Ultrastructure of the pineal gland of the monkey, Macaca fascicularis, with special reference to the presence of synaptic junctions on pinealocytes

Summary The present study described the normal ultrastructure of the monkey pineal gland. The gland was composed of the principal pinealocytes, intramural neurons and glial cells. The nucleus of the pinealocytes was deeply infolded with evenly distributed chromatin materials. The abundant cytoplasm was rich in organelles including the well-developed Golgi apparatuses, multivesicular bodies, dense-cored vesicles and widely scattered free and polyribosomes. A variety of axon terminals was observed and the majority of them contained pleomorphic agranular vesicles with a few large dense-cored vesicles. A few terminals showed flattened vesicles or small dense cored vesicles. Some of the axon terminals formed synaptic contacts with the cell bodies of pinealocytes. These synapses were mainly concentrated in the posterior third of the gland. The occasional intramural neurons observed were postsynaptic to axon terminals containing round agranular vesicles. The sources of the nerve fibres and terminals forming synaptic junctions with pinealocytes and intramural neurons were discussed.     

Nervous system ofSchistosoma mansoni cercaria: Organization and fine structure

As observed by transmission electron microscopy of serially sectionedSchistosoma mansoni cercaria, the nervous system is distributed throughout the three anatomic segments of the larva-i.e., the anterior organ (oral sucker), the body (midsegment), and the tail. The central ganglion, a neuropile surrounded by cell bodies, is located in the anterior area of the body segment. It tapers anteriorly into two lobes from which a pair of anterior central nerve trunks extend longitudinally. The posterior region of the central ganglion tapers into a pair of nerve trunks (posterior central nerve trunks). Twelve peripheral nerve trunks are evenly distributed around the ganglion. Six trunks course anteriad (anterior peripheral nerve trunks) and six course posteriad (posterior peripheral nerve trunks). A pair of dorsal and ventral nerve trunks, positioned opposite each other, extend the length of the tail. All nerve trunks are unsheathed. The nervous system contains three types of vesicles. Type I vesicles average 47.66±2.57 nm in diameter, vary in electron density, and have electron-lucent peripheries. Type II vesicles have a mean diameter of 18.41±2.57 nm, are electron-lucent and are concentrated mostly in the presynaptic area of the synaptic and neuromuscular junctions. The mean diameter of Type III vesicles is 57.47±16.08 nm. They are electron-dense and are concentrated mostly in the tegumental ciliated papillac and their accompanying dendrites. Two types of synaptic junctions are present. Type1 synapse has dense material incorporated in its postsynaptic membrane, while Type2 synapse has dense material of various dimensions incorporated in its presynaptic membrane and usually in its postsynaptic membrane. Synaptic and neuromuscular junctions are similar.     

An electron microscopic study of astroglia and oligodendroglia in the lateral geniculate nucleus of aged rats

The ultrastructural features of glial cells in the lateral geniculate nucleus of aged rats have been studied. Abundant filaments as well as heterogeneous dense bodies are observed in the majority of astrocytes. They frequently surround both axons and nerve terminals showing signs of degeneration. In addition, some degenerating myelinated axons are seen in phase suggestive of engulfment by astrocyte processes. Oligodendrocytes display broad processes containing an organelle-rich cytoplasm and a continuity between their plasma membrane and the outer myelin lamellae which partially ensheath the adjacent axons. Multivesicular bodies and pleomorphic dense inclusions, composed of amorphous material as well as laminated structures, are also present in oligodendrocytes. The significance of these morphological features is discussed in relation to process of normal ageing.     

An ultrastructural study of nerve and glial cells by freeze-substitution

Summary The ultrastructure of nerve and glial cells in the cerebral and cerebellar cortices of mice was studied after rapid freezing followed by substitution fixation. The cerebral and cerebellar cortices were frozen by bringing them into contact with a polished pure copper block cooled at a temperature of about –196 ° C. The tissues were fixed and substituted in acetone containing 2–4% OsO₄ at –78 ° C for 2–3 days and then prepared for electron microscopy. Tissue fixed by this method displayed the following characteristics. (1) The contour of cells, processes and intracellular membrane systems was smooth. (2) The extracellular spaces were of variable widths. (3) Microtubules were well preserved and were often observed to extend into nerve terminals and to run close to presynaptic membranes. (4) The matrix of cytoplasm and mitochondria was electron dense. Dense granules, possibly binding sites of divalent cations, were often found in the mitochondrial matrix. (5) The plasma membrane of neuronal processes was thicker than that of glial processes. (6) The plasma membranes of nerve fibres and glial processes appeared asymmetrical, the inner leaflet being slightly thicker than the outer leaflet, whereas membranes of cell organelles such as smooth endoplasmic reticulum, Golgi bodies, lysosomes, multivesicular bodies, mitochondria and synaptic vesicles, were symmetrical.     

