Studies on perineural junctional complexes and the sites of uptake of microperoxidase and lanthanum in the cockroach central nervous system

Starvation-induced regulation of carbohydrate transport at the blood-brain barrier is TGF-β-signaling dependent

During hunger or malnutrition, animals prioritize alimentation of the brain over other organs to ensure its function and, thus, their survival. This protection, also-called brain sparing, is described from Drosophila to humans. However, little is known about the molecular mechanisms adapting carbohydrate transport. Here, we used Drosophila genetics to unravel the mechanisms operating at the blood-brain barrier (BBB) under nutrient restriction. During starvation, expression of the carbohydrate transporter Tret1-1 is increased to provide more efficient carbohydrate uptake. Two mechanisms are responsible for this increase. Similar to the regulation of mammalian GLUT4, Rab-dependent intracellular shuttling is needed for Tret1-1 integration into the plasma membrane; even though Tret1-1 regulation is independent of insulin signaling. In addition, starvation induces transcriptional upregulation that is controlled by TGF-β signaling. Considering TGF-β-dependent regulation of the glucose transporter GLUT1 in murine chondrocytes, our study reveals an evolutionarily conserved regulatory paradigm adapting the expression of sugar transporters at the BBB.

Glial cells in the posterior sub-esophageal mass of the brain in Sepia officinalis (Linnaeus, 1758) (decapodiformes-sepiida): ultrastructure and cytochemical studies

Electron microscopy revealed that glial cells in the posterior sub-esophageal mass of the brain in Sepia officinalis had a well-developed rough endoplasmic reticulum formed by long coverslips with rectilinear or curvilinear arrangements. The coverslips appeared dilated and have a large amount of adhered polysomes. Vesicular lamellae coexisted with the elongated lamellae of RER and dictyosomes of Golgi apparatus. Endocytosis was evidenced through the pale vesicles which were appeared next to the apical border of microvilli in some glial cells. Sub-cellular features of endocytosis, predominantly the fluid phase, were observed in the apical glial cell cytoplasm. Glial cells were related to phagocytosis of apoptotic neurons, endocytosis, pinocytosis and adsorption. These functions were proposed based on their ultrastructure characteristics and a significant number of vesicles with different shapes (oval to polygonal), sizes 0.052-0.67 µm and contents. Glycogen, MPS and lipid were detected in the glial cells. Alkaline phosphatase was not observed, while an activity of acid phosphatase was bound to lysosomes. ATPases were present in the glial cells along the lateral and basal plasma lemma as well as on the membranes of cell organelles. Unspecific esterase was clearly recognizable by electron microscopy. The monoamine and cytochrome oxidase activities were demonstrated, while the succinate dehydrogenase showed a weak enzyme activity.

Fine structure of the central brain in the octopod Eledone cirrhosa (Lamarck, 1798) (Mollusca-Octopoda)

This study aims to investigate the fine structure of the different cell types in the central brain of Eledone cirrhosa; the organelles in the neurons and the glial cells; the glial hemolymph-brain barrier; the neuro-secretions and the relationships between glial and nerve cells. The brain is surrounded by a non-cellular neurilemma followed by a single layer of perilemmal cells. Ependymal cells, highly prismatic glial cells, astrocytes, oligodendrocytes and epithelial processes were observed. The perikarya of the neurons are filled with slightly oval nuclei with heterochromatin, a strongly tortuous ER, numerous mitochondria and Golgi apparatus with two types of vesicles. In the cellular cortex, glial cells are much less numerous than the neurons and they are located preferably at the border between perikarya and neuropil. Furthermore, they send many branching shoots between the surrounding neuron perikarya and the axons. The glial cytoplasmic matrix appears more electrodense than that of the neurons. Only few ribosomes are attached to the membranes of the ER; the vast majorities are free. In the perikarya of the glial cells, mitochondria, multi-vesicular bodies, various vacuoles and vesicles are present. The essential elements of the hemolymph-brain barrier are the endothelial cells with their tight junctions. The cytoplasm contains various vesicles and mitochondria. However, two other cell types are present, the pericytes and the astrocytes, which are of great importance for the function of the hemolymph-brain barrier. The cell-cell interactions between endothelial cells, pericytes and astrocytes are as close as no other cells.

