STRUCTURE OF THE PERIPLAST OF CRYPTOMONAS OVATA VAR. PALUSTRIS1,2

The periplast of Cryptomonas ovata var. palustris is composed of polygonal plates which are delineated by shallow ridges. A small ejectosome is located at each corner of the plate area. The plate areas vary in size; they are smallest at the anterior and posterior ends and are largest in the middle of the cell with an average length of 0.5 μ and of width 0.4 μ. In cross section a plate area is composed of 2 distinct layers, an outer plasma membrane layer with a fine particulate, appearance, and an inner layer consisting of two sheets. The sheets of the inner layer have a striated lattice pattern with a periodicity of about 20 nm. In negatively stained preparations one lattice appears to underlie another at certain angles. Protease digestion removed polygonal shaped inner layer.     

A freeze-fracture study of the body wall of adult, in utero larvae and infective-stage larvae of Trichinella (Nematoda)

The surface layers of the cuticle, the hypodermal membranes and the muscle membranes of the adult, the in utero larvae and the infective-stage larvae of the nematode Trichinella spiralis have been studied by means of the freeze-fracturing technique. The surface of the cuticle of both adults and larvae fractures in ways different from membranes of internal cells. The surface coat on top of the epicuticle is probably the layer that changes antigenically. Reticulate ridges, with associated particles, on the E face of the outer hypodermal membrane of the adult are probably sites of attachment of the hypodermis to the cuticle. Longitudinally arranged ridges, with associated particles, of the outer hypodermal membrane are probably points of attachment to the cuticle in the in utero and infective larvae. Rectilinear arrays of particles are present on the P face of the inner hypodermal membrane and the P face of the muscle membrane adjacent to the hypodermis of adults and larvae and probably play a role in adhesion of the muscle membrane to the hypodermis. Particle-free areas of membrane lie external to the Z bundles of the muscle cell and are similar to the sites of attachment of Z lines in insect muscles.     

The fine structure of the pellicle ofEuglena gracilis as revealed by freeze-etching

Summary The ultrastructure of the pellicle ofEuglena gracilis (Klebs) Z strain was studied using the freeze-etching technique and the results were correlated with data obtained from thin sections of fixed material.Examination of freeze-etched pellicles reveals an outer particulate layer and an inner striated layer. The particles of the outer layer measure approximately 150 Å in diameter. The striations of the inner layer are about 50 Å wide and are separated from each other by about 35 Å. A broad repeating pattern is also visible with a periodicity of about 450 Å. When deep etching is employed, a smooth outer layer is seen covering the particulate layer. This is probably the outer surface of the plasma membrane. Mucilage is present on the outer surface of the cell and is seen as a substructure of threads superposed on the smooth layer of plasma membrane.Thin sectioning also shows a striated layer interior to the plasma membrane. This appears to be identical to the striated layer seen after freeze-etching.     

A new cell wall structure in a symbiotic and a free-living strain of the blue-green alga genus Chroococcidiopsis (Pleurocapsales)

Freeze etching studies in a symbiotic and a freeliving strain of Chroococcidiopsis revealed a specific layer in the outer cell wall not described so far from Cyanophyta. The layer showed a complex organisation: The main unit are ribbons, 2–3 nm thick, striated at right angle to the longitudinal axis. They are interwoven to a patchwork-like leaflet. The ribbons are virtually composed of globular particles associated in parallel rows. The cytoplasmic membrane and the cell walls of the symbiotic and the free-living strain were compared.Abbreviations cm cytoplasmic membrane - CW 1,2,3 cell wall layer 1,2,3 - EF exoplasmic fracture face - PF protoplasmic fracture face     

Freeze-fracture study of malaria sporozoites: antibody-induced changes of the pellicular membrane.

Plasmodium cynomolgi, Plasmodium knowlesi, and Plasmodium berghei sporozoites, before and after incubation with immune serum, were studied after freeze-fracture by electron microscopy. There were evenly distributed numerous intramembranous particles (IMP) on the P face of the outer membrane. The E face of the plasma membrane had fewer IMP than its P face. The E face of the intermediate membrane had few IMP and also linear arrays of slightly raised ridges running the length of the parasite. The P face of the intermediate membrane had many IMP aligned along the long axis of the sporozoite. On the P face of the inner membrane, IMP were arranged in very distinct rows conforming to the long axis of the parasite; the E face of this membrane had a few randomly distributed IMP. A prominent change in the sporozoite incubated in immune serum was the appearance of a layer of aggregated particles around the parasite. The P face of the plasma membrane had several clear areas devoid of IMP and IMP aggregates. No changes were seen in the other fractured faces of the pellicle. These observations suggest that immune serum acts only on the P face of the plasma membrane.     

