OBSERVATIONS ON THE FINE STRUCTURE OF SPHEROPLASTS OF RHODOSPIRILLUM RUBRUM

Spheroplasts of the photosynthetic bacterium Rhodospirillum rubrum were prepared from cultures grown in either the presence or absence of light. Cells were converted into spheroplasts by using lysozyme and Versene and fixed in a sucrose-veronal-acetate buffer mixture containing osmium tetroxide. Some preparations were shadow-cast and examined whole; others were embedded in Epon 812 and sectioned. The action of lysozyme and Versene appears to result in removal of the cell wall in strips. The relationship of the chromatophores to the cytoplasmic membrane is readily visualized in sections of broken spheroplasts, and in areas the chromatophores are seen to be continuous with the membrane. In all preparations examined, no definite connections between individual chromatophores were observed. In some cells large spherical granules were evident which either possessed or lacked a clearly visible limiting membrane. On serial sectioning, all granules appeared bounded by a single membrane 40 A wide. The granule membrane was well defined only if the section came from the center of the granule. Sections at other levels showed either a diffuse membrane or no membrane at all. The reasons for this are discussed.     

THE FINE STRUCTURE OF RHODOSPIRILLUM RUBRUM

The fine structure of Rhodospirillum rubrum grown under a series of defined conditions has been examined in thin sections prepared by the methods of Ryter and Kellenberger. In cells grown anaerobically at different light intensities, the abundance of 500 A membrane-bounded vesicles in the cytoplasm is inversely related to light intensity, and directly related to cellular chlorophyll content. When the chlorophyll content of the cell is low, the vesicles are exclusively peripheral in location; they extend more deeply into the cytoplasm when the chlorophyll content is high. Typical vesicles also occur, though rarely, in cells grown aerobically in the dark, which have a negligible chlorophyll content. When synthesis of the photosynthetic pigment system is induced in a population of aerobically grown cells by incubation under semianaerobic conditions in the dark, the vesicles become increasingly abundant with increasing cellular chlorophyll content, and the cells eventually acquire the cytoplasmic structure that is characteristic of cells growing anaerobically at a high light intensity. Poststaining with lead hydroxide reveals that the membranes surrounding the 500 A vesicles are indistinguishable in structure from the cytoplasmic membrane, and continuous with it in some areas of the sections. The bearing of these observations on current notions concerning the organization of the bacterial photosynthetic apparatus is discussed.     

OBSERVATIONS ON THE STRUCTURE OF RHODOSPIRILLUM MOLISCHIANUM

The lamellae of the bacterium Rhodospirillum molischianum originate as extensions of the cytoplasmic membrane into the cytoplasm of the cell. Initially, these extensions are narrow folds and occur independently of one another. The first lamellae to appear average about 80 A in width, representing one side of the infolded cytoplasmic membrane, or 160 A when the two sides of the fold are closely appressed. The 160-A lamellae increase in number and may associate to form larger lamellae, which represent varying degrees of association between adjacent folds. Later, the space within each fold increases; the two appressed regions of the cytoplasmic membrane in each fold separate to form distinct invaginations, and the lamellae observed at this stage are formed by an association of the sides of adjacent invaginations.     

FURTHER OBSERVATIONS ON THE STRUCTURE OF SELAGINELLITES CRASSICINCTUS

Leisman , Gilbert A. (Kansas State Teachers Coll., Emporia.) Further observations on the structure of Selaginellites crassicinctus. Amer. Jour. Bot. 48(3): 224–229. Illus. 1961.—Specimens of Selaginellites crassicinctus Hoskins and Abbott in coal balls from southeastern Kansas have yielded new information concerning the anatomy of this strobilus. The axis contains an exarch protostele apparently suspended in an air-cavity by endodermal trabeculae. Sporophylls are attached in alternating whorls of 4 each. The strobilus is compared with modern Selaginella and a related compression fossil species.     

