Biomedical subjects

S D Carlson

Publications and source records attributed to S D Carlson.

At least 19 recordsLinked to original sources

PubMed2001

synaptotagmin mutants reveal essential functions for the C2B domain in Ca2+-triggered fusion and recycling of synaptic vesicles in vivo.

Synaptotagmin has been proposed to function as a Ca(2+) sensor that regulates synaptic vesicle exocytosis, whereas the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex is thought to form the core of a conserved membrane fusion machine. Little is known concerning the functional relationships between synaptotagmin and SNAREs. Here we report that synaptotagmin can facilitate SNARE complex formation in vitro and that synaptotagmin mutations disrupt SNARE complex formation in vivo. Synaptotagmin oligomers efficiently bind SNARE complexes, whereas Ca(2+) acting via synaptotagmin triggers cross-linking of SNARE complexes into dimers. Mutations in Drosophila that delete the C2B domain of synaptotagmin disrupt clathrin AP-2 binding and endocytosis. In contrast, a mutation that blocks Ca(2+)-triggered conformational changes in C2B and diminishes Ca(2+)-triggered synaptotagmin oligomerization results in a postdocking defect in neurotransmitter release and a decrease in SNARE assembly in vivo. These data suggest that Ca(2+)-driven oligomerization via the C2B domain of synaptotagmin may trigger synaptic vesicle fusion via the assembly and clustering of SNARE complexes.

Adaptor Protein Complex alpha Subunits↗

PubMed2000

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.

Animals↗

PubMed1998

Temperature-sensitive paralytic mutations demonstrate that synaptic exocytosis requires SNARE complex assembly and disassembly.

The neuronal SNARE complex is formed via the interaction of synaptobrevin with syntaxin and SNAP-25. Purified SNARE proteins assemble spontaneously, while disassembly requires the ATPase NSF. Cycles of assembly and disassembly have been proposed to drive lipid bilayer fusion. However, this hypothesis remains to be tested in vivo. We have isolated a Drosophila temperature-sensitive paralytic mutation in syntaxin that rapidly blocks synaptic transmission at nonpermissive temperatures. This paralytic mutation specifically and selectively decreases binding to synaptobrevin and abolishes assembly of the 7S SNARE complex. Temperature-sensitive paralytic mutations in NSF (comatose) also block synaptic transmission, but over a much slower time course and with the accumulation of syntaxin and SNARE complexes on synaptic vesicles. These results provide in vivo evidence that cycles of assembly and disassembly of SNARE complexes drive membrane trafficking at synapses.

Amino Acid Sequence↗

PubMed1997

Ultrastructure and blood-nerve barrier of chordotonal organs in the Drosophila embryo.

Chordotonal organs of Drosophila embryos have become models for studies of developmental biology and molecular genetics due to their consistent segmental placement and mutability. Our first goal was to find the origin and anatomical correlate of the blood-nerve barrier of this PNS proprioreceptor in wild type embryos. The concept of a blood-nerve barrier for the PNS of the Drosophila embryo is new, and the present data are the first in this regard. A second goal was to reveal the ultrastructure of these four-celled stretch receptors, focusing particularly on the 'core' of this organ: the bipolar neuron enclosed by a scolopale cell. These latter data have resulted in a graphic reconstruction of the chordotonal organ which reveals how the four consistent cells fit together. At Stage 13 we first observed a clearly recognizable scolopale cell with an enclosed neuron. Surprisingly, an operative blood-nerve barrier, comprised of occlusive pleated-sheet septate junctions, exists at this relatively early stage. A blood-brain barrier is not yet functioning in the CNS during this same stage, as the perineurium is not present until Stage 17. Cross-sectional views of a more mature chordotonal organ show that the neuron's inner segment has a 'tongue-in-groove' formation which fits the dendrite into the scolopale cell. Other newly discovered fine structural features are: hemidesmosomes linking individual scolopale rod bundles to the inner dendrite, and a cap cell matrix bonding with the tip of the ciliary dendrite. Functional aspects of these findings are discussed.

Animals↗

PubMed1997

First developmental signs of the scolopale (glial) cell and neuron comprising the chordotonal organ in the Drosophila embryo.