Metabolite transport across the mammalian and insect brain diffusion barriers

The nervous system in higher vertebrates is separated from the circulation by a layer of specialized endothelial cells. It protects the sensitive neurons from harmful blood-derived substances, high and fluctuating ion concentrations, xenobiotics or even pathogens. To this end, the brain endothelial cells and their interlinking tight junctions build an efficient diffusion barrier. A structurally analogous diffusion barrier exists in insects, where glial cell layers separate the hemolymph from the neural cells. Both types of diffusion barriers, of course, also prevent influx of metabolites from the circulation. Because neuronal function consumes vast amounts of energy and necessitates influx of diverse substrates and metabolites, tightly regulated transport systems must ensure a constant metabolite supply. Here, we review the current knowledge about transport systems that carry key metabolites, amino acids, lipids and carbohydrates into the vertebrate and Drosophila brain and how this transport is regulated. Blood-brain and hemolymph-brain transport functions are conserved and we can thus use a simple, genetically accessible model system to learn more about features and dynamics of metabolite transport into the brain.

Insects as model systems in cell biology

For almost 100 years, insects have been favorable "model systems" in biology. Just to mention a few examples: fruit flies in genetics and developmental biology; bugs and caterpillars in hormone research; houseflies, blowflies, and locusts in neurobiology; silk moths in pheromone research; honeybees and crickets in neuroethology. For more than 50 years the electron microscope (EM) has been a valuable tool in analyzing the structure of cells and organs of these creatures. However, progress in specimen preparation was relatively slow compared with mammalian material and, in 1970, it was taken for granted that insects were much more difficult to fix than mammals. Since then, methods have dramatically improved, and satisfactory results can now be obtained routinely with chemical as well as cryofixation. In this chapter we briefly demonstrate what can be achieved with insect material, and help the researcher to find the most appropriate method for her/his systems and scientific questions.

Glial cells in peripheral nerves of the cockroach, Periplaneta americana

The ultrastructural organization and the junctional complexes of peripheral nerves have been investigated in the cockroach Periplaneta americana. Nerve 5 is surrounded by a layer of connective tissue, the neural lamella, beneath which is a layer of perineurial glial cells wrapping the axons. Adjacent perineurial cells are joined to one another by septate, gap and tight junctions. Septate and gap junctions were observed in freeze-fracture replicas of main trunk nerve 5. Septate junctions were found as rows of PF particles mainly in perineurial cell membranes. Gap junctions exhibited EF macular aggregates in perineurial and subperineurial glial cells. During incubations in vivo with extracellularly applied ionic lanthanum, the lanthanum did not penetrate beyond the perineurium. Where nerve 5 branches and contacts the muscle, lanthanum penetrated freely between the muscle fibres and the nerve branches. In small peripheral branches where the axons are surrounded by single a glial layer, lanthanum is unable to penetrate to the axolemma.

Organization and function of the blood-brain barrier in Drosophila

The function of a complex nervous system depends on an intricate interplay between neuronal and glial cell types. One of the many functions of glial cells is to provide an efficient insulation of the nervous system and thereby allowing a fine tuned homeostasis of ions and other small molecules. Here, we present a detailed cellular analysis of the glial cell complement constituting the blood-brain barrier in Drosophila. Using electron microscopic analysis and single cell-labeling experiments, we characterize different glial cell layers at the surface of the nervous system, the perineurial glial layer, the subperineurial glial layer, the wrapping glial cell layer, and a thick layer of extracellular matrix, the neural lamella. To test the functional roles of these sheaths we performed a series of dye penetration experiments in the nervous systems of wild-type and mutant embryos. Comparing the kinetics of uptake of different sized fluorescently labeled dyes in different mutants allowed to conclude that most of the barrier function is mediated by the septate junctions formed by the subperineurial cells, whereas the perineurial glial cell layer and the neural lamella contribute to barrier selectivity against much larger particles (i.e., the size of proteins). We further compare the requirements of different septate junction components for the integrity of the blood-brain barrier and provide evidence that two of the six Claudin-like proteins found in Drosophila are needed for normal blood-brain barrier function.

