AN ELECTRON MICROSCOPE STUDY OF THE CYTOLOGY OF THE PROTOZOAN EUPLOTES PATELLA

1. Structurally the "sensory bristles" in Euplotes patella are typical cilia, but no ciliary rootlets connect their bases. 2. The "neuromotor fibrils" are composed of filaments 21 mµ in diameter. At the point of junction of the filaments with the peripheral ciliary fibrils a granular structure 65 to 90 mµ in diameter is seen which has dense central and peripheral zones separated by a less dense layer. Information on the interconnection of organelles is expanded. 3. A system of subpellicular fibrils is described. The external fibrillar system described by others could not be found. 4. The motorium is shown to be a mass of intertwining rootlet filaments. 5. The micronucleus is shown to have a spongy, dense material in a less dense material, all of which is surrounded by a double-layered membrane. 6. The double-layered macronuclear membrane contains annuli whose outside diameter is 70 mµ; the macronuclear bodies are sometimes closely applied to the membrane. In the macronuclear reorganization bands, the solution plane is a fine network, while the reconstruction plane is devoid of structure at the level of resolution observed. 7. The mitochondria are composed of tubules, only occasionally oriented, usually embedded in a surrounding material of lower density. 8. Microbodies whose diameters are 250 to 350 mµ are frequently observed in close association with mitochondrial surfaces. 9. The food vacuoles, contractile vacuoles, and ciliary vacuoles are bounded by single-layered membranes. In the food vacuoles, the bacteria are surrounded by membranes individually or in small groups. 10. Cytoplasmic rods localized in the oral region, and cytoplasmic granules dispersed at random, are described. No typical ergastoplasm, endoplasmic reticulum, or Golgi material was observed.     

THE FINE STRUCTURE AND FUNCTION OF THE FOOT OF NASSARIUS KRAUSSIANUS, A GASTROPOD MOVING BY CILIARY LOCOMOTION

Light and electron microscope studies of the oval foot of Nassariuskraussianus (Dunker) showed that the columellar muscle is asingle extrinsic muscle and dorsoventral, transverse and longitudinalfibres are the principal intrinsic muscles. The fine structureof the columellar muscle and the intrinsic musculature is describedand shown to be similar to that of other gastropods. The presenceof abundant mitochondria in a snail moving by cilia may be relatedto the rapid and powerful movements of the foot made in escapingpredators. All pedal muscle consists of smooth muscle fibresand collagen was observed to be particularly associated withthe intrinsic fibres. The foot maintains a constant area ofpedal sole during ciliary locomotion by means of antagonismbetween the intrinsic muscle fibres. The possible role of collagenin this process is discussed. (Received 6 March 1989; accepted 15 June 1989)     

THE FINE STRUCTURE OF OOGENESIS IN MARCHANTIA POLYMORPHA

Archegonium development, beginning with the archegonial initial and culminating in the mature egg, was studied with the electron microscope. The ultrastructural features of the beginning stages in development of the archegonium are relatively similar to one another. Plasmodesmata occur between all adjacent cells at this time. After the secondary central cell is formed these protoplasmic connections are lost, and both axial and parietal cell lineages begin to show signs of ultrastructural differentiation. The mature egg is characterized by cytoplasm rich in ribosomes and larger organelles. Mitochondria and simplified plastids commonly display a juxtaposed association. As far as could be ascertained the numerous plastids and mitochondria in the egg of Marchantia arise through division of preexisting organelles and are not formed anew from evaginations of the nucleus. Blebbing of the nucleus produces polymorphic organelles which appear to be pinched off into the cytoplasm. The mature egg also contains vacuoles and lipid bodies toward its periphery, while dictyosomes and extensive endoplasmic reticulum occur throughout. The space between the wall cells and the mature egg appears to contain an amorphous substance. No extra membrane was observed around the mature egg.     