OBSERVATIONS ON THE FINE STRUCTURE OF LUTEIN CELLS

Corpora lutea from the period of delayed implantation and from early postimplantation stages of the armadillo, mink, and rat were fixed in buffered osmium tetroxide-sucrose or potassium permanganate. After rapid dehydration, the portions of the corpora lutea were embedded in either methacrylate or epoxy resin. Examination of the lutein cells by electron microscopy revealed the presence, in the better preserved material, of an extensive development of tubular agranular endoplasmic reticulum. Although the membranes of the endoplasmic reticulum are the most striking feature of the lutein cells of both stages of the three animals examined, very numerous large mitochondria with cristae that exhibit a variety of forms tending toward villiform, and protrusions and foldings of the lutein cell margins on the pericapillary space are also characteristic of these cells. Certain minor differences in the lutein cells of the species examined are also noted. No indications of conversion of mitochondria into lipid, of accumulation of lipid in the Golgi area, or of the protrusion of lutein cells into spaces between the endothelial cells, as suggested by other authors, were noted in these preparations. Some of the difficulties inherent in the visualization of the secretory activity of cells producing steroid hormones are briefly discussed.     

OBSERVATIONS ON THE FINE STRUCTURE OF CALLIXYLON WOOD

Secondary wood of three species of Callixylon of Lower Mississippian age, preserved by three different modes (fusinization, silicification, and phosphatization), have been studied and characterized in detail. Problems of interpretation at the SEM level of permineralized woods, both containing (silicified wood of Callixylon erianum) and essentially lacking (phosphatized wood of C. arnoldii) original organic cell wall material, are analyzed and discussed. In particular, it is concluded that the flat to curved surfaces showing no evidence of apertures, observed in bordered pit pairs, commonly represent pit membrane surfaces. It is accepted, however, that some concave surfaces might be the mineral accretion surfaces of incomplete pit cavity casts as proposed by Leo and Barghoorn (1976). Regions between groups of pits, previously interpreted as crassulae, may be artifacts of preservation. The fusinized wood has the general appearance of charcoal, but unlike commercially produced charcoal provides evidence of its original microfibrillar structure. The origin of fusain in the fossil record is discussed, and it is concluded that it probably had several origins, including forest fire. Since charcoal can be produced naturally in the absence of O₂ (Brown and Davis, 1973), the suggestion that fusain (charcoal) in the geologic column provides a basis for “assessing oxygen levels in paleoatmospheres” (Cope and Chaloner, 1980) is not supported. Natural sites of fusain production in the absence of O₂ are regions of vulcanism and organic sediments inhabited by anaerobic microorganisms. A circular pattern of crystal orientation in the pit borders of C. arnoldii is interpreted to represent the original microfibrillar pattern. Pit apertures in C. arnoldii are shown to be circular to slightly elliptical. Interpretive evidence of heterogeneous pit membranes in C. arnoldii suggests but does not prove the presence of a torus. The distinctive central region in some pit membranes of the fusinized wood of Callixylon sp. might represent accumulations of waste metabolites. It is argued that a torus would be highly adaptive in large pits with circular apertures.     

OBSERVATIONS ON THE FINE STRUCTURE OF THE MACRONUCLEUS OF TOKOPHRYA INFUSIONUM

The macronucleus in Tokophrya infusionum is composed of numerous Feulgen-positive chromatin bodies (about 0.5 µ in diameter) which appear in thin sections as a dense spongework, homogeneous throughout. The same appearance characterizes metaphase chromosomes of higher forms. Some chromatin bodies of the macronucleus were found to possess a highly organized structure in certain old organisms. This structure appears in cross-sections as a honeycomb and in longitudinal sections as parallel lines about 120 A in diameter evenly spaced (about 230 A). As far as is known this is the first time a regular structure has been found in bodies of chromosomal character at the dimensional level presently explored by electron microscopy. The demonstration that OsO₄ can preserve order in chromatin material is another significant aspect of these findings.     

