THE FINE STRUCTURE OF SIEVE ELEMENTS OF NEREOCYSTIS LÜTKEANA

The sieve elements of Nereocystis from the base of phylloids contain numerous small vesicles, cytoplasm, ribosomes, and the usual organelles and membrane systems, including nuclei, plastids, mitochondria, dictyosomes, and endoplasmic reticulum. They have a thick secondary wall layer which is deposited along the longitudinal walls and at the sieve plate excluding the sieve pores. The sieve pores range in diameter from 100 to 400 nm and are lined by plasmalemma. The sieve elements from the hollow basal parts of the pneumatocyst show essentially the same features but have larger and fewer vesicles, relatively little cytoplasm, larger sieve pores, 400–900 nm in diameter, and may lack a nucleus. In old sieve elements there are large deposits of callose on the sieve plate and along the longitudinal wall; the vesicles seem to break down, and the protoplast appears necrotic. It is concluded that the trumpet hyphae and sieve tubes are basically the same type of cell, and that the trumpet-shape of the sieve elements is due to their passive stretching during extension growth of the organ in which they occur. There are minor but significant differences among the sieve elements from different regions of the thallus which may reflect possible levels of structural specialization of the sieve elements within the same plant.     

FINE STRUCTURE OF DIFFERENTIATING MOUSE PANCREATIC EXOCRINE CELLS IN TRANSFILTER CULTURE

Fine structural observations have been made in the 11-day embryonic mouse of exocrine cells in pancreatic epithelium developing in tissue culture transfilter from salivary gland mesenchyme of the 13-day embryonic mouse. After 2 days in culture, the exocrine cells show increased cytoplasmic density, abundant ribosomes in aggregate or "rosette" form, and expanded profiles of rough-surfaced endoplasmic reticulum. After 3 and 4 days in culture, the cells exhibit continued expansion of the profiles of endoplasmic reticulum, increased amounts of Golgi membranes, and large areas of light density (prozymogen granules). After 5 days in culture, dense zymogen granules are present in the most highly differentiated cells. In addition, at the filter-epithelial surface, at 2 days, small fibers can be discerned which, after 4 days in culture, show obvious periodicity and are thought to be collagen. The significance of these changes, in relation to the mesenchymal effect, to the onset of specific synthesis and to the stabilization of differentiation is discussed.     

THE FINE STRUCTURE OF DIFFERENTIATING SPERMATOZOA AND SERTOLI CELLS IN THE GONAD OF THE POND SNAIL, LYMNAEA STAGNALIS

The fine structure has been examined, of spermatogonia, spermatocytes,early spermatids, late spermatids and early spermatozoa nestlingagainst Sertoli cells in the gonad of Lymnaea stagnalis. Changes in the Sertoli cells are linked with the phases of spermdifferentiation. Details on differentiation particularly ofthe head of the sperm, are presented. (Received 14 March 1981;     

THE FINE STRUCTURE OF NEURONS

1. Thin sections of representative neurons from intramural, sympathetic and dorsal root ganglia, medulla oblongata, and cerebellar cortex were studied with the aid of the electron microscope. 2. The Nissl substance of these neurons consists of masses of endoplasmic reticulum showing various degrees of orientation; upon and between the cisternae, tubules, and vesicles of the reticulum lie clusters of punctate granules, 10 to 30 mµ in diameter. 3. A second system of membranes can be distinguished from the endoplasmic reticulum of the Nissl bodies by shallower and more tightly packed cisternae and by absence of granules. Intermediate forms between the two membranous systems have been found. 4. The cytoplasm between Nissl bodies contains numerous mitochondria, rounded lipid inclusions, and fine filaments.     

