Junctional complexes, perineurial and glia-axonal relationships and the ensheathing structures of the insect nervous system; a comparative study using conventional and freeze-cleaving techniques

Cause of mortality in insects under severe stress

Mortality in the host armyworm larvae Pseudaletia separata parasitized by the parasitic wasp Cotesia kariyai was dramatically increased when they were simultaneously infected by the entomopathogen Serratia marcescens. Previous studies have shown that this strong insecticidal effect is due to a metalloprotease-like insecticide (MPLI) released from S. marcescens enterobacter. This study was conducted to elucidate the exact cause of the mortality resulting from MPLI. Injection of MPLI caused a sharp increase in hemolymph dopamine concentration followed by elevated levels of brain dopamine in armyworm larvae. [3H]Dopamine injected into the hemocoel, was incorporated into the brains of MPLI-injected larvae to a level eight times greater than in BSA-injected control larvae. Transmission electron microscopy showed an obvious decrease in thickness and density of the brain sheath in insects injected with MPLI. This was probably due to the MPLI-induced elevation of hemocyte metalloprotease activities. Further, electron microscopic and TUNEL staining analyses showed a significant increase in apoptotic cells in the brain 12 h after the injection. Injection of 3-iodotyrosine (a tyrosine hydroxylase inhibitor) before MPLI completely prevented the increase in hemolymph dopamine in test larvae and their following death. From these observations, we conclude that MPLI-injected larvae may have suffered mortal damage through increased apoptosis of brain cells caused by an influx of dopamine from the hemolymph.

Neurons in the third abdominal ganglion of the early postnatal crayfish: a quantitative and ultrastructural study

Ultrastructural data on the third abdominal ganglion of the crayfish was heretofore only available for adult individuals. The fine structure of neurons in the adult that are involved in the escape response has been described in detail, but no similar data existed for the postnatal individual. An increase in the number of neurons in the third abdominal ganglion during postnatal stages had been reported, which suggested that several changes in the features of neurons may occur. Here we describe the general anatomy and ultrastructure of the early postnatal third abdominal ganglion, with emphasis on neurons, and we compare their characteristics to those of the adult. Abdominal ganglia of 56 crayfish of 0, 8, 10, 18, 25, 50, 110, and 150 postnatal days were processed under cacodylate buffered aldehyde fixatives, osmicated, embedded in plastic, sectioned, and examined by light and electron microscopy. The anatomy of postnatal ganglia is homologous to the anatomy of the adult ganglia except that the perineurium is not developed in postnatals. The area of neurons within the postnatal ganglion shows no stratification, but neurons are grouped in nuclei according to their size. Neurons constitute a homogeneous population in different stages of maturity, as revealed particularly by the ultrastructure of the nucleolus. Postnatal development is evident in the perineurium, which may provide structural support to the ganglion.

Morphology of glial blood-brain barriers

Glial cells, in certain situations in the CNS, may become modified to form the structural basis of the blood-brain barrier. This occurs in more primitive vertebrates, such as the elasmobranch fish, and in some higher invertebrates. In the latter, the outermost glial sheath, often called the perineurium in avascular ganglia, substitutes functionally for the vascular endothelium of higher organisms. The intercellular junctions between the lateral borders of these modified glial or perineurial cells may be of several types. In nearly all cases, adhesive and communicating (gap) junctions are found together with an occluding junctional structure. The latter is assumed to be the morphologic basis of the observed blood-brain barrier. It varies in nature and may be one in which the adjacent cell membranes fuse, partially or completely, to form a classical tight junction, or it may be one in which the cell membranes remain separated by a distinct intercellular cleft. If the latter, the cleft may be straddled by columns or septal ribbons, between which a charged matrix substance may be found. Restrictive linker junctions, recently found to be the basis of the interglial barrier in cephalopod CNS, as well as that of myriapods, are characterized by cross-striations or columns which, in combination with charged residues, inherent either in them or in the associated extracellular matrix, slow down the entry of exogenous molecules. Septate junctions, which occur between glial cells in certain other invertebrates, exhibit intercellular septal ribbons, which do not prohibit paracellular transport of all substances but may slow down the passage of some by virtue of charged moieties. There is an association of cytoskeletal components with these septate, linker, and tight junctions; the role of the cytoskeleton in tight junctions, which can be seen by freeze fracture to be based on simple ridges in insects or a more complex network of them in arachnids, may also be important in the regulation of paracellular permeability. The structural details of the junctions in different groups are summarized and their physiologic implications discussed.

