Fine structure of the neural sheath, glia and neurons in the brain of the housefly, Musca domestica

Membrane Junctions Between Neurons in the Brain of the Housefly, Musca Domestica

Invertebrate organisms are increasingly being employed for basic neurophysiological studies, however, the cytological features of the invertebrate brain are not very well understood. The present report describes the interneuronal membrane junctions in the brain of the housefly, Musca domestica. Soma of the neurons in Musca brain are located on the periphery of the brain and are completely surrounded by glioplasm which precludes theexistence of synaptic junctionsl. Synaptic sites occur in the neuropil mass which constitutes the core of the brain and is formed by.the intermingling of axons and glial processes. Glial processes only partially surround the axons.

Metabolic enzymes in glial cells of the honeybee brain and their associations with aging, starvation and food response

The honey bee has been extensively studied as a model for neuronal circuit and memory function and more recently has emerged as an unconventional model in biogerontology. Yet, the detailed knowledge of neuronal processing in the honey bee brain contrasts with the very sparse information available on glial cells. In other systems glial cells are involved in nutritional homeostasis, detoxification, and aging. These glial functions have been linked to metabolic enzymes, such as glutamine synthetase and glycogen phosphorylase. As a step in identifying functional roles and potential differences among honey bee glial types, we examined the spatial distribution of these enzymes and asked if enzyme abundance is associated with aging and other processes essential for survival. Using immunohistochemistry and confocal laser microscopy we demonstrate that glutamine synthetase and glycogen phosphorylase are abundant in glia but appear to co-localize with different glial sub-types. The overall spatial distribution of both enzymes was not homogenous and differed markedly between different neuropiles and also within each neuropil. Using semi-quantitative Western blotting we found that rapid aging, typically observed in shortest-lived worker bees (foragers), was associated with declining enzyme levels. Further, we found enzyme abundance changes after severe starvation stress, and that glutamine synthetase is associated with food response. Together, our data indicate that aging and nutritional physiology in bees are linked to glial specific metabolic enzymes. Enzyme specific localization patterns suggest a functional differentiation among identified glial types.

The morphology and fine structure of the giant interneurons of the wood cricket Nemobius sylvestris

The structural and ultrastructural characteristics of giant interneurons in the terminal abdominal ganglion of the cricket Nemobius sylvestris were investigated by means of cobalt and fluorescent dye backfilling and transmission electron microscopy. The projections of the 8 eight pairs of the biggest ascending interneurons (giant interneurons) are described in detail. The somata of all interneurons analyzed are located contralateral to their axons, which project to the posterior region of the terminal ganglion and arborise in the cercal glomerulus. Neuron 7-1a is an exception, because its arborisation is restricted to the anterior region of the ganglion. The fine structure of giant interneurons shows typical features of highly active cells. We observed striking indentations in the perineural layer, enabling the somata of the giant interneurons to be very close to the haemolymph. The cercal glomerulus exhibits a high diversity of synaptic contacts (i.e. axo-dendritic, axo-axonic, dendro-axonic, and dendro-dendritic), as well as areas of tight junctions. Electrical synapses seem to be present, as well as mixed synapses. The anatomical organization of the giant interneurons is finally discussed in terms of functional implications and on a comparative basis.

Organization and postembryonic development of glial cells in the adult central brain of Drosophila

Glial cells exist throughout the nervous system, and play essential roles in various aspects of neural development and function. Distinct types of glia may govern diverse glial functions. To determine the roles of glia requires systematic characterization of glia diversity and development. In the adult Drosophila central brain, we identify five different types of glia based on its location, morphology, marker expression, and development. Perineurial and subperineurial glia reside in two separate single-cell layers on the brain surface, cortex glia form a glial mesh in the brain cortex where neuronal cell bodies reside, while ensheathing and astrocyte-like glia enwrap and infiltrate into neuropils, respectively. Clonal analysis reveals that distinct glial types derive from different precursors, and that most adult perineurial, ensheathing, and astrocyte-like glia are produced after embryogenesis. Notably, perineurial glial cells are made locally on the brain surface without the involvement of gcm (glial cell missing). In contrast, the widespread ensheathing and astrocyte-like glia derive from specific brain regions in a gcm-dependent manner. This study documents glia diversity in the adult fly brain and demonstrates involvement of different developmental programs in the derivation of distinct types of glia. It lays an essential foundation for studying glia development and function in the Drosophila brain.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