Surface architecture of the plant cell: Biogenesis of the cell wall,with special emphasis on the role of the plasma membrane in cellulose biosynthesis

Cell wall structure and biogenesis in the unicellular green alga, Oocystis apiculata, is described. The wall consists of an outer amourphous primary layer and an inner secondary layer of highly organized cellulosic microfibrils. The primary wall is deposited immediately after cytokinesis. Golgi-derived products contribute to this layer. Cortical microtubules underlie the plasma membrane immediately before and during primary wall formation. They function in maintaining the elliptical cell shape. Following primary wall synthesis, Golgi-derived materials accumulate on the cell surface to form the periplasmic layer. This layer functions in the deposition of coating and cross-linking substances which associate with cellulosic microfibrils of the incipient secondary wall. Secondary wall microfibrils are assembled in association with the plasma membrane. Freeze-etch preparations of untreated, living cells reveal linear terminal complexes in association with growing cellulosic microfibrils. These complexes are embedded in the EF fracture face of the plasma membrane. The newly synthesized microfibril lies in a groove of the outer leaflet of the plasma membrane. The groove is decorated on the EF fracture face by perpendicular structures termed “ridges.” The ridges interlink with definitive rows of particles associated with the PF fracture face of the inner leaflet of the plasma membrane. These particles are termed “granule bands,” and they function in the orientation of the newly synthesized microfibrils. Microfibril development in relation to a coordinated multienzyme complex is discussed. The process of cell wall biogenesis in Oocystis is compared to that in higher plants.     

Freeze-Fracture Study of Malaria Sporozoites: Antibody-Induced Changes of the Pellicular Membrane*

Plasmodium cynomolgi, Plasmodium knowlesi, and Plasmodium berghei sporozoites, before and after incubation with immune serum, were studied after freeze-fracture by electron microscopy. There were evenly distributed numerous intramembranous particles (IMP) on the P face of the outer membrane. The E face of the plasma membrane had fewer IMP than its P face. The E face of the intermediate membrane had few IMP and also linear arrays of slightly raised ridges running the length of the parasite. The P face of the intermediate membrane had many IMP aligned along the long axis of the sporozoite. On the P face of the inner membrane. IMP were arranged in very distinct rows conforming to the long axis of the parasite; the E face of this membrane had a few randomly distributed IMP. A prominent change in the sporozoite incubated in immune serum was the appearance of a layer of aggregated particles around the parasite. The P face of the plasma membrane had several clear areas devoid of IMP and IMP aggregates. No changes were seen in the other fractured faces of the pellicle. These observations suggest that immune serum acts only on the P face of the plasma membrane.     

Freeze-Etching of the Cell Envelope of an Acinetobacter Species Which Carries a Regular Array of Surface Subunits

Freeze-etching was applied to preparations, with and without glycerol, of Acinetobacter sp. strain MJT/F5/199A, consisting of intact cells after normal growth or after incubation with chloramphenicol, spheroplasts, and isolated cell walls and outer membranes. Etched preparations show that a regular array of subunits forms the surface of normal cells. Near the zones of constriction in dividing cells, blebs and irregularities are seen, and some blebs, consisting of both surface subunits and outer membrane, are released from the cells. The cross-fractured cell envelope shows four layers which are related to the structures seen in section as follows: cw1, which is not visible in section, contains the surface subunits; cw2 consists of all or part of the outer membrane; cw3 includes the intermediate and dense, peptidoglycan-containing layers; within these cell wall layers is the plasma membrane. Internal fracture of the plasma membrane occurs under all conditions tested, but the fracture plane in the cell wall is only revealed in chloramphenicol-treated cells or normal cells freeze-fractured with glycerol present; the characteristic fracture faces are not seen in spheroplasts or isolated outer membranes. The concave fracture face cw2 consists of densely packed granules, while the convex face cw3 is fibrillar. The probable location of this fracture plane is discussed. After incubation with chloramphenicol, the outer surface of the cells is obscured by extracellular material, the dense peptidoglycan-containing layer is increased in thickness, and the cytoplasm contains rounded bodies bounded by one or more unit membranes.     