FINE STRUCTURE OF CHROMOSOMES

Electron micrographs of staminate hair cells of Tradescantia reflexa indicate that early prophase chromosomes are composed of a number of helically arranged chromonemata. Favorable preparations reveal as many as 64 identifiable subsidiary strands, assumedly arranged as intertwined pairs to form a hierarchy of pairs of pairs. The helices of the smallest discernible units have a diameter of about 125 A, with highly electron-scattering material disposed peripherally around a less dense "core." The wall of this peripheral ring has a thickness of about 40 A, and apparently represents another pair of coiled threads surrounding a 40 A central axis. The implications of the findings are discussed briefly.     

DEVELOPMENT OF CHLOROPLAST FINE STRUCTURE IN ASPEN TISSUE CULTURE

Chloroplast ontogeny has been examined in 42-day etiolated triploid aspen callus (Populus tremuloides Michx.) subjected to two different light conditions. White and low-intensity red illumination showed little differences in their stimulatory effects on plastid development, the red light-irradiated plastids developing only slightly more slowly. Asynchronous plastid development was noted in both lighting systems. Etioplasts contained an interconnected tubular net, phytoferritin aggregates, electron-transparent vesicles which seem to invaginate from the inner plastid membrane, membrane-bound homogeneous spheroids and starch grains. Irradiation caused various morphological changes within the proplastids; the tubular complex became transformed into the more ordered prolamellar body-like structure from which radiated membrane-bound sacs filled with electron-dense material. These sacs, characterized as thylakoid precursors, were transformed into a thylakoidal system typical of mature chloroplasts. This ontogenetic scheme represents an additional pathway for the development of photosynthetic lamellae. Other light-induced changes in the developing plastid include disappearance of phytoferritin particles and homogeneous spheroids, decrease in starch content, and appearance of osmiophilic droplets.     

THE FINE STRUCTURE OF SYMPATHETIC NEURONS IN X-IRRADIATED FROGS

The effects of whole body x-irradiation on the fine structure of sympathetic neurons were studied in 15 unanesthetized adult frogs (Rana pipiens), as seen at intervals ranging from 1 hour to 2 weeks after single exposures to 1000 r and 2000 r. Using standard procedures, the lumbar sympathetic ganglia of experimental and 20 control animals were prepared for electron microscope examination. Radiation produced conspicuous but irregular and variable deterioration, swelling, and clearing of neuronal lysosomes. These changes may have been due to an increased permeability of lysosomal membranes, causing the entry of fluid into lysosomes and their swelling and deterioration, but a pronounced escape of lysosomal enzymes into the cytoplasm was questionable. Less frequent were the dilatation and the parallel layering or complete fusion and tight packing of the rough-edged endoplasmic reticulum. The number of vacuoles, probably derived from Golgi cisternae, was somewhat increased. These vacuoles were conjectured to serve the "sequestration" of damaged cytoplasmic areas. Abnormal amounts of presumptive glycogen granules occupied some axons of myelinated and unmyelinated fibers, especially of presynaptic nerve fibers. This was assumed to be due to a decreased breakdown of glycogen and probably caused the interruption of the transmission of nerve impulses in presynaptic fibers. The maximal incidence of these alterations seemingly occurred 8 days after exposure to 1000 r, and 1 hour after x-irradiation with 2000 r. Signs of recovery appeared 2 weeks after exposure to 2000 r.     

FINE STRUCTURE OF LOACH OOCYTES DURING MATURATION IN VITRO

The morphological changes during in vitro maturation of Misgurnus anguillicaudatus oocyte are described. The process of oocyte maturation can be divided into three provisional stages based on morphological events. Fully-grown, immature oocytes are opaque yellowish-white. The morphological characteristics of their ooplasm are the existence of annulate lamellae, a mass of long mitochondria and an electron dense layer beneath the vitelline surface. Three hr after a 1 hr exposure to corticosterone, these structures disappear and the cortical ooplasm becomes semi-transparent. In this stage of the maturation process (Stage I), the germinal vesicle, without a nucleolus, moves toward the animal pole, and scattered cytoplasmic inclusions approach the vitelline surface. Six hr after exposure to the hormone (Stage II), the whole ooplasm becomes semi-transparent and large yolk platelets are seen in the animal pole region. Tubular endoplasmic reticula develop throughout the ooplasm and some cortical alveoli (CA) become aligned beneath the vitelline surface. Nine hr after exposure to the hormone (Stage III), the oocyte chorion separates from the follicle cells. Most CA align beneath the vitelline surface and cytoplasm accumulates in the cortical region of the animal hemisphere.     