Ultrastructure of the nerve plexuses of the mammalian intestine: The enteric glial cells

The ultrastructure of the glial cells in the enteric plexuses of the rat, guinea-pig, rabbit, cat and sheep has been investigated by freeze-fracture and by thin-section electron microscopy. In all the ganglia studied, glial cells outnumber neurons. They are readily identified by their shape, position and ultrastructure (particularly the abundant amount of gliofilaments) but could not be subdivided into separate types. They provide a partial sheath to the ganglion neurons (but large areas of neuronal membrane lie directly beneath the basal lamina and collagen fibrils) and have long laminar processes extending between nerve processes. Most nerve processes are in direct membrane-to-membrane contact with each other; the glial cells only separate groups of them and rarely form a sheath around an individual neurite.The gliofilaments are anchored to conspicuous dense bodies beneath the cell membrane at the surface of ganglia. The possible significance of these systems of gliofilaments (and the high number of intermediate junctions) is discussed in the light of the severe mechanical stresses imposed on the ganglia by the contractile activity of the gut wall.Numerous specialized contacts, of unknown significance, are found between vesicle-containing nerve varicosities and glial cell bodies or glial processes. In freeze-fracture preparations (cat and guinea-pig), a specific pattern of intramembrane particles allows the cell membrane of the enteric glial cells to be readily identified.     

Neuronal distribution and subcellular localization of HCNP-like immunoreactivity in rat small intestine

A novel peptide, hippocampal cholinergic neurostimulating peptide (HCNP), originally purified from young rat hippocampus, affects the development of specific cholinergic neurons of the central nervous system in vitro. In this study, HCNP-like-immunoreactive nerve processes and nerve cell bodies were identified by electron microscopic immunocytochemistry in the rat small intestine. Labeled nerve processes were numerous in the circular muscle layer and around the submucosal blood vessels. In the submucosal and myenteric plexuses, some HCNP-like-immunopositive nerve cell bodies and nerve fibers were present. The reaction product was deposited on the membranes of various subcellular organelles, including the rough endoplasmic reticulum, Golgi saccules, ovoid electron-lucent synaptic vesicles in axon terminals associated with submucosal and myenteric plexuses, and the outer membranes of a few mitochondria. The synaptic vesicles of HCNP-like-positive terminals were 60–85 nm in diameter. The present data provide direct immunocytochemical evidence that HCNP-like-positive nerve cell bodies and nerve fibers are present in the submucosal and myenteric plexuses of the rat small intestine. An immunohistochemical light microscopic study using mirror-image sections revealed that in both the submucosal and myenteric ganglia, almost all choline acetyltransferase (ChAT)-immunoreactive neurons were also immunoreactive for HCNP. These observations suggest (i) that HCNP proper and/or HCNP precursor protein is a membrane-associated protein with a widespread subcellular distribution, (ii) that HCNP precursor protein may be biosynthesized within neurons localized in the rat enteric nervous system, and (iii) that HCNP proper and/or HCNP precursor protein are probably stored in axon terminals.     

Synaptic organization of the outer plexiform layer of the turtle retina: an electron microscope study of serial sections

Summary Neural connections in the outer plexiform layer of thePseudemys turtle retina have been studied by electron microscopy of serial ultrathin sections. While the distinguishing features of the photoreceptors have been described elsewhere, in this paper we describe the patterns of connectivity between identified second order neurons and identified photoreceptors or amongst second order neurons themselves. Basal telodendria emitted from double cone pedicles interconnect the two members of the double cone. Three morphologically different types of junction are made between bipolar cells and cone pedicles. H1 horizontal cells can be distinguished from H2 horizontal cells and synapses occur between them. Axon terminals of H1 cells are presynaptic to H1 cell bodies. Photoreceptors, H1 cell bodies and H1 axon terminals engage in electrical junctions while chemical synapses occur from both types of horizontal cell to bipolar cells. On rare occasions, bipolar cell dendrites were seen to be presynaptic to other bipolar cell dendrites. The significance of some of these contacts for the electrophysiological findings on the OPL of the turtle retina is discussed.     