Pathophysiology of the blood-brain barrier: animal models and methods

The specialized cerebral microvascular endothelium interacts with the cellular milieu of the brain and extracellular matrix to form a neurovascular unit, one aspect of which is a regulated interface between the blood and central nervous system (CNS). The concept of this blood-brain barrier (BBB) as a dynamically regulated system rather than a static barrier has wide-ranging implications for pathophysiology of the CNS. While in vitro models of the BBB are useful for screening drugs targeted to the CNS and indispensable for studies of cerebral endothelial cell biology, the complex interactions of the neurovascular unit make animal-based models and methods essential tools for understanding the pathophysiology of the BBB. BBB dysfunction is a complication of neurodegenerative disease and brain injury. Studies on animal models have shown that diseases of the periphery, such as diabetes and inflammatory pain, have deleterious effects on the BBB which may contribute to neurological complications associated with these conditions. Furthermore, genetic and/or epigenetic abnormalities in constituents of the BBB may be significant contributing factors in disease etiology. Research that approaches the BBB as a dynamic system integrated with both the CNS and the periphery is therefore critical to understanding and treating diseases of the CNS. Herein, we review various methodological approaches used to study BBB function in the context of disease. These include measurement of transport between blood and brain, imaging-based technologies, and genomic/proteomic approaches.

Octopamine mimics the effects of parasitism on the foregut of the tobacco hornworm Manduca sexta

The parasitic braconid wasp Cotesia congregata lays its eggs inside the body of the larval stage of its host, the moth Manduca sexta. The Cotesia congregata larvae develop within the hemocoel of their host until their third instar, when they emerge and spin cocoons and pupate on the outer surface of the caterpillar. From this time until their death approximately 2 weeks later, the Manduca sexta larvae show striking behavioral changes that include dramatic declines in spontaneous activity and in the time spent feeding. Coincident with these behavioral changes, it is known that octopamine titers in the hemolymph of the host become elevated by approximately 6.5-fold. Octopamine is an important modulator of neural function and behavior in insects, so we examined hosts for neural correlates to the behavioral changes that occur at parasite emergence. We found that, in addition to the changes reported earlier, after parasite emergence (post-emergence), Manduca sexta larvae also showed marked deficits in their ability to ingest food because of a disruption in the function of the frontal ganglion that results in a significant slowing or the absence of peristaltic activity in the foregut. This effect could be produced in unparasitized fifth-instar larvae by application of blood from post-emergence parasitized larvae or of 10(−6)mol l(−1)d,l-octopamine (approximately the level in the hemolymph of post-emergence larvae). In contrast, blood from parasitized larvae before their parasites emerge or from unparasitized fifth-instar larvae typically had no effect on foregut activity. The effects of either post-emergence parasitized blood or 10(−6)mol l(−1) octopamine could be blocked by the octopamine antagonists phentolamine (at 10(−5)mol l(−1)) or mianserin (at 10(−7)mol l(−1)).

Blood barriers of the insect

The blood-brain barrier (BBB) ensures brain function in vertebrates and insects by maintaining ionic integrity of the neuronal bathing fluid. Without this barrier, paralysis and death ensue. The structural analogs of the BBB are occlusive (pleated-sheet) septate and tight junctions between perineurial cells, glia and perineurial cells, and possibly between glia. Immature Diptera have such septate junctions (without tight junctions) while both junctional types are found in the imago. Genetic and molecular biology of these junctions are discussed, namely tight (occludin) and pleated-sheet septate (neurexin IV). A temporal succession of blood barriers form in immature Diptera. The first barrier forms in the peripheral nervous system where pleated-sheet septate junctions bond cells of the nascent (embryonic) chordotonal organs in early neurogenesis. At the end of embryonic life, the central nervous system is fully vested with a blood-brain barrier. A blood-eye barrier arises in early pupal life. Future prospects in blood-barrier research are discussed.

Analog of vertebrate anionic sites in blood-brain interface of larval Drosophila

The blood-brain barrier ensures brain function in vertebrates and in some invertebrates by maintaining ionic integrity of the extraneuronal bathing fluid. Recent studies have demonstrated that anionic sites on the luminal surface of vascular endothelial cells collaborate with tight junctions to effect this barrier in vertebrates. We characterize these two analogous barrier factors for the first time on Drosophila larva by an electron-dense tracer and cationic gold labeling. Ionic lanthanum entered into but not through the extracellular channels between perineurial cells. Tracer is ultimately excluded from neurons in the ventral ganglion mainly by an extensive series of (pleated sheet) septate junctions between perineurial cells. Continuous junctions, a variant of the septate junction, were not as efficient as the pleated sheet variety in blocking tracer. An anionic domain now is demonstrated in Drosophila central nervous system through the use of cationic colloidal gold in LR White embedment. Anionic domains are specifically stationed in the neural lamella and not noted in the other cell levels of the blood-brain interface. It is proposed that in the central nervous system of the Drosophila larva the array of septate junctions between perineurial cells is the physical barrier, while the anionic domains in neural lamella are a "charge-selective barrier" for cations. All of these results are discussed relative to analogous characteristics of the vertebrate blood-brain barrier.