THE FINE STRUCTURE OF INHIBITORY SYNAPSES IN THE CRAYFISH

Physiological investigations have shown that the synaptic input to the sensory neuron of the stretch receptor in the abdominal muscles of the crayfish is purely inhibitory. This neuron was chosen, therefore, as a site in which to study the fine structure of inhibitory synaptic endings. It was hoped that this fine structure might (a) provide a morphological prototype for the study of more complex synaptic systems and (b) reflect the inhibitory mechanisms. Stretch receptors were fixed in situ in buffered OsO₄, dehydrated, and embedded in Araldite. Both cross and longitudinal sections were examined after staining with phosphotungstic acid. The inhibitory endings were easily identified by their great similarity to previously described excitatory endings. Small circular profiles (synaptic vesicles) about 460 A in diameter and an accumulation of mitochondria were consistently observed within the presynaptic endings. An increased osmiophilia of pre- and postsynaptic membranes, where they were in apposition, was also seen. The only observed difference between these inhibitory endings and excitatory endings, described by other authors, was the variable presence of a latticework of 230 A tubules in the connective tissue immediately adjacent to the inhibitory endings. Inhibitory endings were observed on all parts of the sensory neuron except the axon.     

THE FINE STRUCTURE OF ACOUSTIC GANGLIA IN THE RAT

Nerve cell bodies in the spiral and vestibular ganglia of the adult rat are surrounded by thin (about ten lamellae) myelin sheaths which differ in several respects from typical axonal myelin. In some instances lamellae surrounding perikarya appear as typical major dense lines, and in others as thin Schwann cell sheets in which cytoplasm persists. Discontinuities and irregularities appear in the structure of perikaryal myelin. Lamellae may terminate anywhere within the sheaths; they may bifurcate; they may reverse their direction; or they may merge with each other. The number of lamellae varies from one part of a sheath to another. In addition, the myelin of a single perikaryal sheath may receive contributions from more than one Schwann cell, which overlap and interleave with each other. The ganglion cells are of two types: those which are densely packed with the usual cytoplasmic organelles but have few neurofilaments (granular neurons), and those which exhibit large areas containing few organelles but have a high concentration of neurofilaments (filamented neurons). The latter cell type is ensheathed by myelin which is generally more compact that that surrounding the former. The formation and the physiologic significance of perikaryal myelin are discussed.     

THE FINE STRUCTURE OF BRUNNER''S GLANDS IN THE MOUSE

Examined with the electron microscope, the secretory cells of the submucosal glands of Brunner in the mouse present a curious combination of the fine-structural features of both serous and mucus-secreting cells. The cells have numerous mitochondria, abundant basal ergastoplasm, dense secretory granules that bear a superficial resemblance to pancreatic zymogen granules, and an unusually extensive Golgi apparatus. The prominence of the lamellar, vesicular, and vacuolar elements of the Golgi complex facilitates detailed observation of these components. More evident than in other glandular cells, aggregates of small vesicles appear to represent the transitional elements and are vehicles for transport of the product between the ergastoplasm and the Golgi complex. The numerous vesicular evaginations of smooth-surfaced regions on cisternae of the rough-surfaced endoplasmic reticulum and accumulations of innumerable vesicles of similar size in the area between the nearest profiles of the ergastoplasm and the Golgi complex support this contention. The cytological characteristics and physiologic properties of Brunner's glands in various species are discussed briefly. It is concluded that the submucosal glands of the mouse are excellent material for exploration of the ultrastructural correlates of both protein and carbohydrate secretion, and it is suggested that their secretion may have functions other than those generally attributed to them, namely, chemical and mechanical protection of the duodenal surface epithelium.     