Freeze-fracture studies on Pneumocystis carinii. I. Structural alteration of the pellicle during the development from trophozoite to cyst

Pneumocystis carinii has generally been distinguished in three developmental stages, namely, trophozoite, precyst and cyst. The fine structure of the pellicle--the plasma membrane and the outer layer existing outside this plasma membrane--of each stage was studied by freeze-fracture technique. By this technique, P. carinii was cleaved through the cytoplasm or through the hydrophobic region of the plasma membrane, and the cross-fractured face of the outer layer was revealed on the replicas. The outer layer, which is electron-dense in the thin section, consisted of numerous fine granules about 15 nm in diameter in freeze-fracture images, whereas the electron-lucent middle layer which appeared in the precyst and cyst was less granular. Measurement of the intramembranous particles (IMP) also was carried out. The number of IMP per square micrometer of the plasma membrane of the trophozoite was 1,512 +/- 125 on the P face and 417 +/- 44 on the E face. In the precyst, the IMP density decreased, and 1,037 +/- 56 on the P face and 262 +/- 22 on the E face. In the cyst, it further decreased, nd 875 +/- 59 and 150 +/- 20 respectively. It is generally assumed that the density of IMP is related to the physiological activity of the cell membrane, so that the present results obtained in P. carinii suggest that the trophozoite is the most active stage, and that metabolic activity of the pellicle gradually decreases with the progress of development to the precyst then to the cyst.     

Organization of the cell membrane inEuglena

Summary The cell membrane ofEuglena gracilis has been investigated with the freeze-fracture technique. When split, this membrane produces two fracture faces which are striking in their non-complementarity. The P fracture face is covered with a high density of 110 Å (average diameter) particles, while the E face is made up of a complex series of striations occurring at regular angles to the pellicle ridges which encircle the organism. Under certain conditions, however, the structure of the P fracture face assumes a more ordered configuration, and striations are visible on this fracture face which are precisely complementary to those observed on the E face. These observations suggest that the cortical cell membrane ofEuglena may be organized along the lines of a two dimensional crystal. However, this pattern of organization is restricted to the cortical region of the cell membrane; as the membrane invaginates near the anterior end of the cell the fracture faces change abruptly, and organization more typical of other cell membranes is observed. This invagination forms an extensive reservoir in the anterior of the cell, and the membrane bounding it is distinctly fluid in structure, with clear examples of endo- and exocytosis observable. These differences suggest that the cell membrane inEuglena is divided into two distinct but contiguous regions, each specialized with regard to structure and function.     

Freeze-fracture studies of the plasmalemma of Candida albicans after treatment with econazole-nitrate.

In electron microscopic studies the interior of the plasmalemma of Candida albicans was revealed by means of the freeze-fracture technique. The superficial structures of the extracellular (E) and protoplasmic (P) fracture faces differed negligibly from structures on the corresponding fracture faces of Saccharomyces cerevisiae. Following treatment with 2.2 x 10(-5) M econazole nitrate a layer, present on the P face in the form of a tight matrix of globular proteins, dissolved into isolated groups of particles whose globular elements sometimes formed hexagonal patterns. As the damage progressed, fissure-shaped membrane invaginations on the P face disappeared. Parts of the outer lipid layer of the plasmalemma were torn off the cell wall and adhered in fragments to the P face. The ultrastructural changes in the plasmalemma induced by econazole nitrate temporally correlate with an increase in the permeability of the cell envelope found in physiological studies performed by other authors.     

Specialised plasma membranes in the preovulatory follicle op the fowl

Summary In the hen's follicle processes from the zona granulosa cells extend into the zona radiata. The terminal plasma membrane of these processes has a total thickness of around 160 Å and consists of five layers. Small granules spaced at regular intervals are attached to the cytoplasmic aspect of the inner layer by short stalks. In the 2 mm and 7 mm follicles the plasma membrane of the ovum facing the specialised terminal membranes has a striated appearance and shows a regular arrangement of granules attached by stalks to both its inner and outer aspects. The terminal and striated membranes are separated by an interval although there are areas of closer contact. In the 15 mm and 35 mm pre-ovulatory follicles the plasma membrane round the whole surface of the ovum is now typical striated membrane with bristles and attached granules. No explanation can be given at present of the function of the terminal membranes of the granulosa processes. They may indicate some change in the permeability permitting the intercellular diffusion of particles. It is suggested that the striated ovum plasma membrane is associated with the adsorption and transport of substances into the ovum for yolk synthesis.     