Innervation of dipteran eclosion muscles: ultrastructure, immunohistochemistry, physiology and death

The thoracic eclosion muscles of flies die by cytotoxic attack under neural control. We have investigated the innervation, ultrastructure and immunohistochemistry of the ventral eclosion muscle of Glossina. Two neurons located in the thoracic ganglion innervate this muscle. One of these is immunoreactive for serotonin and does not provide motor innervation. It appears to terminate near the attachment of an immunocyte involved in the dismantling of the muscle. The neuromuscular junction has features that distinguish it from any other chemical junction. A narrow, 3 nm gap separates pre- and post-synaptic membranes and this apparently acts to limit diffusion into and out of the junction. The immunocyte may use neuromuscular innervation as a path-finder to all muscle fibres and may even receive direct input from this source. Neuromuscular transmission is probably chemical as decreasing temperature results in decreasing amplitude of the (graded) muscle potential.

Fine structure and blood-brain barrier properties of the central nervous system of a dipteran larva

Using scanning and transmission electron microscopy, we studied basic ultrastructure, membrane specializations, and blood-brain barrier properties of the ventral ganglion and abdominal nerves of the last (third) instar larva of a dipteran fly, Delia platura. Both ganglion and nerves are covered with a non-cellular neural lamella. A monolayer of flattened perineurial cells lies beneath the neural lamella. Perineurial cells contain stores of metabolites and nutrients and these cells extensively interdigitate with one another. An extensive extracellular series of channels pervades perineurial cells. Glial cells beneath the perineurium envelope but do not entwine axons. In a minority of cases, adjacent axons in nerve and neuropil appear to be contiguous without glial intervention. Extensive (pleated) septate junctions with triangular septa are present between perineurial cells. Hemidesmosomes, half desmosomes (a first report for invertebrates), and desmosomes were also observed. Although no tight junctions were discovered, an effective blood-brain barrier exists, and tracer (ionic lanthanum) in no case reached neuronal surfaces. Extracellular tracer halted within the extensive septate junctions between perineurial cells. We postulate that in the absence of tight junctions the functional blood-brain barrier is effected by the septate junctions in the central nervous system of the Delia larva.

Fine structure of the blood-brain interface in the cuttlefish Sepia officinalis (Mollusca, Cephalopoda)

The blood-brain interface was studied in a cephalopod mollusc, the cuttlefish Sepia officinalis, by thin-section electron microscopy. Layers lining blood vessels in the optic and vertical lobes of the brain, counting from lumen outwards, include a layer of endothelial cells and associated basal lamina, a layer of pericytes and a second basal lamina, and perivascular glial cells. The distinction between endothelial cells and pericytes breaks down in small vessels. In the smallest microvessels, equivalent to capillaries, and in venous channels, and endothelial and pericyte layers are discontinuous, but a layer of glial cells is always interposed between blood and neural tissue, except where neurosecretory endings reach the second basal lamina. In microvessels in which cell membranes of the entire perivascular glial sheath could be followed, the glial layer was apparently 'seamless', not interrupted by an intercellular cleft, in ca 90% (27/30) of the profiles. Where a cleft did occur, it showed an elongated overlap zone between adjacent cells. The walls of venous channels are formed by lamellae of overlapping glial processes. In arterial vessels, the pericyte layer is thicker and more complete, with characteristic sinuous intercellular clefts. Arterioles are defined as vessels containing 'myofilaments' within pericytes, and arteries those in which the region of the second basal lamina is additionally expanded into a wide collagenous zone containing fibroblast-like cells and cell processes enclosing myofilaments. The 'glio-vascular channels' observed in Octopus brain are not a prominent feature of Sepia optic and vertical lobe. The organization of cell layers at the Sepia blood-brain interface suggests that it is designed to restrict permeability between blood and brain.

