Gap junctions between photoreceptor cells in the vertebrate retina

Axonless horizontal cells of the rabbit retina: synaptic connections and origin of the rod aftereffect

An axonless horizontal cell (AHC) of the rabbit retina was penetrated with a microelectrode and stained with horseradish peroxidase after recording its light responses. The cell was then serially sectioned and its connections examined with the electron microscope. Physiologically, the cell exhibited cone-dominated responses and a minor rod influence known as rod aftereffect. Electron microscopy showed that this AHC was only connected to cones. Therefore, the rod aftereffect could only invade the cell through the gap junctions between the synaptic endings of rod and cone photoreceptors. In the synaptic invaginations of the cone pedicles contacting the cell, only one of the lateral elements was stained. This suggests that the two lateral elements of each cone-invaginating synapse belong to two different horizontal cells. By staining intracellularly adjacent AHCs, we showed that the two lateral processes may originate from two horizontal cells belonging to the same morphological type.

Telodendrial contact of HRP-filled photoreceptors in the turtle retina: pathways of photoreceptor coupling

Synaptic contacts of photoreceptors in the turtle retina were studied by intracellular injection of horseradish peroxidase (HRP) and electron microscopy. Both cone and rod photoreceptors radiated basal processes (telodendria) from their terminal endings. These telodendria ran laterally in the outer plexiform layer. The telodendria of cones gave rise to many fine branches that penetrated synaptic cavities of several neighboring cones. Tips of these branches terminated near the walls of synaptic cavities. Some of the telodendrial contact formed two types of basal junction: symmetrical and punctate. The distribution of cones that made telodendrial contacts with the HRP-filled cone were quantitatively investigated. Green-sensitive cones (n = 3) made telodendrial contacts with neighboring red- and blue-sensitive cones, blue-sensitive cones (n = 4) with red- and green-sensitive cones, and red-sensitive cones (n = 9) with red- and green-sensitive cones. In contrast to these cone connections, rod telodendria did not penetrate neighboring photoreceptors. Direct synaptic contacts were not found between rods and cones. Our results clarify the variety of cone couplings in turtle retina: the three chromatic classes of cones are selectively coupled by the basal junctions at the ends of telodendrial processes.

Interphotoreceptor contacts at the inner segment level in primate retinas

Just sclerad to the external limiting membrane of the retina of the monkey (M. fasciculata), short processes from some foveolal and foveal cone inner segments made dense contacts with each other and at the same level of peripheral human retina similar rod-cone and rod-rod contacts were observed.

Intercellular junctions in the gill epithelium of the Atlantic hagfish, Myxine glutinosa

The intramembrane organization of the occluding junctions in the gill epithelium of the Atlantic hagfish, Myxine glutinosa, was studied by means of freeze-fracture electron microscopy. Mitochondria-rich cells, characterized by assemblies of rod-shaped particles in the luminal plasma membrane and by an extensive intracellular amplification of the basolateral plasma membrane, are singly distributed between the pavement cells in the gill epithelium of this marine and stenohaline cyclostome. The occluding junctions between mitochondria-rich cells and pavement cells do not differ from those between adjacent pavement cells, concerning the number of superimposed strands (median 6, range 4-9) and their geometrical organization. These observations suggest that, in contrast to marine teleosts, the paracellular pathway plays a minor role in transepithelial ion movements in the hagfish gill epithelium. The findings are in agreement with the absence of hypoosmoregulatory mechanisms in hagfish, as have been evolved in various marine vertebrates. In addition, small communicating junctions are demonstrated between pavement cells; they possibly serve for a coordinated synthesis and secretion of mucus by the pavement cells.

Spectral sensitivity of the electroretinogram b-wave in dark-adapted goldfish

The action spectrum of the ERG b-wave was measured under dark-adapted conditions in intact goldfish (Carassius auratus). It is substantially broader than the absorption spectrum of goldfish rod porphyropsin. Neither prolonged dark adaptation nor removal of possible efferent neural activity affected its shape. Moreover, a 682-nm background did not produce a selective loss of sensitivity to long wavelengths. The results imply that the spectral sensitivity of the b-wave in dark-adapted goldfish reflects the influence of at least two photoreceptor types which act as a single univariant mechanism near absolute threshold.