Genes expressed during imaginal disc morphogenesis: IMP-E1, a gene associated with epithelial cell rearrangement

A unique set of genes that encodes hormone-inducible transcripts found on membrane-bound polysomes is expressed during ecdysone-dependent morphogenesis of Drosophila imaginal discs cultured in vitro. The pattern of expression of one of these genes, IMP-E1, was analyzed in tissues from late third instar larvae and white prepupae by hybridization of asymmetric RNA probes to tissue sections. The IMP-E1 transcript was detected in all anterior and posterior imaginal discs except the ommatidial region of the imaginal eye disc. Within the imaginal leg disc, the IMP-E1 transcript is expressed more abundantly in the proximal than in the distal portions of the epithelium. The distribution of transcripts is consistent with the hypothesis that the IMP-E1 gene product is involved in the cell rearrangements associated with morphogenesis of the disc epithelium. The IMP-E1 transcript is also expressed at pupariation in glial cell layers that ensheath the brain. This glial cell transcript is the same size (7.5 kb) and polarity as the imaginal disc transcript, and is also transcribed in response to 20-hydroxyecdysone. Similarities between the morphogenetic changes in imaginal disc and glial cell layers during metamorphosis are discussed.

Distinctions between gap junctions and sites of intermediate filament attachment in the leech C.N.S

Freeze-fracture studies on the nerve cord of the leech Hirudo medicinalis reveal that the plasma membranes of various cells, including glial and muscle cells, contain at least two distinct types of aggregated intramembrane particles, identified as hemidesmosomes and gap junctions. Hemidesmosomes consist of angular particles irregularly arranged in circular or elongate patches in external leaflets (E-faces), and are associated with a bundle of intermediate filaments extending into the cytoplasm. Hemidesmosomes of specific axons abut on extracellular space at openings in the surrounding glial sheath. Gap junctions are patches of rounder particles in cytoplasmic leaflets (P-faces) and are more uniformly spaced; they have a corresponding array of pits in the complementary E-face. Gap junctions connect processes of adjacent smooth muscle cells, and apparently interconnect glial processes. Thus, different types of cells in the leech C.N.S. have similar intramembrane specializations. Moreover, the hemidesmosomes and gap junctions might, on superficial examination, be confused.

Ultrastructural comparison of the perisympathetic organs in three Coleoptera: Chrysocarabus auronitens F., Oryctes rhinoceros L. and Tenebrio molitor L

Ultrastructural comparison of different types of perisympathetic organs (POs) in three species of Coleoptera (Chrysocarabus auronitens, Oryctes rhinoceros, and Tenebrio molitor) showed that the structure of these organs was not related to their morphological types but to their topography. Two kinds of PO structure may be distinguished: compact median and diffuse lateral. They were similar in that both were surrounded by thin neural lamellae and exhibited numerous glial cells originating in the perineurium (type I perineurial cells) as well as abundant neurosecretory endings. They were different in as much as in median POs, the neurosecretory endings were generally surrounded by perineurial processes but in transverse POs, these endings were sheathless. Only one type of neurosecretory axon was distinguished in the median organs but three or four in the transverse. The nature of the processes by which neurosecretory granules are released may depend on the type of neurosecretory axon. For instance, exocytosis always occurred for dense spherical granules, and granule fragmentation was visualized for granules of smaller size.