Modulation of a glial blood-brain barrier

CNS interstitial fluid homeostasis by the glial perineurial blood-brain barrier in the crayfish and cockroach is dependent on glial uptake mechanisms, low paracellular permeability, and the cation-binding properties of the extracellular matrix. Potassium selective permeability of the crayfish perineurium is modulated by a Ca(2+)-dependent mechanism at the basolateral membranes of the glial barrier and is ion and voltage dependent. In addition, extracellular charged sites are significant in perineurial K+ and Ca2+ homeostasis and may be modified by changes in pH. In the cockroach, and probably the crayfish, perineurial K+ transport may also be modulated by receptor-mediated changes in glial membrane permeability. The factors acting at the crayfish and cockroach blood-brain barrier are summarized in FIGURE 8 and would be well suited for providing efficient K+ spatial buffering of the CNS. Analogous processes have been described in vertebrate glial cells and in the endothelial blood-brain barrier, which implies a common primary function. The CNS is protected from large fluctuations in the body fluids by the blood-brain barrier, whereas glial uptake mechanisms control the composition of the brain interstitial fluid, and modulation of both barrier permeability and glial transport by the altered chemical environment following neuronal activity allows precise adjustment of the brain extracellular fluids to the changing needs of the CNS. The insect and crustacean ventral nerve cord and perineurial blood-brain barrier provide an excellent preparation in which the interactions between these factors can be investigated in intact CNS tissue.

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.

Temporal changes in activity during destruction of the thoracic ventral eclosion muscle of the tsetse fly

The spontaneous intracellular activity of the thoracic ventral longitudinal eclosion muscle (VLEM) of Glossina is described for the period from eclosion up to a short time before the final breakdown of recorded fibres. The VLEM comprises a single motor unit with no inhibitory input. The firing frequency of the motor unit declines over 5 h after eclosion and leg release. Over a period of inactivity lasting between 19-24 h in the sample fibres, there is no loss of resting membrane potential and occasional miniature potentials. The inactivity is ended by the sudden onset of biphasic potentials very different in form to the motor potentials and having a greatly reduced amplitude. These potentials fired at 6 Hz. lasted 2-4 h and ended with a rise in frequency to 25 Hz. No further activity is recorded and the fibres are observed to lose their striations and rigor. Experiments to characterize the ionic basis of activity in the VLEM have been done on spontaneous and evoked activity. Like other insect muscles, the VLEM has a major Ca2+ potential but unlike insect skeletal muscles, it also appears to have a TTX-sensitive component. This Na+ component is revealed by pretreating the system in Na(+)-free-choline saline, or by treatment with TEA in Ca(2+)-free saline. Neither EGTA nor cobalt abolish this potential. Addition of EGTA does not inhibit nerve evoked activity suggesting that the VLEM neuromuscular junction is in some way protected. The similarity of this muscle to insect visceral muscles is discussed.

Periaxonal ensheathment of lobster giant nerve fibres as revealed by freeze-fracture and lanthanum penetration

Sheath structure and permeability have been studied in the nerve fibres of lobster (Panulirus argus) walking limbs, in particular the individually ensheathed larger giant fibres, 100-150 microns in diameter, of which there are five or six in a peripheral bundle. They are easily distinguished and can be separated from neighbouring fibre bundles in which smaller giant axons (65-80 microns diameter) and many axons of much smaller diameter (5-15 microns) are ensheathed together. Each of the larger giant axons is enveloped by a Schwann cell layer outside of which is a multilayered sheath consisting of one-cell thick belts of flattened cells and interleaved zones of collagen fibrils and extracellular matrix. The cells in each belt lack basal lamina and, after freeze-fracture, as well as in thin sections, exhibit intercellular gap junctions and incomplete, fascia type, tight junctions; their most striking aspect is an exceedingly large number of exo-endocytic profiles. Permeability to lanthanum chloride in the bathing medium studied before or during fixation both in intact nerves and in nerves with surgically breached (slit) epineurium showed penetration of lanthanum tracer between the cells around the giant fibres, but the electron-dense tracer was excluded from the Schwann cell layer and the periaxonal space unless the epineurium had been slit. The extent of lanthanum diffusion was evaluated by transmission electron microscopy of thin sections and confirmed by X-ray microanalysis (EDAX) of comparable selected areas in such sections. The results indicate structural similarities but distinct permeability differences between the multilayered sheath surrounding the lobster giant axons and the vertebrate nerve perineurium. Other ultrastructural details provided by the freeze-fracture replicas concern the distribution of intramembrane particles in the axolemma and the Schwann and sheath cell membranes.