FURTHER OBSERVATIONS ON THE FINE STRUCTURE OF THE PARIETAL EYE OF LIZARDS

An electron microscopical study of the third eye of the Western Fence Lizard, Sceloporus occidentalis, fixed with 1 per cent osmium tetroxide, pH 7.4–7.6, for 16 to 20 hours at 0°C., revealed the following new facts. The fibrillar system of the retinal photoreceptor consists of nine double fibrils enclosed in a sheath. Pigment cells and lens cells possess similar systems. Two short cylindrical centrioles are associated with the fibrillar apparatus: one, from which striated rootlets extend inward, lies at the base of the fibrils, with the other at an oblique angle to the axis of the system. A Golgi complex, whorls of endoplasmic reticulum, lipid (?) droplets, and other organelles and inclusions in the photoreceptors are described. An axon leads from the base of the photoreceptor into the nervous layer of the retina which consists of many nerve fibers and large ganglion cells. Although the pattern of neural connections has not yet been determined, some synapses were found. The parietal nerve consists of about 250 non-medullated fibers. The capsule of the eye usually has a layer of iridocytes, which contain rows of guanine (?) rods. A few parietal eyes of the Granite Night Lizard, Xantusia henshawi, were also examined. Large lipid (?) droplets occur in the bases of their receptoral processes.     

SOME OBSERVATIONS ON THE FINE STRUCTURE OF THE SYMPATHETIC GANGLION OF BULLFROG

Electron microscope observations of abdominal sympathetic ganglia of American bullfrogs, Rana catesbiana, have demonstrated the presence of specific areas of cytoplasm in the superficial zone of the perikaryon which are devoid of granulated endoplasmic reticulum. These areas are occupied almost exclusively by granules 200 to 400 A in diameter which can be stained intensely with lead hydroxide but faintly with uranyl acetate. Each granule shows subgranular internal structure after the lead staining. Granules of similar properties are found in synapses also, and may be glycogen. From the satellite cell there extends a number of leaf- or finger-like cytoplasmic projections around the root portion of the nerve process. Some of these projections directly cover the surface of the nerve process. Many others, however, are separated from the neuron by a fairly wide interspace. Multivesicular bodies of the neuron are occasionally observed in a configuration which suggests that they are being extruded from the root of the nerve process into the interspace. Filaments about 100 A in thickness are found in the satellite cell cytoplasm. They are arranged more or less parallel to each other and are especially well developed around synapses and nerve fibers.     

ELECTRON MICROSCOPE OBSERVATIONS ON THE STRUCTURE AND DISCHARGE OF THE STENOTELE OF HYDRA

Sections of the stenotele type of nematocyst of Chlorohydra hadleyi have revealed that the stenotele, upon firing, completely everts its stylets and spines and the long, thin tubule, much as the eversion of the tubule of the nematocyst of the jewel anemone (Picken, 1953; Robson, 1953). Alternative mechanisms for supplying the energy necessary to forcefully discharge the stenotele contents are discussed as well as the possible significance of several regions containing highly ordered periodic structure. The origin of nematocysts as kinetosomal derivatives is discussed as a possibility suggested by the symmetry of the stenotele contents and the structure, location, and function of the cnidocil.     

FURTHER OBSERVATIONS ON THE FINE STRUCTURE OF THE MACRONUCLEUS IN TOKOPHRYA INFUSIONUM

In a previous paper (8) an organized structure was described in the macronuclei of certain old organisms of Tokophrya infusionum. It was found that the same honeycomb structure appears in great abundance in the macronuclei of overfed organisms. This permitted a better three-dimensional reconstruction of the described structure. Since the defined structure may be experimentally induced, it offers an opportunity for further more detailed studies as to its nature and meaning.     