Morphology of neuroglia in the antennal lobes and mushroom bodies of the brain of the honeybee

We investigated the distribution and anatomical organization of glial cells in the antennal lobes and mushroom bodies of the honeybee. Reconstructions from serial sections, prepared according to the ethyl gallate method, revealed the entire morphology of glial cells in neuropiles, tracts, and the soma rind. The distribution of the glial cell bodies in the neuropiles was derived from the staining of cell nuclei with a fluorescent dye. There are glial cells of different shape in the soma rind which are wrapped around the neuronal cell bodies of the antennal lobes and the Kenyon cells of the mushroom bodies. Glial cells surround neuropilar areas such as the external and lateral sides of the glomeruli of the antennal lobes. Whereas we could not detect glia in the glomerular neuropile, glial cells with long processes are located in the core of the antennal lobe. Extensions of these glial cells also invade tracts containing the olfactory projection neurons. A layer of glial cells separates the mushroom body neuropile from the surrounding protocerebral neuropile. The neuropile of the mushroom bodies is clearly compartmented by glial cells. There is a high density of astrocyte-like glia in a column of the pedunculus which can be followed to the ventral part of the alpha-lobe. A network of mushroom body intrinsic glial cells separates the alpha-lobe from the beta-lobe and the pedunculus. This anatomical description of glial cell types in olfactory information processing pathways of an insect brain provides a framework for further physiological studies of neuroglia in dissociated cell culture.

Distribution, classification, and development ofDrosophila glial cells in the late embryonic and early larval ventral nerve cord

To facilitate the investigation of glial development inDrosophila, we present a detailed description of theDrosophila glial cells in the ventral nerve cord. A GAL4 enhancer-trap screen for glial-specific expression was performed. Using UAS-lacZ and UAS-kinesin-lacZ as reporter constructs, we describe the distribution and morphology of the identified glial cells in the fully differentiated ventral nerve cord of first-instar larvae just after hatching. The three-dimensional structure of the glial network was reconstructed using a computer. Using the strains with consistent GAL4 expression during late embryogenesis, we traced back the development of the identified cells to provide a glial map at embryonic stage 16. We identify typically 60 (54-64) glial cells per abdominal neuromere both in embryos and early larvae. They are divided into six subtypes under three categories: surface-associated glia (16-18 subperineurial glial cells and 6-8 channel glial cells), cortex-associated glia (6-8 cell body glial cells), and neuropile-associated glia (8-10 nerve root glial cells, 14-16 interface glial cells, and 3-4 midline glial cells). The proposed glial classification system is discussed in comparison with previous insect glial classifications.

Symmetry analysis of cell nuclei

An algorithm is described that is used to analyze the two-dimensional spatial symmetry of cell nuclei. The method provides two symmetry features: the symmetry index (SI), which estimates the precise spatial symmetry of a given chromatin component, Cn, and the quadrant symmetry index (QSI), which estimates the number of quadrants being occupied by Cn. A previous analysis is used to show that age-related change in Malpighian tubule nuclei from the adult housefly is associated with significant alterations in the spatial symmetry of low-, medium-, and high-density chromatin components (LDC, MDC, HDC). This included a seven-fold increase in the spatial symmetry of HDC and a shift in the symmetry profile (from highest to lowest degree of symmetry) from LDC-MDC-HDC to MDC-LDC-HDC. The increased spatial symmetry of HDC suggests that it occurs at new nuclear sites as the fly ages and that these sites are distributed over approximately 60% of the chromosome population.