Ultrastructural evidence that horizontal cell axon terminals are presynaptic in the human retina

The organization of the rod spherule and of the horizontal cell axon terminals within the invagination of the rod spherule in the human retina was examined in serial sections by electron microscopy. Twenty-one rod spherules were reconstructed in this study. Axon terminal processes of type I horizontal cells consistently make one or two small punctate synapses onto each rod spherule within the invagination. In addition, these axon terminal processes make distinct synapses upon rod bipolar dendrites outside the spherule before both processes enter the invagination. This is the first positive description of a synapse from a horizontal cell axon terminal process onto a photoreceptor terminal and the first identification of a synapse from a horizontal cell to a rod bipolar cell in the mammalian outer plexiform layer. We speculate that the axon terminal-to-rod synapse is responsible for feedback while the synapse upon the rod bipolar cell is feed-forward and serves to expand the receptive field of the rod bipolar cell beyond its dendritic field. Alternatively, the latter may contribute to a center-surround organization of the rod bipolar's receptive field.

Neuronal and synaptic organization of the outer plexiform layer of the pigeon retina

The organization of the outer plexiform layer (OPL) of the pigeon retina is described by electron microscopy and Golgi impregnation. Six types of photoreceptor, four types of horizontal cell, eight types of bipolar cell, and an interplexiform cell type were found by Golgi impregnation. The OPL was tri-stratified due to the endings of the photoreceptors at three different levels. This stratification was reflected in the laminar arrangement of the dendrites of the horizontal and bipolar cells. Electron microscopy showed that the synaptic endings of the photoreceptors made ribbon synapses, both triads and dyads, and basal junctions with the process of second-order neurons. Horizontal cells formed conventional chemical synapses, while horizontal cell axon terminals were extensively linked by gap junctions.

Variations in the structure of nexuses in the myocardium of the golden hamster Mesocricetus auratus

The structure of nexuses in the atrioventricular node of the golden hamster was studied with the transmission electron microscope, using thin sections and freeze-fracture replicas, and was compared with that of nexuses in the working myocardium of the right ventricular wall. Whereas ventricular myocardium contained macular nexuses only, nodal tissue contained annular and linear configurations as well as maculae of varying size. The significance of such variations in nexus pattern is not clear although several hypotheses are discussed in the literature. Measurements made on electron micrographs, after allowing for tilt of the specimen, yielded a particle diameter of 10.59 nm for nodal myocardium and 10.95 nm for ventricular myocardium, both measurements being substantially higher than figures generally cited in the literature. In each area the measurements had a normal distribution suggesting a single type of particle. The small but significant difference in particle size between the two areas is more likely to be caused by dissimilarities in packing arrangement rather than by differences in intrinsic structure or in functional state.

Glucose alters configuration of gap junctions between pancreatic islet cells

In rat pancreatic islets, gap junctional subunits (GJS) occur under two different configurations, namely in linear single strands and in polygonal particle aggregates. The present freeze-fracture study demonstrates that GJS can rapidly (dis)assemble into one of these membrane specializations without changes in their total number. Isolation of the pancreatic gland and its perfusion at 2.8 mM glucose is accompanied by a decrease in polygonally packed GJS from 46 to 16%. A rise in medium glucose concentration is followed, within 10 min, by a dose-dependent increase in the percent polygonal particles. This glucose effect on gap junction configuration is calcium dependent and reversible upon glucose removal; it is still entirely detectable when protein synthesis is blocked by cycloheximide. These results indicate that islet gap junctions are dynamic structures that rapidly adjust their configuration to extracellular regulators of beta-cell function. In the light of previous observations, it is suggested that this rapid (dis)assembly of gap junctional structures be considered as a component in the ionic and metabolic coupling between insulin-containing beta-cells of the pancreas.

Telodendrites of cone photoreceptors: structure and probable function

Intracellular dye injection and compartmental modeling were used to analyze the structure and function of telodendrites of cones in the retina of the walleye. After identifying the spectral type of an impaled cone on the basis of its response to red and green light, horseradish peroxidase and/or Lucifer Yellow were injected for 1 to 25 minutes. In 38 of 58 recovered cells, dye spread into the telodendrites; so in many cases, the detailed pattern of the telodendritic arbor could be reconstructed from serial sections. Typically, five telodendritic processes, about 1 micron in diameter and 18 micron in length, radiated from the cone pedicle. A majority of the processes terminated at pedicles of neighboring cones. Some of the Lucifer Yellow injections provided evidence for electrical coupling between cones via telodendrites. Calculations from a compartmental model, based on the measured dimensions of cones and telodendrites, indicate that the signal arising in the inner segment spreads with little loss to the end of a telodendrite, whereas about half of the signal is lost in transmission from telodendrite to inner segment. Assuming that each contact point within the telodendritic network is an electrical synapse of 2,500 M omega, the model shows spatial interaction over a field of some 80 micron, which is comparable to that measured experimentally. Although our anatomical data indicate that orange- and green-sensitive cones may be interconnected via telodendrites, model calculations indicate that such connections do not appreciably distort the intrinsic spectral sensitivity of walleye cones. This outcome agrees with previous experimental results.