Different effects of the biogenic amines dopamine, serotonin and octopamine on the thoracic and abdominal portions of the escape circuit in the cockroach

1. The escape behavior of the cockroach, Periplaneta americana, is known to be modulated under various behavioral conditions (Camhi and Volman 1978; Camhi and Nolen 1981; Camhi 1988). Some of these modulatory effects occur in the last abdominal ganglion (Daley and Delcomyn 1981a, b; Libersat et al. 1989) and others in the thoracic ganglia (Camhi 1988). Neuromodulator substances are known to underlie behavioral modulation in various animals. Therefore, we have sought to determine whether topical application of putative neuromodulators of the escape circuit enhance or depress this circuit, and whether these effects differ in the last abdominal vs. the thoracic ganglia. 2. Topical application of the biogenic amines serotonin and dopamine to the metathoracic ganglion modulates the escape circuitry within this ganglion; serotonin decreases and dopamine enhances the response of leg motoneurons to activation of interneurons in the abdominal nerve cord by electrical or wind stimulation. 3. The neuropil of the thoracic ganglia contains many catecholamine-histofluorescent processes bearing varicosities, providing a possible anatomical substrate for dopamine release sites. 4. Topical application of octopamine to the terminal abdominal ganglion enhances the response of abdominal interneurons to wind stimulation of the cerci. In contrast, serotonin and dopamine have no effect at this site. 5. It is proposed that release of these biogenic amines may contribute to the known modulation of the cockroach escape response.

Nonsynaptic regulation of sensory activity during movement in cockroaches

Here we describe a nonsynaptic mechanism for filtering out potentially perturbing sensory feedback during locomotion. During flight, the cockroach moves its cerci, two abdominal sensory appendages, about their joint with the body and holds them in place. The cerci bear highly sensitive wind-receptive hairs, which would be strongly stimulated by flight wind. Such wind could cause habituation of the synaptic connections from these cercal receptors onto interneurons responsible for the running escape response to an approaching predator. We have found that the cercal displacement blocks one-third to one-half of the action potentials along the sensory nerve, possibly aiding in protection against such habituation. This block occurs if one experimentally displaces a cercus, and the block persists in the complete absence of any connections with the central nervous system. The block appears to be nonsynaptic and to result instead from mechanical pressure on the nerve near the joint. The results suggest that activity in peripheral nerves in other animals may also be affected by the position or movement of joints through which the nerves pass.

Glial membrane specializations and the compartmentalization of the lamina ganglionaris of the housefly compound eye

Membrane specializations in the lamina ganglionaris of the housefly are investigated using conventional thin-section EM, freeze-fracture replication and the diffusion of colloidal lanthanum. All glial cells in the lamina are coupled by gap junctions. Desmosomes also link all glia except the epithelial glia. Extensive glia-glial and glia-neuronal septate junctions are present in the pseudocartridge zone and nuclear layer. Septate junctions in the nuclear layer intermingle with bands of interglial and glia-neuronal tight junctions. Tight junctions are also found between satellite and epithelial glia at the border of the nuclear and plexiform layers, between adjacent epithelial glial cells in the plexiform layer, between epithelial and marginal glia at the proximal boundary of the optic neuropil, between marginal glial cells, and between marginal glia and axons. Colloidal lanthanum, introduced through an incision in the cornea, penetrates the retina but is occluded from the neuropil by septate junctions in the pseudocartridge zone. The disposition of tight and septate junctions is described in relation to the compartmentalization of the lamina. Two major compartments are delineated. The first represents the nuclear layer and contains the cell bodies of second-order visual neurons (monopolar neurons). The second compartment constitutes the plexiform layer of the lamina. Within the plexiform layer, each optic cartridge is partitioned into a separate subcompartment. Also, tracheoles and axons of long visual fibres are isolated from the optic cartridges by glial tight junctions. Morphological evidence for compartmentalization is correlated with previously established electrical properties of the insect lamina ganglionaris.

Differential accessibility to two insect neurones does not account for differences in sensitivity to alpha-bungarotoxin

The nicotinic acetylcholine receptor probe alpha-bungarotoxin (1.0 x 10(-7) M) blocks the depolarising response to ionophoretic application of acetylcholine onto the cell body membrane of the fast coxal depressor motoneurone (Df) of desheathed cockroach (Periplaneta americana) metathoracic ganglia, but at the same concentration is completely ineffective in blocking the depolarising action af acetylcholine on dorsal unpaired median (DUM) neurones in the same ganglion. The possibility that this is due to differences in accessibility of the toxin to the neurones has been tested by a combination of ionophoretic injection of horseradish peroxidase into single neurones with a study of the distribution of the exogenous tracer lanthanum, which is of similar effective size to alpha-bungarotoxin. The peripherally located cell body membranes and the fine axonal processes of Df and DUM neurones of desheathed metathoracic ganglia are equally accessible to lanthanum. Differential accessibility to the two cell types does not account therefore for the differences in sensitivity to alpha-bungarotoxin.