Myelinating glia of earthworm giant axons: thermally induced intramembranous changes

The median and lateral giant axons in the ventral nerve cord of the earthworm Lumbricus terrestris are ensheathed by extensive spiral glial cell wrappings which resemble vertebrate myelin. The other, smaller, axons are encompassed by attenuated glial processes, as is typical of invertebrates. The fine structural details of the glial cells have been studied in thin sections and in replicas produced by freeze-fracturing where the intramembranous particle (IMP) populations within the lipid bilayer are visible. These consist of both low-profile IMPs as well as prominent ones 6-8 nm in diameter, scattered at random over the lipid interface in the myelinating glia. The larger IMPs on both P and E faces number about 80/mum2 at 16 degrees C in contrast to the IMP density of 400/mum2 in the other glial membranes. After acclimation to 5, 16 and 26 degrees C, the loose myelin glial membranes show variations in the density of their larger IMP population; in animals acclimated over 3 or more weeks to 5 degrees C, the number of these IMPs is significantly (P less than 0.001) less per unit area than in animals acclimated to 16 or 26 degrees C. The size of the particles at 5 degrees C is significantly (P less than 0.001) smaller than those at 16 or 26 degrees C. When animals are subjected to a sudden differential in ambient temperature, from 26 or 16 to 5 degrees C, or from 5 to 26 degrees C, and their giant axons with encompassing glia are fixed and frozen 30 min after this temperature change, the IMP population of the glial membranes remaining does not appear to alter. The differences in the IMP population of the myelinating glial membranes at different temperatures may reflect the extent to which they insulate and/or influence the velocity of impulse propagation.

The perineurium of the adult housefly: ultrastructure and permeability to lanthanum

The ultrastructure of the perineurial cells of Musca overlying the first optic neuropile was examined by transmission electron microscopy. These cells are somewhat similar to those of other insects but cytoplasmic flanges seem to be absent, and mitochondria are relatively large and sinuous. The intercellular channel system on the lateral border of the cells is relatively spacious and highly meandering. Perineurial cells are joined by septate, gap, and tight junctions, hemidesmosomes, and desmosomes. Tight and septate junctions bond perineurial cells and glial cells. These data are evaluated on the basis of tracer studies with lanthanum. This material penetrates the extracellular space between perineurium and underlying glial and nerve cells, between epithelial glial cells and retinular axon terminals (capitate projections), and between the alpha-beta fiber pair in the optic cartridge (gnarls). If no damage occurs to the perineurial cells during tissue preparation, this passage of lanthanum to neuronal surfaces indicates that the blood brain barrier is incomplete in this restricted area. Supportive evidence for such permeance is based on electrophysiological data, considerations of membrane specializations in the optic neuropile, and Na+/K+ ratios of dipteran hemolymph.

Membrane specializations in the first optic neuropil of the housefly, Musca domestica L. II. Junctions between glial cells

Membrane specializations between the three types of glial cells in the first optic neuropil (lamina ganglionaris) of the housefly were determined from thin sections and freeze-fracture replicas. Three strata of glia cells are present in the lamina. A relatively thin layer of satellite glia covers the distal (perikaryal) rind of the lamina and these cells wrap retinular axons that enter the lamina. The central synaptic fields of the lamina neurons are enclosed by epithelial glia, while the proximal surface of the lamina is capped by marginal glial cells. Satellite glia bond to each other via desmosomes, septate and gap junctions. Freeze-fracture replicas show gap junctions as aggregations of E face particles and P face pits on the intramembranous surfaces. Parallel rows of P face particles are indicative of septate junctions. Angulated, intersecting, P face particle ridges are arranged in circumferential bands around retinular axons at the glia-axon interface. Thin section correlates of these junctions are presented. Epithelial glia are characterized by elaborate series of parallel membranes which appear to be suspended in the cytoplasm but may be the invaginated plasma membranes of a neighbouring glial cell. An intermembranous cleft of 40-50 A is noted and this area has an appreciable electron density which give the appearance of a gap junction. When cleaved, these membranes show plaques of particles on the P face. The marginal glial cells are relatively large and are joined by a newly discovered junction which is characterized (from freeze-fracture data) by numerous, undulating, uninterrupted, parallel P face ridges which sometimes become circular and form enclosures. In thin section, electron-dense material fills the membrane appositional areas and in tangential sections faint diffuse parallel striae are seen. This specialized cell contact may be a variant of a continuous junction although, based on fracture replicas, there are obvious similarities to tight junctions. These membranes specializations are related, in the three dimensions of the optic cartridges, to functions in a possible blood-eye barrier system.