FINE STRUCTURE OF PROTEIN-STORING PLASTIDS IN BEAN ROOT TIPS

The fine structure of leucoplasts in root tip cells of Phaseolus vulgaris L. has been studied in material fixed in glutaraldehyde followed by osmium tetroxide and poststained in uranyl acetate and lead citrate. Plastid development has been followed from the young stages in and near the meristematic region, through an ameboid stage, to the larger forms with more abundant storage products in the outermost cells. The plastids contain a dense stroma penetrated by tubules and cisternae arising from the inner membrane of the plastid envelope. Also located in the stroma are lamellae, ribosome-like particles, phytoferritin granules, and fine fibrils in less dense regions. In some elongate plastids microfilaments run lengthwise in the stroma near the surface. The same plastids store both starch and protein, but in a strikingly different manner. The starch is deposited in the stroma, while the protein always is accumulated within membrane-bounded sacs. These sacs arise as outgrowths from a complex of interconnected tubules which in turn appears to originate by coalescence and proliferation of tubules and cisternae arising from the inner plastid membrane. This "tubular complex" bears a strong resemblance to the prolamellar body of etiolated chloroplasts, but is smaller and ordinarily less regularly organized, and is apparently light-insensitive. Crystallization of the protein commonly occurs in the sacs and occasionally takes place within the tubules of the complex as well. The fine structure of the leucoplasts is discussed in relation to that of etiolated chloroplasts. Suggestions are made concerning the function of the tubular complex, role of the ameboid plastid forms, and manner of accumulation of the storage protein in the plastids.     

THE FINE STRUCTURE OF MITOSIS IN RAT THYMIC LYMPHOCYTES

The fine structure of rat thymic lymphocytes from early prophase to late telophase of mitosis is described, using material fixed at pH 7.3 either in 1 per cent OsO4 or in glutaraldehyde followed by 2 per cent OsO4. The structure of the centriolar complex of interphase thymocytes is analyzed and compared with that of centrioles during division. The appearance of daughter centrioles is the earliest clearly recognizable sign of prophase. Daughter centrioles probably retain a secondary relation to the primary centriole, while the latter appears to be related, both genetically and spatially, to the spindle apparatus. The nuclear envelope persists in recognizable form to help reconstitute the envelopes of the daughter nuclei. Ribosome bodies (dense aggregates of ribosomes) accumulate, beginning at late prophase, and are retained by the daughter cells. Cytokinesis proceeds by formation of a ribosome-free plate at the equator with a central plate of vesicles which may coalesce to form the new plasma membrane of the daughter cells. Stages in the formation of the midbody are illustrated.     

蜱类须肢感器的细微结构

用扫描电镜观察了1种软蜱和4属8种硬蜱的成虫以及血蜱属3种若虫的须肢感器的外部形态;同时用透射电镜观察了波斯锐缘蜱、中华革蜱和亚洲璃眼蜱3种成虫须肢感器的内部结构.结果表明,须肢感器的外部形态在硬蜱各属成虫之间存在着一定的差异,主要表现在感器形状、顶端感毛数目、类型及形态,基部感毛数目及形态等几个方面.同属各种间差异不大.血蜱属幼期和波斯锐缘蜱幼期之间也有一定的差异,主要表现在感器形状、顶端感毛数目以及基部感毛数目、形态和排列方式.须肢顶端感毛的内部结构在三种蜱中都可分为A、B两种类型.A型具两个腔:其一为圆形腔,内有树突;另一为新月形腔,无树突.靠近感毛顶端有通道通向外界.B型感毛在三种蜱中差异较大,其共同点是只具有一个含有树突的腔.     