Neuro-epitheliomuscular cell and neuro-neuronal gap junctions inHydra

Summary Gap junctions have been described ultrastructurally between neurons and epitheliomuscular cells and between neurons and their processes in the hypostome peduncle and basal disc ofHydra. All gap junctions examined inHydra exhibit two apposed plasma membranes having a 2–4 nm gap continuous with the extracellular space. The gap junctions are variable in length from 0.1–1.6 m and appear linear or V-shaped in section. Neuronal gap junctions inHydra occur infrequently as compared to chemical synapses. Electron microscopy of serial sections has demonstrated the presence of adjacent electrical and chemical synapses (neuromuscular junctions) formed by the same neuron. In addition multiple gap junctions were present between two neurons. This is the first ultrastructural demonstration of electrical synapses in the nervous system ofHydra. Such synapses occur in neurons previously characterized as sensory-motor-interneurons on the basis of their chemical synapses; these neurons appear to represent a type of stem cell characterized by having both electrical and chemical synapses.     

An electron microscopic study of the mediodorsal cerebral cortex in the lizard Agama agama

Summary An electron microscopic analysis was made of the small-celled part of the mediodorsal cortex of the lizard Agama agama. This cortex consists of four layers: Superficial plexiform layer, cellular layer, deep plexiform layer and fiber layer. In the superficial plexiform layer one type of solitary neuron with smooth dendrites is present.Three types of axon terminals can be observed: terminals with a moderately electron dense matrix packed with spherical vesicles (S1 type), axon terminals with an electron lucent matrix containing fewer spherical synaptic vesicles than the S1 type (S2 type) and axon terminals with an electron lucent matrix and scattered pleomorphic synaptic vesicles (F type). F type axon terminals are larger than S terminals. At the pial surface endfeet of tanycytic processes form a limiting glial layer, contacting one another by means of gap junctions. In the cellular layer perikarya of pyramidal neurons are densely packed. The karyoplasm of these neurons shows either evenly dispersed or discretely clumped chromatin. Spiny dendrites arise from the perikarya and extend into both the superficial and deep plexiform layers. The structure of the deep plexiform layer is roughly similar to that of the superficial plexiform layer. The fiber layer contains the majority of the afferent and efferent axons of the mediodorsal cortex. The axons are myelinated and unmyelinated. Between the fibers, scattered solitary neurons are present, often accompanied by glial cells.The lateral ventricle beneath the fiber layer is lined by a single row of ependymal tanycytes. Tanycytic processes traverse the cortical layers and may form endfeet at the pial surface. Protoplasmic excresenses from some ependymal cells protrude into the ventricle.     

A lectin, peanut agglutinin, as a probe for the extracellular matrix in living neuromuscular junctions

Summary The extracellular matrix plays important roles in the differentiation of synapses. To identify molecules concentrated specifically in the synaptic extracellular matrix, fluorescently-labelled lectins were applied to neuromuscular junctions. A lectin, peanut agglutinin (PNA), stains the neuromuscular region selectively and irreversibly (up to at least 3 weeksin situ), outlining the periphery of the nerve terminal arborization in the frog. Snake neuromuscular junctions also stain intensely with fluorescent PNA, while mouse diaphragm staining is faint. At the electron microscopic level, the reaction products of horseradish peroxidase-conjugated PNA are found primarily in the extracellular matrix flanking Schwann cells in the frog endplate regions. Fluorescently labelled PNA does not affect synaptic potentials and can serve as a simple stain for correlating functional studies of living neuromuscular junctions. Moreover, it can be combined with a presynaptic dye to observe nerve terminals and synaptic extracellular matrix in the same junctionsin situ. This report reveals the existence of synapse-specific carbohydrates associated with Schwann cell extracellular matrix in the frog neuromuscular junction. The specific binding and its physiological compatibility make PNA a useful probe for further investigation of synaptic differentiation, plasticity and maintenance.     