Structural and histochemical features of the avian blood-brain barrier

We have investigated the structural and histochemical features of the blood-brain barrier (b-bb) in both adults and embryos of chicken (Gallus domesticus, White Leghorn) and quail (Corturnix coturnix japonica). We found that brain endothelial cells of both species are characterized structurally by tight junctions, a low density of pinocytotic vesicles, and a moderately elevated density of mitochondria. Both alkaline phosphatase and butyryl cholinesterase were found in adult quail brain capillaries, but only alkaline phosphatase was found in adult chick brain capillaries. Aromatic amino acid decarboxylase was not found in brain capillaries of either species. In the chick embryo alkaline phosphatase appeared during the time when b-bb matures functionally; i.e., during the third week of development. However, an elevation in mitochondrial density was not apparent until after hatching. In the quail, alkaline phosphatase and butyryl cholinesterase appeared during the last week of embryonic development. By 2 days posthatching the structural characteristics of the brain capillaries were similar to those in the adult.

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.

Lack of restriction at the blood-brain interface in Limulus despite atypical junctional arrangements

Tracer and freeze-fracture techniques are used to evaluate the capacity of the central and peripheral nervous system of the horseshoe crab, Limulus polyphemus to admit or exclude molecular or ionic constituents of the blood intercellularly. Both the peripheral and central nervous systems are contained within blood sinuses so there is intimate contact between the haemolymph and the neural lamella. No discrete perineurium exists so any protection afforded to the nerve cells must be provided by the ensheathing glial cells and any junctions between them. Using ionic lanthanum as a pre-fixation incubation medium the system is seen to be completely "open', with the tracer gaining access to all regions of the nervous tissue. Cellular association in the peripheral nervous system, as revealed by thin-section and freeze-fracture, consist only of small scattered gap junctions between glial cells which afford no restriction to tracer entry. Gap junctions are again present between glial cells in the C.N.S. but here they are far more numerous, sometimes forming extensive sheets of almost continuous gap junctional plaques. Between certain glial cells there also exists a junctional system of linear PF ridges and complementary EF grooves; these may associate with or surround, often discontinuous arrays, the gap junctional plaques. Given their characteristics and the freedom of tracer entry, they seem unlikely to represent either typical occluding tight junctions or septate junctions.

A vertebrate-like blood--brain barrier, with intraganglionic blood channels and occluding junctions, in the scorpion

India ink and ionic lanthanum injections have revealed that the central nervous system (CNS) of the scorpion possesses a highly vascularized cephalothoracic ganglionic mass. It, together with other abdominal ganglia which form a ventral nerve cord, are all ensheathed by an outer layer of modified glial, or perineurial, cells. These cells resemble those which line the blood channels permeating the CNS, in exhibiting both inverted gap and tight junctions. Although the latter show close or fused membrane appositions, lanthanum appears to penetrate past a number, but not all, of them. Freeze-fracturing reveals that these junctions are composed of E-face particles aligned into a network of rows, or ridges, which are frequently discontinuous, especially near the periphery of the perineurium. This produces a somewhat 'leaky' system but occlusion to tracers occurs ultimately, for in the CNS none can be found beyond the perineurium. The existence of this perineurial blood-brain barrier is also demonstrable electrophysiologically where cations such as Mg2+ are unable to penetrate beyond the perineurial layer although they can, it seems, leak in via the blood vascular system. Relative differences in tightness between the perineurium and the cells lining the blood channels may be attributed to differences in the relative number of discontinuous ridges. This is borne out by the observation that the peripheral nervous system has a highly attenuated perineurium with many fewer junctions, and some of these nerves tend to be leaky with respect to tracer penetration. In fixed material the junctional ridges may fracture on to the E-face or partly on both the EF and PF, while in unfixed tissue they are usually found on the PF. In both cases they exhibit complementary grooves that are coincident with the ridges across membrane transitions; in such cases the cell membranes are fused with concomitant obliteration of the intercellular space. These tight junctions, often closely associated with EF gap junctional particle aggregates which may be very loosely clustered, appear to form the basis of the observed blood-brain barrier in the scorpion CNS.