The chordotonal (scolopidial) organ is a segmental stretch receptor (proprioceptor) and a four-celled organ of the peripheral nervous system of Drosophila. This organ has become a model in studies of embryogenesis involving molecular genetics and developmental biology. We determined how three glial cells and a bipolar neuron (the chordotonal organ) develop and assemble to become a sensitive stretch receptor. Our focus was on the scolopale cell-neuron association which is the core of this organ. The first anatomical appearance of these developing organs appears in Stage 12 embryos: a central fissure forms in the scolopale cell and two intracellular adherence loci redirect the cleft to wall off a cylindrical sector. This portion hollows out and the dendrite of a sensory neuron enters the cavity. Cytoplasmic walls of the cylinder then regress to leave a more prominent lymph space, within which is the cilary portion of the sensory dendrite. A cap (glial) cell then covers and connects the distal portion of the scolopale-neuron pair. Chordotonal organs are formed in about two hours and assembled throughout Stages 13 to 15. We have drawn schematics of these developmental phases with the resultant four-celled organ. Lanthanum tracer in the embryonic hemocoel never gained access to the neuron housed by the scolopale cell. Thus a blood-nerve barrier forms as early as Stage 13 in the peripheral nervous system. Paracellular clefts, sealed by occlusive septate junctions between accessory (glial) cells, are the probable basis for barrier properties.

Animals↗

PubMed1994

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.

Animals↗

PubMed1992

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.

Abdomen↗

PubMed1989

Ultrastructure of the ocellar visual system in normal and mutant Drosophila melanogaster.

Between the two compound eyes on the vertex on the adult head are the three simple eyes, ocelli. Transmission and scanning electron microscopy were used to investigate the corneal lenses, ocellar photoreceptors, and axonal projections in normal and mutant Drosophila melanogaster. In wild type flies, the cornea consists of about 45 lamellae. It has corneal nipples distally and is underlaid with a monolayer of corneagenous cells. Retinula cells have open rhabdomeres of about 2 microns (diameter) x 7 microns (length). Rhabdomeres extend to the distal extent of the cell and do not have caps. Microvilli have a rodlet within. Retinula cells are joined by belt desmosomes on the lateral borders. Eye color pigment granules are housed within the retinula cells of normal flies, not in accessory cells. The granules do not migrate in response to light. No screening pigment granules exist in the white mutant. Each ocellus has about 80 retinula cells whose axons project to corresponding ganglia from which 4 giant afferent interneurons (per ganglion) project to the brain. receptor terminals are invested with capitate projections from glia. Receptors synapse onto dyads of follower cells, usually interneuron processes, at sites of T shaped presynaptic ribbons. These "T bars" are surrounded by indistinct flattened vesicles. Interneurons make feed back synapses onto receptor terminals at T bars clustered with distinct round vesicles. Three mutants with abnormal ocelli were investigated. The none mutant has unusual compound eye and ocellar corneas. The compound eye is devoid of differentiated photoreceptors but some axons from undifferentiated cells from synapses. No receptors were found in the ocelli of none. The oc mutant has no ocelli, although sometimes an ocellar cornea like that of none is seen; the compound eye is normal. The rdo mutant is also specific to ocelli with smaller ocelli having half the wild type allotment of receptor cells; the number of giant afferents is unaffected. Mutants best known for their compound eye defects were examined. The norpA mutant loses its ocellar rhabdomeres with age but has normal feed forward and feed back synapses. This normal synaptology prevails despite the electrophysiological defects in norpA ocelli reported earlier. The rdgABS12 mutant has poorly formed ocellar receptors which show some degeneration with age but synapses survive. The trp. rdgBKS222 and rgdAPC47 mutants are essentially normal with respect to structure and survival of ocellar receptors and synapses.

Animals↗

PubMed1989

Photoreceptor maintenance and degeneration in the norpA (no receptor potential-A) mutant of Drosophila melanogaster.

The norpA (no receptor potential) mutant of Drosophila melanogaster has a visual transduction deficit. This study determines whether lack of function leads to structural repercussions in photoreceptor cells of the compound eye and their synapses. For this purpose, we examined thin sections and freeze fracture replicas of norpA using transmission electron microscopy. Ultrastructurally, retinula cells in the compound eye and all aspects of the first optic neuropil (lamina ganglionaris) are essentially normal in newly emerged flies. However, as expected, intraretinular pigment granules fail to show their light elicited aggregation; further, the P face particle density is somewhat lower than in wild type. We confirm that there are unusual membrane specializations on the plasmalemma of the retinula cell dubbed "zippers." Zippers appear to increase with age and can cause a distorted geometry of ommatidia. Only a few retinula cells ultimately degenerate in norpA, and the proportion may not differ from that of wild type. Despite the absence of the receptor potential in norpA, many aspects of the turnover of rhabdomeric membrane appear to be as in wild type.