Membrane specializations in the first optic neuropil of the housefly, Musca domestica L. I. Junctions between neurons

Thin section and freeze-fracture replicas of the first optic neuropil (lamina ganglionaris) of the fly Musca were studied to determine the types, extent and location of membrane specializations between neurons. Five junctional types are found, exclusive of chemical synapses. These are gap, tight and septate junctions, close appositions between retinular (R) axons and capitate projections (in which an epithelial glial cell invaginates into an R axon). Junctional types and their cellular associations follow: gap junctions, between lamina (L) interneurons, L1-L2; tight junctions, between L1-L2; L3-L4; L4-epithelial glial cell; and R7-R8. Septate junctions, between L1-L2, L3-L4, L3-beta, L4-beta, alpha-beta, and an unidentified fibre making septate junctions with L1 and L2. Close appositions are found between R axons in the distal portion of the optic cartridges of this neuropil prior to extensive R chemical synapses with L1, L2. These loci (seen in freeze-fracture replicas) have rhomboidal patches of hexagonally arrayed P face particles. Intermembranous clefts between R axons are about 50 A and are invariably electron lucent. These points of near contact between R terminals are probably the sites of low electrical resistance measured by Shaw (1979). Capitate projections are for the first time revealed in freeze fracture surfaces. Here epithelial glia send many, short, mushroom-shaped processes invaginating into R axons forming a tenacious structural bond. All four membrane leaflets (P and E faces of R axon and glial membrane) in the capitate projection possess particles in higher densities than in the surrounding nonspecialized regions. The known, general functions of each membrane specialization were correlated with the functional capacities of those lamina neurons possessing them in an effort to interpret better the integrative capacity of this neuropil. These data provide some fine structural bases for a putative 'blood-brain' barrier between lamina and haemolymph, between lamina and peripheral retina, and possibly between lamina and second optic neuropil.

Gap and tight junctions in tunicates. Study in conventional and freeze-fracture techniques

Intercellular junctions have been investigated in epidermis and pharyngeal epithelium of larvae and adults of various species of tunicates with conventional and freeze-fracture techniques. Gap and tight junctions were found, similar to those observed in vertebrate tissues. Gap junctions were frequent in glandular epithelia and in larval tissues. They were interpreted as ways of intercellular communication in these developing tissues. They were also particularly numerous in Phallusia pharyngeal cells. Tight junctions were found preferentially in adult pharyngeal and epidermal epithelia, where they were arranged in strands of distinct particles forming a belt-like network at the apical part of cells. These junctions were interpreted as providing a tight barrier between the internal medium and the external environment. In larvae, tight junctions were found only between epidermal cells of the tail. These junctions thus characterized completely differentiated tissues, where they might play, in tunicates and in vertebrates, the same role as septate junctions do in invertebrates.

Intramembranous particles in the form of ridges, bracelets or assemblies in arthropod tissues

Non-junctional intramembranous particle arrays in the form of ridges, bracelets or rectilinear assemblies have been found by freeze-fracturing in the cytoplasmic half or P face of the plasma membrane in a variety of arthropod tissues. These tissues include both excitable cells, nerve and muscle, and such other cells as those from the intestinal tract, the tracheal system and the connective tissue. The intramembranous ridges are short rows of fused particles about 10 nm in diameter; comparable particles comprise the bracelets and the rectilinear aggregates, although the former are of lower profile. In cells sending out cytoplasmic projections during migration and development, for example, axons in embryonic, newly hatched or pupal tissues, tracheoles or fibroblasts, the intramembranous ridges are always aligned parallel to the longitudinal axis of the cellular process. The physiological significance of these may be that they play some role in recognition during development, perhaps by contact guidance. The ridges and rectilinear arrays found in the gut could also be involved in recognition and/or adhesion. In muscle, bead-like ridges are intimately associated with the transverse tubular system and may have a receptor function. Irregular and circular low-profile ridges occur in the tissues of the horseshoe crab, Limulus, and 'bracelet' forms are found in the inner membrane of insect pupal tracheae. The latter may play a part in the initiation and development of small tracheoles.