Extracellular matrix of the superior olivary nuclei in the dog

Summary The extracellular matrix around nerve cell bodies in canine lateral and medial superior olivary nuclei was examined by conventional electron microscopy, Golgi impregnation and histochemical techniques. Each neuron is surrounded by a region of myelin-free neuropil embedded amongst the myelinated fibres of the trapezoid body. In the myelin-free neuropil there are astrocytes, axons, synaptic boutons and extracellular matrix. The extracellular matrix fills the spaces between slender axons near the terminals, synaptic boutons and glial processes, but not the synaptic cleft. Golgi impregnation selectively stains the perineuronal nets which cover some or all of the nerve cell bodies and dendrites. The Golgi-EM method revealed that the impregnated profiles of the nets are restricted to the extracellular matrix. Synaptic boutons are situated in the holes of the perineuronal nets. Peanut (PNA) and soybean (SBA) agglutinins bound the extracellular matrix but not the synaptic boutons, glial processes, nerve cell bodies or basal lamina of blood capillaries. Light microscopic immunohistochemistry of the glial fibrillary acidic protein (GFAP) and S-100 protein did not stain a layer corresponding to the extracellular matrix and synapses but showed an intensely positive reaction immediately outside this layer. These data suggest the existence of a unique microenvironment associated with glycoconjugates around nerve cell bodies in canine superior olivary nuclei.     

Cytoplasmic Syncytial Connections Between Neuron Bodies in the CNS of Adult Animals

Studies of neurons in the dentate gyrus and hippocampal fields CA1 and CA2 and cerebellar granule cells were performed to test the hypothesis that there are syncytial connections between the bodies of neurons in adult higher vertebrates. Electron microscopic investigations showed that these cells were densely packed and had incomplete glial coatings. The outer cell membranes of these cells were found to be in contact, and membrane contacts in the form of tight junctions and gap junctions were seen. These areas showed membrane perforation and the establishment of syncytial connections between neurons, with all the expected ultrastructural characteristics. These connections could form between several contacting neurons, resulting in a unified functional cell cluster. These studies support the hypothesis that cytoplasmic syncytial interneuronal connections, along with synaptic and contact-type electrical connections, form not only in tissue cultures and the autonomic nervous system during early postnatal ontogenesis, but also in the CNS in adult vertebrates.     

The fine structure of parasympathetic nerve cells in the otic ganglia of the rabbit

The ultrastructure of parasympathetic neurons in the otic ganglia of the rabbit is described with the object of providing a basis for comparisons of parasympathetic with sympathetic nerve cells. The neuronal cytoplasm contains Nissl substance, mitochondria, dense bodies, agranular Golgi membranes, neurofilaments (60 to 100 Å in diameter) and microtubules having diameters about 200 Å and filled with irregularly dense material. Granulated vesicles (about 900 Å in diameter) consisting of a limiting membrane and enclosing a dense spherical droplet of about 500 Å diameter are also present in the cytoplasm and processes of these neurons and in presynaptic terminals, thus disproving the hypothesis put forward by some authors that the presence of granulated vesicles indicates a sympathetic nervous pathway. Long, tubular bodies with a dense central core are described which are possibly peculiar to parasympathetic neurons. The nuclear membrane is double, porous and sometimes exhibits localized, deep invaginations of its surface. Junctional complexes are described between apposing membranes of neurons and satellite cells which are morphologically similar to zonulae adhaerentes. Small gaps exist in the cytoplasmic sheath surrounding the neurons. Over these areas only a basement membrane separates the nerve cell from the connective tissue space and the significance of this finding is discussed.     

The neuronal architecture of the anteroventral cochlear nucleus of the cat in the region of the cochlear nerve root: Electron microscopy

We have studied the posterior division of the anteroventral cochlear nucleus, where the cochlear nerve root enters the brain, in the cat. In Nissl preparations, this region contains two types of neuronal cell bodies: globular and multipolar. The two types can be identified in the electron-microscope by comparing Nissl substance and rough endoplasmic reticulum. Globular cell bodies receive many synaptic terminals, which cover 85% of the surface. In contrast, multipolar cell bodies are almost entirely wrapped by thin glial sheets—synaptic terminals contact less than 15% of the surface and tend to cluster at the bases of dendrites. Synaptic terminals are of three kinds, types 1, 2, and 3, which contain large round, small round-to-oval, and small flattened synaptic vesicles, respectively. Terminals of all three kinds synapse on both types of cell bodies. However, only globular cell bodies receive the largest type 1 terminals, which correspond to end-bulbs, seen in Golgi impregnations to arise from cochlear nerve axons. Cochlear ablation leads to degeneration of type 1, but not type 2 or 3 terminals.We conclude that neurons with globular cell bodies receive heavy somatic input from the cochlear nerve, as well as from other sources. Neurons with multipolar cell bodies receive very little input to their perikarya—giving their dendrites a more important role in determining their response properties. We suggest a morphological basis for correlating individual kinds of neurons with certain electrophysiological response types.     