Animals↗

PubMed1989

Cardiovascular reactivity and interpersonal influence: active coping in a social context.

Previous studies have demonstrated that effortful attempts to secure positive outcomes or avoid negative outcomes produce significant increases in systolic blood pressure (SBP), diastolic blood pressure (DBP), and heart rate (HR). Although these effects of active coping on cardiovascular reactivity are central in current psychosomatic theories, virtually all of the research to date has used impersonal, asocial tasks. Our two studies examined the cardiovascular effects of effortful attempts to influence other people. In Study 1, male subjects attempting to influence the opinions of their discussion partner to improve their own chances of winning money displayed significantly greater SBP, DBP, and HR reactivity. In Study 2, we obtained similar effects on SBP and DBP reactivity in men and women, while both preparing an influence attempt and making that attempt. Furthermore, reactivity levels were larger as the magnitude of incentive for successful persuasion increased. Implications of this interpersonal equivalent of active coping for the development of cardiovascular disease are discussed.

Blood Pressure↗

PubMed1986

Ultrastructure of capitate projections in the optic neuropil of Diptera.

Photoreceptor axons in the first optic neuropil of the dipteran flies Musca domestica and Drosophila melanogaster was examined with electron microscopy. The objective was to determine ultrastructure, persistence and glial source of the capitate projections found within these neurons. Capitate projections are simple or compound processes of epithelial glial cells which profusely insert into form-fitting folds of axon terminals of the peripheral retinular cells (R1-6) in the synaptic plexus portion of the first optic neuropil. These neuro-glial junctions may be simple indentations, have a head with a single stalk, or possess a single, circular stalk from which 3 or 4 bulbous (glial) heads are elaborated. Using serial thick sections of Drosophila neuropil for HVEM we were able to observe that the stalks connecting nearly all capitate projections led directly to a glial cell. Thus no disembodied heads were found suspended in axoplasm. Capitate projections appeared to be persistent structures, present in young as well as senescent adults. No evolution of form was found; thus 3 distinct expressions of these glial processes (without transitional forms) are present. From freeze-fracture replicas and serial HVEM sections it was determined that there were approximately 3 capitate projections per micron 2 in Drosophila and Musca, respectively. About 800 capitate projections exist per Musca axon terminal or about 5 times the number of chemical synapses. Cp's were slightly larger in Drosophila than in Musca, although the Musca retinular axon has twice the diameter and length of that of the fruit fly. The evidence was reviewed in light of the likely supportive function of capitate projections on the R1-6 terminals.

Animals↗

PubMed1985

Interneuronal and glial-neuronal gap junctions in the lamina ganglionaris of the compound eye of the housefly, Musca domestica.

The cell-body layer of the lamina ganglionaris of the housefly, Musca domestica, contains the perikarya of five types of monopolar interneuron (L1-L5) along with their enveloping neuroglia (Strausfeld 1971). We confirm previous reports (Trujillo-Cenóz 1965; Boschek 1971) that monopolar cell bodies in the lamina form three structural classes: Class I, Class II, and midget monopolar cells. Class-I cells (L1 and L2) have large (8-15 microns) often crescent-shaped cell bodies, much perinuclear cytoplasm and deep glial invaginations. Class-II cells (L3 and L4) have smaller perikarya (4-8 microns) with little perinuclear cytoplasm and no glial invaginations. The 'midget' monopolar cell (L5) resides at the base of the cell-body layer and has a cub-shaped cell body. Though embedded within a reticulum of satellite glia, the L1-L4 monopolar perikarya and their immediately proximal neurites frequently oppose each other directly. Typical arthropod (beta-type) gap junctions are routinely observed at these interfaces. These junctions can span up to 0.8 micron with an intercellular space of 2-4 nm. The surrounding nonspecialized interspace is 12-20 nm. Freeze-fracture replicas of monopolar appositions confirm the presence of beta-type gap junctions, i.e., circular plaques (0.15-0.7 micron diam.) of large (10-15 nm) E-face particles. Gap junctions are present between Class I somata and their proximal neurites, between Class I and Class II somata and proximal neurites, and between Class II somata. Intercartridge coupling may exist between such monopolar somata. The cell body and proximal neurite of L5 were not examined. We also find that Class I and Class II somata are extensively linked to their satellite glia via gap junctions. The gap width and nonjunctional interspace between neuron and glia are the same as those found between neurons. The particular arrangement and morphology of lamina monopolar neurons suggest that coupling or low resistance pathways between functionally distinct neurons and between neuron and glia are probably related to the metabolic requirements of the "nuclear" layer and may play a role in wide field signal averaging and light adaptation.