Polymeric cryoprotectants in the preservation of biological ultrastructure. III. Morphological aspects

Two high molecular weight polymers, polyvinylpyrrolidone (PVP) and hydroxyethyl starch (HES), have been used as cryoprotectants for preparing specimens to be freeze fractured. Solutions of 25% (w/w) suppress the formation of intracellular ice in single cells and tissue blocks from both plants and animals to the extent that fine structural details of the cell can be elucidates. The mode of action of these cryoprotectants, together with the structures they reveal and the peculiar advantages attached to their use, is discussed.

Two types of presynaptic configurations in insect central synapses: an ultrastructural analysis

Two structurally distinct types of synapses have been identified in the cockroach metathoracic ganglion. The two synaptic types are distinguished on the basis of (a) the shape and position of the presynaptic density seen by serial thin sectioning and (b) the arrangement and location of vesicle attachment sites (VAS) on the presynaptic membrane obtained from replicas of aldehyde-fixed, freeze-fractured neuropile. Bar-type synapses in thin sections possess a long presynaptic density located in a trough or groove opposite the extracellular space between two contiguous postsynaptic processes. In freeze-fracture, this trough is flanked by two rows of vesicle attachment sites. The second synaptic conformation consists of rows of discrete dense projections located on the convexities of the presynaptic membrane, i.e., directly opposite a single postsynaptic process. This conformation has been correlated with groups of VAS linearly arranged also on the convexities of the presynaptic membrane. These structurally different synapses may represent functionally different contacts within the insect ganglion.

Structural and functional changes in an identified cricket neuron after separation from the soma. I. Structural changes

The morphological effects of separation from the soma were examined in isolated arborization and isolated axon segments of an identified motor neuron in the Polynesian field cricket, Teleogryllus oceanicus. The identified neuron, the contralateral dorsal longitudinal motor neuron of the metathoracic ganglion (CDLM), possesses an arborization most of which lies contralateral to its soma within the metathoracic ganglion. Midline surgical lesions in the metathoracic ganglion separated CDLM into a distal segment composed of the axon and most of the arborization, and a proximal segment comprised of the remaining arborization, neuritie, and soma. Isolated axonal segments were produced by cutting the nerve root containing the axon of CDLM close to the ganglion. The normal anatomy of CDLM was determined by axonal dye-fills using cobaltous chloride. Morphological changes in the isolated arborization of CDLM were examined by axonal dye-fills at successive time intervals. Changes in the isolated CDLM axon were examined via dissection and histological cross-sections of the distal nerve at graded time intervals. In one example, a remnant of the isolated CDLM arborization survived to 168 days postoperative, a time comparable to the longest previously-reported physiological and morphological survival times of distal axonal segments of invertebrates. In general the isolated arborization does not survive this long. Normally-occurring branches of the arborization can be preserved about 0 to 50 days. After this period branches of the arborization seem to be lost in progressive fashion from smaller to larger, leading to complete loss of the arborization and axon in most cases at 100 or more postoperative days. There is evidence for the presence of supernumerary fibers in the isoalted CDLM arborization between 0 to 63 days postoperative. Such supernumerary fibers indicate an independent capacity for outgrowth of the isolated arborization without connection to the nucleus. The distal axonal segment of CDLM degenerates physiologically and morphologically within 4 to 15 days after peripheral nerve section. This time course is close to that of Wallerian degeneration of vertebrate peripheral nerve axons.

A freeze-fracture study of adult Calliphora salivary glands

A freeze-fracture study of adult Calliphora salivary glands has revealed a high density (approx. 4500/mu2) of intramembraneous particles (80-110 A) in both the apical and basal membranes. Most of the particles were associated with the A face. The density of the stalked surface particles which coat the cytoplasmic surface of the apical membrane. The possible significance of these particles in ion transport is discussed.