Immunocytochemical localization of the glucose transporter 2 (GLUT2) in the adult rat brain. II. Electron microscopic study

Following a former immunohistochemical study in the rat brain [Arluison, M., Quignon, M., Nguyen, P., Thorens, B., Leloup, C., Penicaud, L. Distribution and anatomical localization of the glucose transporter 2 (GLUT2) in the adult rat brain. I. Immunohistochemical study. J. Chem. Neuroanat., in press], we have analyzed the ultrastructural localization of GLUT2 in representative and/or critical areas of the forebrain and hindbrain. In agreement with previous results, we observe few oligodendrocyte and astrocyte cell bodies discretely labeled for GLUT2 in large myelinated fibre bundles and most brain areas examined, whereas the reactive glial processes are more numerous and often localized in the vicinity of nerve terminals and/or dendrites or dendritic spines forming synaptic contacts. Only some of them appear closely bound to unlabeled nerve cell bodies and dendrites. Furthermore, the nerve cell bodies prominently immunostained for GLUT2 are scarce in the brain nuclei examined, whereas the labeled dendrites and dendritic spines are relatively numerous and frequently engaged in synaptic junctions. In conformity with the observation of GLUT2-immunoreactive rings at the periphery of numerous nerve cell bodies in various brain areas (see previous paper), we report here that some neuronal perikarya of the dorsal endopiriform nucleus/perirhinal cortex exhibit some patches of immunostaining just below the plasma membrane. However, the presence of many GLUT2-immunoreactive nerve terminals and/or astrocyte processes, some of them being occasionally attached to nerve cell bodies and dendrites, could also explain the pericellular labeling observed. The results here reported support the idea that GLUT2 may be expressed by some cerebral neurones possibly involved in glucose sensing, as previously discussed. However, it is also possible that this transporter participate in the regulation of neurotransmitter release and, perhaps, in the release of glucose by glial cells.     

Ultrastructure of substance P immunoreactive elements in the periaqueductal gray matter of the rat

Substance P (SP) is a non-opioid peptide that generates a potent, analgesia when injected into the periaqueductal gray matter (PAG)The aim of this study was to investigate the fine neuronal structures and synaptic circuits involved in SP action in rats by means of electron microscopy, using immunocytochemical (ICC) pre-embedding methods. A conventional ultrastructural study, carried out to interpret the ICC data correctly, shows small sized nerve cell bodies with a high nucleus–cytoplasmic ratio; absence of an extensive granular endoplasmic reticulum; and few axo-somatic contacts having symmetrical and asymmetrical junctions in equal proportions. The large neuropil is characterized by numerous thin unmyelinated axons and axodendritic synapses mainly showing pleomorphic vesicles and asymmetrical junctions. The ICC analysis showed moderately labeled nerve cell bodies with the same structural, synaptic, and dimensional features as the negative cells. In the neuropil SP immunoreactivity is shown by dendrites, synapses, and thin elements which are unidentifiable structurally. No SP terminals synapsing on SP nerve cell bodies were found and only occasional SP light labeled terminals synapsing on negative perikarya were seen. The SP boutons generally have pleomorphic vesicles and asymmetrical junctions. On the basis of these data a possible excitatory activity of PAG SP synapses could be hypothesized. This activity would take place on postsynaptic neurons generally at a dendritic level. Our ultrastructural findings give support to an excitatory role carried out by SP neurons of the PAG, as suggested by the role of PAG circuitry on spinal nociception.     

Ultrastructure of substance P immunoreactive elements in the periaqueductal gray matter of the rat.