Animals↗

PubMed1984

Blue and ultraviolet light induced damage to the Drosophila retina: ultrastructure.

Intense ultraviolet (UV) and blue stimulation photolyses rhodopsin through a fluorescent metarhodopsin (M') in the predominant photoreceptor type, R1-6, of the compound eye of white eyed Drosophila melanogaster. We investigated the associated retinal degeneration using High Voltage Electron Microscopy (HVEM). The threshold for UV induced damage was about 19 log quanta/cm2 while for blue, the threshold was about 20. These intensities are toward the upper level of the dynamic range for rhodopsin photolysis. Thus, there is a sensitization for near UV induced degeneration as had been found for photolysis of the visual pigment. Vitamin A deprivation protects against light elicited retinal degeneration, particularly in the UV. Since vitamin A deprivation eliminates the blue absorbing rhodopsin and a UV sensitizing pigment in R1-6, the degeneration is likely mediated through quantal absorption through these photoexcitation pigments. Intense light converts the microvilli of the rhabdomeres (the photopigment containing organelles) into dense strands and the cytoplasm fills with a dense reticulum. Such damage is elicited shortly after stimulation and is permanent. Under most conditions, the second order interneurons are spared. These results are discussed in the context of other animal models of intense light retinal degeneration.

Animals↗

PubMed1983

Ultrastructure of the compound eye and first optic neuropile of the photoreceptor mutant oraJK84 of Drosophila.

The developmental mutant of Drosophila (oraJK84) is characterized by nonfunctional photoreceptor cells (R1-6), while the R7/R8 cells are normal. A fundamental question is: Does the near absence of photosensitive membranes inhibit development of the R1-6 axons and their synapses at the other end of the cell? The retina and first optic neuropile (lamina ganglionaris) were examined with freeze-fracture technique and high voltage electron microscopy. R1-6 have reduced rhabdomere caps; rhabdomeric microvilli have about 50% of the normal diameter and 20% of the normal length. Affected cells exhibit prominent vacuoles which appear to communicate with some highly convoluted microvillar membranes. Almost no P-face particles (putative rhodopsin molecules) are present in the R1-6 rhabdomeres, and particle densities are lower in R7 than previously reported. Near the rhabdomere caps, microvilli of R1-6 are fairly normal, but at more proximal levels they are greatly diminished in length and changed in orientation, while at still more proximal levels they are lost. R1-6, R7, and R8 axons from each ommatidium are bundled into normal pseudocartridges beneath the basement membrane. No abnormalities are found in the lamina ganglionaris, and all synaptic associations as well as the presumed "virgin" synapses (of R1-6) appear normal. No glial anomalies are present, and R7/R8 axons project through the lamina in the usual fashion. These fine structural findings are correlated with known electrophysiological, biochemical, and behavioral correlates of both sets of photoreceptors (R1-6, and R7/R8).

Animals↗

PubMed1983

The fine structure of neuroglia in the lamina ganglionaris of the housefly, Musca domestica L.