THE FINE STRUCTURE OF GREEN BACTERIA

The fine structure of several strains of green bacteria belonging to the genus Chlorobium has been studied in thin sections with the electron microscope. In addition to having general cytological features typical of Gram-negative bacteria, the cells of these organisms always contain membranous mesosomal elements, connected with the cytoplasmic membrane, and an elaborate system of isolated cortical vesicles, some 300 to 400 A wide and 1000 to 1500 A long. The latter structures, chlorobium vesicles, have been isolated in a partly purified state by differential centrifugation of cell-free extracts. They are associated with a centrifugal fraction that has a very high specific chlorophyll content. In all probability, therefore, the chlorobium vesicles are the site of the photosynthetic apparatus of green bacteria.     

THE FINE STRUCTURE OF ELASTIC FIBERS

The fine structure of developing elastic fibers in bovine ligamentum nuchae and rat flexor digital tendon was examined. Elastic fibers were found to contain two distinct morphologic components in sections stained with uranyl acetate and lead. These components are 100 A fibrils and a central, almost amorphous nonstaining area. During development, the first identifiable elastic fibers are composed of aggregates of fine fibrils approximately 100 A in diameter. With advancing age, somewhat amorphous regions appear surrounded by these fibrils. These regions increase in prominence until in mature elastic fibers they are the predominant structure surrounded by a mantle of 100 A fibrils. Specific staining characteristics for each of the two components of the elastic fiber as well as for the collagen fibrils in these tissues can be demonstrated after staining with lead, uranyl acetate, or phosphotungstic acid. The 100 A fibrils stain with both uranyl acetate and lead, whereas the central regions of the elastic fibers stain only with phosphotungstic acid. Collagen fibrils stain with uranyl acetate or phosphotungstic acid, but not with lead. These staining reactions imply either a chemical or an organizational difference in these structures. The significance and possible nature of the two morphologic components of the elastic fiber remain to be elucidated.     

THE FINE STRUCTURE OF ANNULATE LAMELLAE

Certain lamellar structures have been described from snail (Otala) and clam (Spisula) oocytes, the acinar cells of amphibian (Ambystoma) pancreas, and from rat spermatids. These structures are alike in possessing numerous rings or annuli, resembling those in the nuclear membrane. Thus the name "annulate lamellae" has been proposed for them. It is suggested that they may function in the transfer of specificities from nucleus to cytoplasm.     

THE FINE STRUCTURE OF THE PURKINJE CELL

This paper describes the fine structure of the Purkinje cell of the rat cerebellum after fixation by perfusion with 1 per cent buffered osmium tetroxide. Structures described include a large Golgi apparatus, abundant Nissl substance, mitochondria, multivesicular bodies, osmiophilic granules, axodendritic and axosomatic synapses, the nucleus, the nucleolus, and the nucleolar body. A new and possibly unique relationship between mitochondria and subsurface cisterns is described. Possible functional correlations are discussed.     

FINE STRUCTURE OF THE CILIA OF ROTIFERS

The fine structure of the coronal cilia of the rotifer Philodina citrina has been studied in detail. Specimens were fixed with OsO₄ and embedded in butyl—methyl methacrylate, Epon 812, or Vestopal and sectioned with a Porter-Blum microtome. The details of structure of the rootlets, basal bodies, basal plates, and free cilia are described. The general structure of the rotifer ciliary apparatus conforms well to that established for other species. One of the main observations is the difference in structure of the peripheral filaments in the opposing halves of a cross-section of the free cilium. Also, in longitudinal sections evidence is offered for the existence of a helical structure in the peripheral filaments.     

THE FINE STRUCTURE OF DIPLOCOCCUS PNEUMONIAE

The fine structure of an unencapsulated strain of Diplococcus pneumoniae is described. A striking feature of these bacteria is an intracytoplasmic membrane system which appears to be an extension of septa of dividing bacteria. The possible function of these structures and their relationship to the plasma membrane and other types of intracytoplasmic membranes found in pneumococcus is discussed.     