OBSERVATIONS ON THE FINE STRUCTURE OF THE DEVELOPING SPERMATID IN THE DOMESTIC CHICKEN

The kinetic apparatus, the acrosome and associated structures, and the manchette of the spermatid of the domestic chicken have been studied with the electron microscope. The basic structural features of the two centrioles do not change during spermiogenesis, but there is a change in orientation and length. The proximal centriole is situated in a groove at the edge of the nucleus and oriented normal to the long axis of the nucleus and at right angles to the elongate distal centriole. The tail filaments appear to originate from the distal centriole. The plasma membrane is invaginated along the tail filaments. A dense structure which appears at the deep reflection of the plasma membrane is identified as the ring. The fine structure of the ring has no resemblance to that of a centriole and there is no evidence that it is derived from or related to the centrioles. The tail of the spermatid contains nine peripheral pairs and one central pair of tubular filaments. The two members of each pair of peripheral filaments differ in density and in shape: one is dense and circular, and the other is light and semilunar in cross-section. The dense filaments have processes. A manchette consisting of fine tubules appears in the cytoplasm of the older spermatid along the nucleus, neck region, and proximal segment of the tail. The acrosome is spherical in young spermatids and becomes crescentic and, finally, U-shaped as spermiogenesis proceeds. A dense granule is observed in the cytoplasm between acrosome and nucleus. This granule later becomes a dense rod which is interpreted as the perforatorium.     

SOME OBSERVATIONS ON THE FINE STRUCTURE OF THE LUTEIN CELLS OF X-IRRADIATED RAT OVARY

The ovaries of 10- to 13-week-old rats were exteriorized and irradiated with sterilizing doses of X-rays. Following treatment, the animals entered a phase of constant vaginal cornification. Animals were killed 8 to 12 wk after the onset of this phase, and their ovaries were prepared for electron microscopy. Tissue was fixed in glutaraldehyde, postfixed in osmium tetroxide (Millonig's, phosphate-buffered), and embedded in Epon. Lutein cells from these ovaries were compared with those from sham-irradiated controls. The cytoplasm of lutein cells from experimental animals was characterized by an increase in the amount of agranular endoplasmic reticulum and by an increase in the number of mitochondria. These mitochondria are more variable in external form and often possess increased numbers of villiform cristae. Other features noted were a decrease in the amount of cytoplasmic lipid granules and an increase in cell size and surface irregularity. The significance of the morphological findings is discussed in relation to ovarian hormone production in animals sterilized by X-irradiation.     

OBSERVATIONS ON THE FINE STRUCTURE OF PHARYNGEAL MUSCLE IN THE PLANARIAN DUGESIA TIGRINA

Pharyngeal muscle of the planarian Dugesia tigrina was studied by electron microscopy after osmium tetroxide fixation. The muscle cell was observed to contain one myofibril or bundle of myofilaments parallel to its longitudinal axis. The myofilaments were of two types, different in size and distribution. No Z lines or myofilament organization into cross or helical striations were seen. Dense bodies were seen as projections from an invagination of the plasma membrane and as dense lines parallel to the myofilaments. The muscle cells are surrounded by a plasma membrane which is structurally associated with dense body projections, with vesicles and cisternae of sarcoplasmic reticulum, and with synaptic nerve endings. The cell has sarcoplasmic projections perpendicular to its long axis; these projections are seen to contain the nucleus or mitochondria and granules. Mitochondria and granules are also seen in a sarcoplasm rim around the fibril. The dense bodies may serve as attachment for thin myofilaments and function in transmission of stimuli from plasma membrane to the interior of the fibril.     

FURTHER OBSERVATIONS ON THE STRUCTURE OF MYELIN SHEATHS IN THE CENTRAL NERVOUS SYSTEM

Direct evidence has been presented to confirm the existence of a spiral in the myelin sheaths of the central nervous system. An account of some of the variations in structure of central myelin sheaths has been given and it has been shown that the radial component of myelin sheaths has the form of a series of rod-like thickenings of the intraperiod line. These thickenings extend along the intraperiod line in a direction parallel to the length of the axon. The relative position of the internal mesaxon and external tongue of cytoplasm has been determined in a number of transverse sections of sheaths from the optic nerves of adult mice, adult rats, and young rats. In about 75 per cent of the mature sheaths examined, these two structures were found within the same quadrant of the sheath, so that the cytoplasm of the external tongue process tends to lie directly outside that associated with the internal mesaxon. The frequency with which the internal mesaxon and external tongue lie within the same quadrant of the sheath increases both with the age of the animal and with the number of lamellae present within a sheath. The possible significance of these findings is discussed.