FINE STRUCTURE OF NUCLEAR CRYSTALS IN LEAVES OF TWO DIANTHUS SPECIES

Examination of leaf cells of Dianthus barbatus with the election microscope revealed the presence of crystals in the nuclei, but not in the cytoplasm or in other organelles. They were at first thought to be viral crystalline inclusions, but no evidence could be adduced to support this possibility. The crystals were found in the nuclei of cells from all parts of the leaf, including parenchyma cells of the vascular elements. They were usually 0.5–2 μ long, and occasionally up to 5 μ. In longitudinal section they appeared as electron-dense striations, about 10–15 mμ wide, alternating with electron-transparent bands, 6–8 mμ wide. In cross section they had a honeycomb appearance, resulting from closely packed hexagonal elements, with electron-transparent centers. Similar crystals were found in nuclei of D. chinensis leaf cells, but not in other members of the Caryophyllaceae, including two varieties of D. caryophyllus, D. deltoides, D. arenarius, Silene acaulis, Lychnis chalcedonica, Saponaria calibrica, and Stellaria media. This appears be the first fine structural report of a lattice inclusion in nuclei of cells of healthy higher plants.     

FINE STRUCTURE OF PACINIAN CORPUSCLES IN THE MESENTERY OF THE CAT

Pacinian corpuscles in the mesentery of adult cats were fixed with either glutaraldehyde, osmium tetroxide or permanganate solutions by close intra-arterial injection through the mesenteric artery, and were processed, after electron staining and Epon embedding, for electron microscopy. Better resolution of the corpuscle's ultrastructure was obtained than available heretofore. The myelinated segment of the corpuscle contains blood vessels separated from the axon by collagen fibers and 3 to 4 layers of lamellae. No blood vessels are found in the central core, though access from the vessels is afforded by diffusion through the "cleft" of the inner core. Two cell types are discernible in the inner core hemilamellae; the "clear cells" in which pinocytotic vesicles and organelles abound and reflect the greater metabolic activity of these cells, in contrast to the "dark cells." The ultraterminal is ellipsoidal in form with projections into the "cleft" which give this portion an irregular appearance in section. The terminal and ultraterminal are packed with mitochondria, and "synaptic" vesicles are seen in the ultraterminal. The innermost laminae of the inner core cells are in close apposition to the terminal and break their regular pattern of hemilamellation to surround the small ultraterminal projections at the apical part of the corpuscle.     

FINE STRUCTURE OF EPITHELIAL CANAL CELLS IN PETIOLES OF XANTHIUM PENSYLVANICUM

Secretory canals, lined with an epithelium, occur in many families, e.g., Umbelliferae, Compositae. These canals are said to extend continuously through the root and shoot systems and are known, in some cases, to secrete resins, essential oils, etc. In Xanthium the canals arise schizogenously from cells derived from canal initials. Subsequent divisions lead to a ring of 7–12 epithelial cells surrounding a central cavity. During maturation the epithelium becomes crushed and obliterated. Canals were examined in petioles of Xanthium pensyhanicum (Cocklebur) grown under long day illumination to maintain vegetative growth. The fine structure of the canal and its epithelium was studied by electron microscopy of thin sections cut transverse to the principal axis of petioles from leaves in an early stage of development. The canal proper is delimited by walls of epithelial cells which protrude into a scallop shaped cavity. In comparison to the surrounding parenchyma, the epithelial cells are smaller, cytoplasmically more dense, and less vacuolate. The epithelium contains pleomorphic starch-free plastids with planar thylakoids frequently stacked into grana; thus, the plastids are presumed photosynthetically active. Mitochondria are abundant and often dense. The cytoplasm is rich in free polysomes, and smooth endoplasmic reticulum predominates over the rough form. Spheroidal granules averaging about 530 nm in diameter are numerous in the epithelium and appear at lower concentration in neighboring cells. Many features of fine structure of the epithelial cells suggest that a high metabolic activity is present in this tissue during this early stage of development. A possible function of the canals is defense against insect predation and animal grazing.