THE FINE STRUCTURE OF BONE CELLS

An electron microscopic study of Araldite-embedded, undecalcified human woven and chick lamellar bone is presented. The fine structure of the cells of bone in their normal milieu is described. Active osteoblasts possess abundant granular endoplasmic reticulum, numerous small vesicles, and a few secretion droplets. Their long cytoplasmic processes penetrate the osteoid. The transition of osteoblasts into osteoid osteocytes and then into osteocytes is traced and found to involve a progressive reduction of cytoplasmic organelles. Adjoining the osteocytes and their processes is a layer of amorphous material which is interposed between the cell surfaces and the bone walls of their respective cavities. Osteoclasts contain numerous non-membrane-associated ribosomes, abundant mitochondria, and little granular endoplasmic reticulum, thus differing markedly from other bone cells. The brush border is a complex of cytoplasmic processes adjacent to a resorption zone in bone. No unmineralized collagen is seen at resorption sites and it appears that collagen is removed before or at the time of mineral solution. All bone surfaces are covered by cells, some of which lack distinctive qualities and are designated endosteal lining cells. The structure of osteoid, bone, and early mineralization sites is illustrated and 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.     

THE FINE STRUCTURE OF GIARDIA MURIS

Giardia is a noninvasive intestinal zooflagellate. This electron microscope study demonstrates the fine structure of the trophozoite of Giardia muris in the lumen of the duodenum of the mouse as it appears after combined glutaraldehyde and acrolein fixation and osmium tetroxide postfixation. Giardia muris is of teardrop shape, rounded anteriorly, with a convex dorsal surface and a concave ventral one. The anterior two-thirds of the ventral surface is modified to form an adhesive disc. The adhesive disc is divided into 2 lobes whose medial surfaces form the median groove. The marginal grooves are the spaces between the lateral crests of the adhesive disc and a protruding portion of the peripheral cytoplasm. The organism has 2 nuclei, 1 dorsal to each lobe of the adhesive disc. Between the anterior poles of the nuclei, basal bodies give rise to 8 paired flagella. The median body, unique to Giardia, is situated between the posterior poles of the nuclei. The cytoplasm contains 300-A granules that resemble particulate glycogen, 150- to 200-A granules that resemble ribosomes, and fusiform clefts. The dorsal portion of the cell periphery is occupied by a linear array of flattened vacuoles, some of which contain clusters of dense particles. The ventrolateral cytoplasm is composed of regularly packed coarse and fine filaments which extend as a striated flange around the adhesive disc. The adhesive disc is composed of a layer of microtubules which are joined to the cytoplasm by regularly spaced fibrous ribbons. The plasma membrane covers the ventral and lateral surfaces of the disc. The median body consists of an oval aggregate of curved microtubules. Microtubules extend ventrally from the median body to lie alongside the caudal flagella. The intracytoplasmic portions of the caudal, lateral, and anterior flagella course considerable distances, accompanied by hollow filaments adjacent to their outer doublets. The intracytoplasmic portions of the anterior flagella are accompanied also by finely granular rodlike bodies. No structures identifiable as mitochondria, smooth endoplasmic reticulum, the Golgi complex, lysosomes, or axostyles are recognized.     

FINE STRUCTURE OF THE OCTOPUS RETINA

The fine structure of the visual and the supporting cells and of the blood capillaries in the octopus retina is described. Lamellated structures contained in the proximal segment of the visual cell consist of compact arrays of dense membranes each of which is quintuple-layered and divides at its margins into two thinner sheets or membranes which are connected directly with the agranular or granular endoplasmic reticulum. Proximal to the deeper extremities of the rhabdomeres, the lateral plasma membranes of two adjoining visual cells contact each other forming a quintuple-layered compound membrane, which results in occlusion of the intercellular space. The central layer of the compound membrane is of high density, so that the membrane, as a whole, appears to be a single thick layer at low magnifications. The supporting cells are connected with the neighboring visual cells by two types of junctions. Long slender processes extend from the supporting cells to the surface of the retina through narrow spaces among the distal segments of the visual cells. The capillary endothelial cells are characterized by luminal surfaces irregularly contoured and by lateral surfaces elaborately interdigitated. The functional significance of the close contact between adjoining visual cells is discussed.