Definitive evidence for the existence of tight junctions in invertebrates

Extensive and unequivocal tight junctions are here reported between the lateral borders of the cellular layer that circumscribes the arachnid (spider) central nervous system. This account details the features of these structures, which form a beltlike reticulum that is more complex than the simple linear tight junctions hitherto found in invertebrate tissues and which bear many of the characteristics of vertebrate zonulae occludentes. We also provide evidence that these junctions form the basis of a permeability barrier to exogenous compounds. In thin sections, the tight junctions are identifiable as punctate points of membrane apposition; they are seen to exclude the stain and appear as election- lucent moniliform strands along the lines of membrane fusion in en face views of uranyl-calcium-treated tissues. In freeze-fracture replicas, the regions of close membrane apposition exhibit P-face (PF) ridges and complementary E-face (EF) furrows that are coincident across face transitions, although slightly offset with respect to one another. The free inward diffusion of both ionic and colloidal lanthanum is inhibited by these punctate tight junctions so that they appear to form the basis of a circumferential blood-brain barrier. These results support the contention that tight junctions exist in the tissues of the invertebrata in spite of earlier suggestions that (a) they are unique to vertebrates and (b) septate junctions are the equivalent invertebrate occluding structure. The component tight junctional 8- to 10-nm-particulate PF ridges are intimately intercalated with, but clearly distinct from, inverted gap junctions possessing the 13-nm EF particles typical of arthropods. Hence, no confusion can occur as to which particles belong to each of the two junctional types, as commonly happens with vertebrate tissues, especially in the analysis of developing junctions. Indeed, their coexistance in this way supports the idea, over which there has been some controversy, that the intramembrane particles making up these two junctional types must be quite distinct entities rather than products of a common precursor.

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.

Junctional structures in digestive epithelia of a cephalopod

Intercellular junctions have been studied in the epithelia of digestive organs of Sepia officinalis (digestive gland, digestive duct appendages and caecum) by conventional staining, lanthanum tracer and freeze-fracturing techniques. In the three organs studied the same junctional complex occurs, consisting of a belt desmosome, a septate junction and gap junctions. The septate junction isd of pleated-sheet type and the gap junction has its particles on the P face of the fracture. Circular structures have been found in the digestive gland septate junctions. Neither continuous nor tight junctions have been found. These results show that Cephalopods have junctional structures very close to those of other Molluscs and of Annelids. Some small differences between the septate junctions of the three organs could be related to their different physiology.

Intercellular junctions and the development of the blood-brain barrier in Manduca sexta

In early embryonic development of the tobacco horn moth no blood-brain barrier is present, as shown by the unimpeded entry of exogenous tracers into the nervous system. However, later on, just before hatching, lanthanum and horseradish peroxidase (HRP) are unable to move inwardly beyond the level of the perineurium, which is the morphological site of the blood--brain barrier in the adult moth, as well as in other insects. Freeze-fracture studies indicate that in the early embryo, 10 nm particles are scattered about in the perineurial membrane PF, either as separate entities or as short linear arrays. By hatching or just before, however, the 10 nm particles have become aligned into lengthy linear aggregates as PF ridges with EF grooves. These would appear to be the simple, arthropod-form of tight junction, and are presumed to be the basis of the perineurial blood-brain barrier. At about the same time, gap junctional elements appear both between adjacent perineurial cells and between glial cells. In both cell types, the gap junctions form from free 13 nm EF particles which gradually become aligned or clumped into strands and aggregates which ultimately coalesce to form first irregular masses and then the macular plaques typical of mature gap junctions. Many of the latter stages are coincident with the hatching of a motile larvae, so that the perineurial and glial cells are by this stage coupled via the channels of the gap junctional particles. They are therefore able to undergo both ionic and metabolic exchange and cooperation during larval life, in addition to being able to respond to hormonal substances in an integrated way. During the 5 larval instars more gap junctions form as the perineurial layer grows thicker. These junctions become more regular in outline and their particles more tightly packed; these larval structures are compared with junctions found in the adult which tend to be more extensive but otherwise similar. Since no septate junctions are apparent during Manduca embryonic or larval life when the blood-brain barrier forms, nor in adults, the results of this report support the contention that it is the tight junctions rather than septate ones which form the basis of permeability barriers in this, and probably other, arthropod systems.

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.