Six morphologically distinct glial cell layers are described in the housefly lamina ganglionaris, a region previously thought to be composed of only three. 1. The external glial layer abuts the basement membrane of the retina. The cells of this layer have a highly involuted surface membrane and an abundance of ribosomes and rough endoplasmic reticulum (ER) throughout their cytoplasm. They envelop the traversing photoreceptor and mechanoreceptor axons as well as the large tracheoblast cells of the fenestrated layer. They are referred to as the fenestrated layer glia. 2. The second glial layer is composed of large, horizontally elongated cells with large elongate nuclei. They contain large membrane-bounded vacuoles and extensive arrays of parallel-running microtubules and smooth ER. These glia invest the photoreceptor axons through much of the multiple chiasmatic (pseudocartridge) region and are thus designated as the pseudocartridge glia. 3-4. Satellite glia comprise the third and fourth glial layers. Thin cytoplasmic processes of these multipolar glia intervene between the tightly packed monopolar neuron somata and the photoreceptor axons of the nuclear layer. The satellite glia are distinguished into two sub-groups: distal and proximal. The distal satellite glia are exclusively responsible for the large glial invaginations of the type I monopolar cell bodies. Multilaminated processes of the proximal layer of satellite glia surround the photoreceptor axons and the neurite neck of the monopolar neurons prior to their entry into the plexiform layer. The proximal satellite glia also contain prominent lipid deposits. 5. Epithelial glia are columnar cells that occupy the plexiform layer. They envelop the optic cartridges of the neuropil and are the substrate for two characteristic glial-neuronal invaginations; i.e. the capitate projection and the 'gnarl'. The cytoplasm of the epithelial glia is electron dense and contains numerous stacked arrays of infolded membrane. 6. Marginal glia form the proximal boundary of the optic neuropil. They invest the axons entering or leaving through the base of the lamina ganglionaris. Marginal glia contain large numbers of parallel microtubules and numerous polyribosomes. Fine structural evidence is presented relevant to the role of these six glial layers in the maintenance of ionic and metabolic homeostasis across the retina-lamina barrier.

Animals↗

PubMed1983

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.

Animals↗

PubMed1982

Ultrastructural pathology of the compound eye and optic neuropiles of the retinal degeneration mutant (w rdg BKS222) Drosophila melanogaster.

The compound eye and the two most distal optic neuropils (lamina ganglionaris and medulla externa) of the Drosophila mutant w rdg BKS222 were examined with transmission electron microscopes at conventional (60 kV) and high (0.8-1 MV) voltages. Eye tissue was sampled in the newly emerged and at 3, 7, and 21 days following eclosion. This mutant is known to show a light-induced degeneration of the peripheral retinular cells (R1-6); the spectral sensitivity is altered and the threshold is increased reflecting the function of the central cells (R7, 8) which do not degenerate. A totally normal appearing visual system (peripheral retina and optic neuropiles) was found in newly emerged adults. After 3 days the somata of some of the peripheral retinal cells are affected and all of their axons show degeneration. At one week the R1-6 pathology is well advanced in both somal and axonal regions. In affected cells the cytoplasm is more or less uniformly electron dense and contains liposomes, lysosome-like bodies, myeloid figures and vacuoles suggesting autophagy. Such cytoplasm (noted at 3 and 7 days post-eclosion) exhibits an electron dense reticulum and degenerate mitochondria. Microvilli become more electron dense. Retinular axon terminals are electron opaque and lack synaptic vesicles with few if any presynaptic structures. Mitochondrial remains are barely recognizable. Transsynaptic degeneration was not found. After 3 weeks, the structure of R1-6 in the peripheral retina (somata and rhabdomeres) is greatly reduced or lost while R7 and R8 and higher order neurons are not affected. The debris from cell bodies and axon terminals or R1-6 seems diminished, so that some phagocytosis probably takes place along with gliosis in the lamina.

Animals↗

PubMed1982

Synaptic vesicle activity in stimulated and unstimulated photoreceptor axons in the housefly. A freeze-fracture study.

Light-stimulated and unstimulated photoreceptor (retinular) axon terminals in the lamina ganglionaris (first optic neuropil) of the housefly are examined using freeze-fracture replication. The presence of numerous, cross-fractured capitate projections permits unmistakable identification of the retinular axon terminal membrane. Regardless of the conditions of illumination, the protoplasmic face (P-face) of the terminal membrane contains numerous bowtie-shaped particle clusters (active zones) which resemble the en face form and disposition of the presynaptic ribbon found in thin sections. Estimates from freeze-fractured material indicate that each retinular axon possesses at least 175 such active zones. In eyes fixed during illumination, active zones are surrounded by many membrane dimples indicative of vesicle fusion sites. Such synaptic vesicle sites are seldom encountered in terminals which are dark-adapted and fixed in the dark. Results from light-adapted eyes placed in the dark following the onset of fixation suggest that endocytosis may occur in the extrasynaptic regions of this inhibitory synapse. P-face particles are uniformly distributed throughout the extrasynaptic regions of unstimulated terminals. Particle density increases in areas peripheral to the active zones in stimulated eyes, particularly within the regions presumed to be undergoing active endocytosis. These structural findings are discussed in the context of the Heuser-Reese model of vesicle exocytosis and recycling.

Animals↗