Substance P (SP) is a non-opioid peptide that generates a potent analgesia when injected into the periaqueductal gray matter (PAG). The aim of this study was to investigate the fine neuronal structures and synaptic circuits involved in SP action in rats by means of electron microscopy, using immunocytochemical (ICC) pre-embedding methods. A conventional ultrastructural study, carried out to interpret the ICC data correctly, shows small sized nerve cell bodies with a high nucleus-cytoplasmic ratio; absence of an extensive granular endoplasmic reticulum; and few axo-somatic contacts having symmetrical and asymmetrical junctions in equal proportions. The large neuropil is characterized by numerous thin unmyelinated axons and axo-dendritic synapses mainly showing pleomorphic vesicles and asymmetrical junctions. The ICC analysis showed moderately labeled nerve cell bodies with the same structural, synaptic, and dimensional features as the negative cells. In the neuropil SP immunoreactivity is shown by dendrites, synapses, and thin elements which are unidentifiable structurally. No SP terminals synapsing on SP nerve cell bodies were found and only occasional SP light labeled terminals synapsing on negative perikarya were seen. The SP boutons generally have pleomorphic vesicles and asymmetrical junctions. On the basis of these data a possible excitatory activity of PAG SP synapses could be hypothesized. This activity would take place on postsynaptic neurons generally at a dendritic level. Our ultrastructural findings give support to an excitatory role carried out by SP neurons of the PAG, as suggested by the role of PAG circuitry on spinal nociception.     

Different kinds of axon terminals forming symmetric synapses with the cell bodies and initial axon segments of layer II/III pyramidal cells. II. Synaptic junctions

Summary Four different types of axon terminals form symmetric synapses with the cell bodies and initial axon segments of pyramidal cells in layer II/III of rat visual cortex. One type belongs to chandelier cells, and the other three kinds of terminals have origins that have not been established yet. These latter are referred to as large, medium-sized and dense terminals. The purpose of the present study was to examine the synaptic junctions formed by all four types of terminal. The synapses formed by the chandelier cell terminals are readily recognized in thin sections because of the characteristic features of both the terminals and the initial axon segments, which are the neuronal elements postsynaptic to them. In en face views of these axo-axonal synapses the junctions can be seen to have presynaptic dense projections that form a grid in which they are triagonally spaced, and have an average centre-to-centre spacing of 84 nm. As an ensemble the projections form the presynaptic grid, which usually has an oval or round outline, but may be notched on one side where projections are absent. The synaptic junctions of the large, medium-sized and dense terminals were examined by making reconstructions of the terminals from serial thin sections. It was found that at the interfaces between the axon terminals and the cell bodies of pyramidal cells, several separate synaptic junctions may be present, in addition to a number of puncta adhaerentia. Thus, there may be as many as five separate synaptic junctions and as few as one. It was also found that while the proportion of the area of the synaptic interface occupied by synaptic junctions was between 12% and 26% for dense terminals, for medium sized terminals it was 10–15%, and for the one large terminal reconstructed it was only 8%. Thus, there can be multiple synaptic junctions between each of these types of axon terminals and a pyramidal cell, and because many of the terminals forming symmetric junctions are boutons en passant, a number of vesicle release sites exist between the presynaptic axon and its postsynaptic partner. The axon terminals forming symmetric synapses in the cerebral cortex are assumed to be inhibitory, and consequently it is suggested that this arrangement of multiple release sites is designed to ensure that stimulation of the presynaptic axon results in an effective level of hyperpolarization of the postsynaptic neuron.     

Novel arthropod cell junctions with restrictive intercellular ‘linkers’

Summary The peripheral glial cells that surround the components of the avascular CNS in certain groups of primitive arthropods are characterized by unusual intercellular junctions. In the centipedes and millipedes (Myriapoda), these glial cells are associated by interconnecting filamentous linkers which produce a reduction, but not an occlusion, of the intercellular cleft; these are interposed between conventional gap junctions. In replicas, the freeze-cleave images are of loosely aggregated gap junctional connection plaques, fracturing on to the extracellular membrane half leaflet (E face), together with linear alignments of intramembranous particles (IMPs) and furrows; complementary P face ridges also occur. Exogenous tracers appear unable to penetrate beyond these junction-rich glial clefts, possibly by binding to the linkers or extracellular matrix between them. Peripheral glial cells in the cerebral ganglion of the horseshoe crab,Limulus, are also characterized by linear IMP arrays; in this case they primarily exhibit E face grooves and complementary ridges of P face IMPs, which also do not produce complete membrane fusion. These, too, are intimately associated with gap junctional plaques of E face particles or P face pits. These intramembranous particle rows are novel structural modifications, called here linker junctions, and are quite distinct from conventional tight or septate junctions found between the outer glial cells in more highly evolved arthropods such as the insects and arachnids. They seem to represent a new category of intercellular junction.