Cuticle Ultrastructure of Hemicycliophora arenaria,Aphelenchus avenae,Hirschmanniella gracilis and Hirschmanniella belli

Ultrathin sections of all parasitic stages of Hemicycliophora arenaria revealed two major divisions in the body covering. The outermost was a seven-layered sheath and the innermost a five-layered cuticle comprising three zones; an outer trilaminate cortex, a fibrillar matrix and a striated basal layer. The body covering of the nonparasitic males also exhibited two major divisions: the outer, a relatively thin four-layered sheath and the inner, a six-layered cuticle consisting of three zones; an outer trilaminate cortex, a two-layered matrix and a striated basal layer. The cuticles of all stages of Aphelenchus avenae were similar, consisting of five layers divisible into three zones; an outer trilaminate cortex, a fibrillar matrix and a striated basal layer. Hirschmanniella gracilis and H. belli cuticles were also similar in all stages examined, consisting of six layers divisible into three zones; an outer trilaminate cortex, a two-layered matrix and a striated basal layer.     

The lungs of Polypterus senegalus and Erpetoichthys calabaricus: Insights into the structure and functional distribution of the pulmonary epithelial cells

The present article is a comparative, structural study of the lung of Polypterus senegalus and Erpetoichthys calabaricus , two species representative of the two genera that constitute the Polypteriformes. The lung of the two species is an asymmetric, bi‐lobed organ that arises from a slit‐like opening in the ventral side of the pharynx. The wall is organized into layers, being thicker in P. senegalus . The inner epithelium contains ciliated and non‐ciliated bands. The latter constitute the respiratory surface and are wider in E. calabaricus . The air‐blood barrier is thin and uniform in P. senegalus and thicker and irregular in E. calabaricus . In the two species, the ciliated areas contain ciliated cells, mucous cells and cells with lamellar bodies. Additionally, P. senegalus contains polymorphous granular cells (PGCs) and neuroendocrine cells (NECs) while E. calabaricus lacks PGCs but shows granular leukocytes and a different type of NEC. Interestingly, ciliated cells and secretory cells show a dual morphology in E. calabaricus indicating the presence of cellular subtypes and suggesting more complex secretory activity. Also in E. calabaricus , cilia show a novel doublet‐membrane interaction that may control the displacement of the microtubule doublets. The subepithelium is a connective layer that appears thicker in P. senegalus and contains, in the two species, fibroblasts and granulocytes. The outer layer contains bundles of richly innervated striated muscle. This layer is likely involved in the control of lung motion. In the two species, smooth muscle cells constitute a limiting layer between the subepithelium and the striated muscle compartment. The role of this layer is unclear.     

Fracture Faces in the Cell Envelope of Escherichia coli

Freeze-fracturing of Escherichia coli cells in the presence of 30% (v/v) glycerol resulted in a double cleavage of the cell envelope exposing two convex and two concave fracture faces ([Formula: see text], [Formula: see text] and [Formula: see text], [Formula: see text]) with characteristic patterns. Complementary replicas revealed the relationship of the fracture faces to their corresponding fracture planes. The inner fracture plane splits the plasma membrane at one particular level. Apparently the outer fracture plane was located in the outer part of the wall, as it was separated by a layer ([Formula: see text]) from the fractured profile (CW1) presumably corresponding to the murein layer. The outer fracture plane did alternate toward the cell periphery, exposing complementary smooth areas ([Formula: see text] and [Formula: see text]). When cells were freeze-fractured in the absence of glycerol, the outer cell surface appeared as an etching face rather than a fracture face. A schematic representation of the relative location of the different fracture faces in the E. coli cell envelope is given.     

The structure of the egg-shell of Porrocaecum ensicaudatum (Nematoda: Ascaridida)

Wharton D. A. 1979. The structure of the egg-shell of Porrocaecum enslcaudatum (Nematoda: Ascaridida). International Journal for Parasltology9: 127–131. The egg-shell of Porrocaecum ensicaudatum is oval with an opercular plug at either end. The shell consists of three layers: an inner lipid layer, a middle chitinous layer and an outer vitelline layer. The vitelline layer has strands of particulate material attached to its outer surface. The chitinous layer consists of 8.5 nrn fibrils which are made up of a chitin microfibril core surrounded by a protein coat. The fibrils are oriented randomly or in parallel, there being no indication of helicoidal architecture.The chitinous layer varies in thickness to form a pattern of interconnecting ridges on the surface of the egg. This pattern presumably increases the shell's structural strength.     

