonidial formation, there is a gradual rounding up of the cells, and the transverse septa become part of the gonidial wall. The cell membrane contains many invaginations, both along the outer wall and along the transverse septa. Thin sections through rosettes show the holdfast as material which is a heavily-staining amorphous material peripheral to the outer wall layer. Sections through knots show highly contorted cell walls, closely appressed. Fibrillar nuclear material, ribosomes, and storage granules can be seen within the cytoplasmic matrix.     

THE CELL WALL OF SCENEDESMUS QUADRICAUDA

Bisalputra, T., and T. E. Weier. (U. California, Davis.) The cell wall of Scenedesmus quadricauda. Amer. Jour. Bot. 50(10): 1011–1019. Illus. 1963.—Fine structure of the cell wall of Scenedesmus quadricauda fixed in both KMnO₄ and osmium tetroxide is described. The cell wall consists of 3 layers: the inner cellulosic layer which delimits individual cells; the outer pectic layer which binds the cells of the coenobium together; and a thin middle layer, bounded by membranes on either side, which is electron-dense in osmium-fixed material but of medium electron density in KMnO₄. The structure of the outer pectic layer is similar in both fixatives; it consists of a hexagonal network of electron-dense material on the surface, and a system of tubules or “props” which radiate out from the middle layer of the wall to support the net. The pectic layer appears in the daughter coenobia before their liberation from the parent colony.     

SEPTAL PORES IN THE HETEROBASIDIOMYCETIDAE,PUCCINIA GRAMINIS AND P. RECONDITA

The septal pores in uredial mycelium of Puccinia graminis and P. recondita lack the septal swelling and septal pore cap (dolipore-parenthosome configuration) typically associated with the pores of previously investigated Homobasidiomycetidae and the Tremellales among the Heterobasidiomycetidae. The pores in young hyphae of these two species of Puccinia are characterized by the presence of a cytoplasmic matrix which apparently occludes the pore and acts as a plug, thus preventing the migration of organelles from cell to cell. Large vesicles are typically present at the periphery of the pore matrix and the matrix may be very incompletely bounded by a membrane. Nuclei and other cytoplasmic structures migrate from cell to cell through an opening in the septum lateral to the pore. The available evidence indicates that this peripheral gap in the septum results from a breakdown of a portion of an initially complete septum rather than from incomplete septum formation. In addition to the centripetally formed septa, the hyphae of P. graminis and P. recondita are further compartmentalized by shallow infoldings of the lateral wall and limited unilateral septum formation. There is apparent free passage of cellular material between adjacent compartments.     

Wall structure and bud formation in Rhodotorula glutinis

Summary The first stage in the formation of a bud in Rhodotorula glutinis is the production of a tapered plate of new wall material between the existing wall and the plasmalemma. The parent cell wall is lysed, allowing the bud to emerge enveloped in this new wall. Mucilage is synthesised to surround the developing bud. As the bud grows a septum forms centripetally dividing the two cells. When the daughter cell reaches maximum size the septum cleaves along its axis, producing the bud scar on the parent cell and the birth scar on the daughter cell. The birth scar is obliterated later as the wall of the young cell grows. A system of endoplasmic reticulum and vesicles is found in young buds and is thought to be responsible for the transport of wall material precursors.     

Electron Microscopic Studies on the Outer Layers of Staphylococcus aureus Using a Lytic Enzyme from Flavobacterium

The structure of the outer layers (cell wall and membrane) of Staphylococcus aureus was studied by electron microscope using a bacteriolytic enzyme from Flavobacterium sp. called the L-11 enzyme. Comparative studies on the morphology of bacteria before and after treatment with this enzyme and cell wall and membrane fractions obtained from bacteria after the enzyme treatment led to the following conclusions. (1) The cell wall of S. aureus is composed of morphologically distinct two layers which are both susceptible to the L-11 enzyme. (2) Between the cell wall and membrane, there is an electron opaque region which could not be stained using any of the methods tested. (3) Before treatment of bacteria with the enzyme the cell membrane could not be seen clearly. However, after enzyme treatment the membrane was clearly seen. (4) The infolding of the inner layer of the cell wall, forming a structure like a mesosome, was liberated by extensive enzyme treatment.