Effects of increased lifespan on chromatin condensation in the adult male housefly

Analysis of the chromatin condensation pattern in optic lobe nuclei from the adult male housefly has shown significant differences in males from a long lived, low activity (LA) group as compared to a short lived, high activity (HA) group. The rate of chromatin condensation in the LA group was very much less than that observed in the HA group, leaving the nuclei from the former group in a permanently lowered condensation state. Moreover, a detailed comparison of the amount and spatial distribution of a single high density chromatin component (HDC) suggests that the low activity conditions not only lowers the rate of chromatin condensation but also alters the normal program controlling this process. Linear discriminant analysis showed that a consistently higher percentage of nuclei from the LA group, as compared to the HA group, could be classified with a 1-day-old model derived from the HA group. These results are discussed with respect to environmental influences on lifespan and are compared to the response of lipofuscin accumulation in low activity male houseflies.

Undifferentiated cells present in the pars intercerebralis of larval and adult locusts are glial precursors. Autoradiographic and ultrastructural study in vivo and in vitro

The intercerebral part of the protocerebrum from embryos, larvae and imagos of Locusta migratoria was investigated in vivo and after culture of the brain in vitro using light and electron microscopy. The results showed the presence in embryo and persistence in larva and adult of 2 clusters of mitotically active embryonic cells in the inner part of each half of the pars intercerebralis. The fate of these undifferentiated cells was investigated during postembryonic life by in vitro and in vivo labeling with tritiated thymidine combined with counts of nervous cells of the pars intercerebralis. Autoradiographic results confirmed the mitotic activity of the undifferentiated cells and established the pattern of this activity which declines from the third larval instar to adult stage. Mitoses were never seen in neurons and glial cells. Neurons were unlabeled and their number was constant. Glial cells were labeled and their number increases throughout postembryonic life with a pattern of proliferation similar to the pattern of mitotic activity of the undifferentiated cells. These observations indicate that the undifferentiated cells of the pars intercerebralis of the locust represent a source of glial cells and could be called glioblasts.

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.

Perineurial and glial cells in the tick Boophilus microplus (Acarina: Ixodidae): freeze-fracture and tracer studies

In the cattle tick Boophilus microplus, the cells of the perineurium are characterized by accumulations of glycogen which increase dramatically after feeding. Gap junctions couple both these perineurial cells which enshealth the C.N.S. and the underlying glial cells. No tight junctions have been found between perineurial cells and there is in consequence no blood-brain barrier. Using ionic lanthanum as a tracer the extensive gap junctions are shown to have no occluding effect and lanthanum penetrates through the perineurium and glial layers to the level of the axonal surfaces. By colloidal lanthanum impregnation and freeze-fracture studied, the gap junctions appear to be typical of arthropids in that their particles show a characteristic diameter (13 nm in freeze-fracture), are distributed relatively loosely within the junctional plaques and fracture onto the E face of the junctional membranes. Semi-ordered particle arrays are found on E face membranes of adjacent axons and glia which may represent axoglial junctions.

The brain of the horseshoe crab (Limulus polyphemus). I. Neuroglia

The brain of the horseshoe crab, Limulus polyphemus, harbors three populations of neuroglial cells, whose distribution and cellular details are best appreciated by a combination of silver impregnation, scanning, and transmission electron microscopy. Stellate astrocytes envelop neurons as satellite cells, permeate the neuropile, and secrete a framework of sustentacular trabeculae throughout the brain. Velate astrocytes are restricted to Kenyon cells, i.e. small association neurons, of which they harbor up to 150 per neuroglial cell. Vascular neuroglia is composed of glycogen and mitochondria-laden, interlocked cells that form an open meshwork in the hemocoelic spaces of the brain. Aside from supportive functions of neuroglia, the vascular neuroglial cells in particular seem to subserve the role of a metabolic reserve cell for the central nervous system.