Location of the Fracture Faces Within the Cell Envelope of Acinetobacter Species Strain MJT/F5/5

The cell wall of the gram-negative bacterium Acinetobacter species strain MJT/F5/5 shows in thin section an external “additional” layer, an outer membrane, an intermediate layer, and a dense layer. Negatively stained preparations showed that the additional layer is composed of hexagonally arranged subunits. In glycerol-treated preparations, freeze-etching revealed that the cell walls consist of four layers, with the main plane of fracture between layers cw 2 and cw 3. The surface of [Formula: see text] 2 consisted of densely packed particles, whereas [Formula: see text] 3 appeared to be fibrillar. In cell envelopes treated with lysozyme by various methods, the removal of the dense layer has detached the outer membrane and additional layer from the underlying layers, as shown in thin sections. When freeze-etched in the absence of glycerol, these detached outer membranes with additional layers fractured to reveal both the faces [Formula: see text] 2 and [Formula: see text] 3 with their characteristic surface structures, and, in addition, both the external and internal etched surfaces were revealed. This experiment provided conclusive evidence that the main fracture plane in the cell wall lies within the interior of the outer membrane. This and other evidence showed that the corresponding layers in thin sections and freeze-etched preparations are: the additional layer, cw 1; the outer membrane, cw (2 + 3); and the intermediate and dense layers together from cw 4. Because of similarities in structure between this Acinetobacter and other gram-negative bacteria, it seemed probable that the interior of the outer membrane is the plane most liable to fracture in the cell walls of most gram-negative bacteria.     

Topographical relations of intramembrane particle distribution patterns in human sperm membranes

The distribution of intramembrane particles in human sperm membranes has been explored with particular reference to the topographical region of the sperm cell and the membranes' fracture face. Conspicuous differences in the size, arrangement, density, and lateral mobility of intramembrane particles between some topographically distinct membrane domains are demonstrated. The greatest regionality is exhibited by the plasma membrane. In sperm head regions, it shows a significant variability and changes its particle distribution during culture in capacitating medium. In contrast, little variability and no changes during the incubation are seen in the acrosomal and nuclear membranes. Striking is the difference in particle distribution on the E face of the outer acrosomal membrane between the acrosomal and equatorial regions. It is suggested that the invariable regional difference in the organization of the outer acrosomal membrane may bear on the different behavior of its two main domains during sperm capacitation and acrosome reaction.     

Histological organization of the intestine in the crayfish Procambarus clarkii

Six longitudinal ridges span the length of the intestine in the crayfish Procambarus clarkii. A simple columnar epithelium with tetralaminar cuticle lines the lumen. Folds of the epithelium overlie a dense irregular connective tissue packed with mixed acinar (alveolar) glands. Mucous secretions are probably involved with formation and lubrication of faecal strings; neither the nature nor the role of the serous secretions is immediately apparent. Aggregations of cells with large cytoplasmic vacuoles, called bladder cells, appear in the subepithelial connective tissue near the tops of the intestinal ridges. The bladder cells are suitably positioned to bolster the integrity of the ridges. Striated muscle of the intestine occurs in inner longitudinal and outer circular layers. The inner longitudinal layer consists of six strips, with one strip associated with the base of each intestinal ridge. The outer circular layer is essentially complete, but there are periodic apertures in this layer on the left and right sides of the intestine, providing nerves and haemolymph vessels with access to the interior of the gut. Based on histological features, and consistent with reports on other crayfish, we conclude that the intestine of P. clarkii has a proctodeal (ectodermal) origin.     

Intercellular junctions in myriapods

Tissue from the intestinal tract of myriapods, including millipedes, centipedes and pauropods were examined in tracer-impregnated sections and freeze-fracture replicas. The foregut and hindgut of all three classes exhibit pleated septate junctions; these display undulating intercellular ribbons in thin sections. In replicas they show discrete intramembranous particle (IMP) arrays aligned in rows in parallel; with one another. The tissues of the hindgut also possess scalariform junctions, characterized by cross-striated intercellular clefts in sections and IMP-enriched membranes in replicas. Gap junctions occur in all groups, but they are atypical in replicas in that their component IMPs do not always fracture onto the E face, as is characteristic of other arthropods; some IMPs cleave to the P face and others to the E face. The midgut of these organisms exhibits smooth septate junctions with conventional straight septal ribbons and occasional interseptal columns. However the intramembranous appearance in replicas is variable, particularly in centipedes, in that the rows of IMPs in chemically-unfixed propanecryofixed tissues, are prominent and adhere preferentially to the E face, with complementary P face grooves, while in fixed tissues the IMPs are much less distinct and fracture to either P face or E face. They tend not to protrude far beyond the mid-plane of the membrane bilayer and lie in rows which commonly take on the form of a network. Individual rows of the network sometimes curve to run beside a second row, over a short distance, before bending away into another part of the network. The aligned particle rows, which are much more prominent in millipedes, where they frequently lie in close parallel appositions, do not fuse into ridges as often occurs in insect tissues. The myriapod junctions, therefore, are of the same general kind as are found in the gut tract of other arthropod groups, but differ with respect to the subtleties of their intramembranous organization and disposition.