Telodendrial contacts between foveolar cone pedicles in the human retina

The synaptic pedicles of foveolar cones in the human retina contact each other by means of telodendrial processes. Thus direct lateral coupling of photoreceptor terminals exists even in the area of highest acuity function.

Rod-cone interaction in flicker perimetry: evidence for a distal retinal locus

The luminance threshold for the detection of 25 Hz flicker was measured in nine patients with retinal disorders under stimulus conditions that have been shown previously to involve an interaction between rod and cone systems. The disorders studied included congenital stationary nightblindness, X-linked juvenile retinoschisis, hereditary dominant optic atrophy, optic atrophy found in association with neurofibromatosis, retinitis pigmentosa, choroideremia, and an acquired diffuse photoreceptor disorder, all of which involve pathologic changes that are presumed to occur primarily at specific levels of the retina. The results are consistent with a distal (outer) retinal locus for the rod-cone interaction.

A quantitative analysis of interactions between photoreceptors in the salamander (Ambystoma) retina

A quantitative description of the electrical properties of the photoreceptor layer in the salamander retina was obtained from earlier data on the characteristics of isolated rods and cones and on rod-rod coupling, and from new data on rod-cone and cone-cone coupling and on the rod photocurrent. Injecting -1 nA current into a rod elicits hyperpolarizations of about 20 mV in an adjacent rod and 4 mV in an adjacent cone. Responses of more distant receptors are smaller. Injecting -1 nA into a cone elicits hyperpolarizations of about 4 mV in an adjacent rod and 0.4 mV in a nearby cone. Depolarizing current evokes smaller responses. Assuming, in agreement with anatomical evidence, that each rod is electrically coupled to four rods and to four cones around it, and that there is no direct electrical coupling between cones, we found these results could be predicted from the properties of isolated rods and cones if adjacent rods are coupled by a resistance of 300 M omega and adjacent rods and cones are coupled by a resistance of 5000 M omega. The small cone-cone coupling seen is due to coupling via intervening rods. The two halves of double cones are not electrically coupled. The spectral sensitivity of both halves is a maximum around 620 nm wave-length. The rod photocurrent has been characterized by voltage-clamping rods isolated from the retina. In agreement with Bader, MacLeish & Schwartz (1979) we found the time course of the photocurrent to be approximately independent of voltage between -35 and -85 mV. The voltage responses of rods, single cones and double cones isolated from the retina obey the principle of univariance. Responses of receptors in the retina do not obey univariance. The main deviations from univariance observed can be explained if adjacent rods and cones are coupled by a resistance of 5000 M omega. Our data demonstrate that rod-cone coupling is relatively weak. We simplified our description of the photoreceptor network, by omitting cones, to investigate the spatiotemporal processing that the rod network is capable of. Computer simulations predict, as is found experimentally, that the rod voltage response to a large spot of bright light should show a much more pronounced initial transient hyperpolarization than the response to a small spot of light of the same intensity. This difference is produced by the combination of electrical coupling of the rods with the existence of a voltage-gated current, IA, in the rod membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Pattern of synaptic connections in the pineal organ of the ayu, Plecoglossus altivelis (Teleostei)

Synaptic connections were studied by means of electron microscopy in the sensory pineal organ of the ayu, Plecoglossus altivelis, a highly photosensitive teleost species. Three types of specific contacts were observed in the pineal end-vesicle: symmetrically organized gap junctions between the basal processes of adjacent photoreceptor cells; sensory synapses endowed with synaptic ribbons, formed by basal processes of photoreceptor cells and dendrites of pineal neurons; conventional synapses between pineal neurons, containing both clear and dense-core vesicles at the presynaptic site. Based on these findings, the following interpretations are given: (i) The gap junctions may be involved in an enhancement of electric communication and signal encoding between pineal photoreceptor cells. (ii) The sensory synapses transmit photic signals from the photoreceptor cells to pineal nerve cells. (iii) The conventional synapses are assumed to be involved in a lateral interaction and/or summation of information in the sensory pineal organ. A concept of synaptic relationships among the sensory and neuronal elements in the pineal organ of the ayu is presented.

The neuronal organization of the outer plexiform layer of the primate retina

In the primate retina at the level of the first synapse in the visual system, the outer plexiform layer, processes from 15 different types of neurons have so far been described. These are the synaptic spherules of rods, the pedicles of three spectral types of cones, dendrites and axons of two types of horizontal cell, dendrites of seven types of bipolar cell, processes of interplexiform cells, and the outwardly coursing dendritic extensions of biplexiform ganglion cells. The interconnections of these neurons as studied by electron microscopy and Golgi-EM are presented in a summary diagram (Fig. 27). Basal processes from cone pedicles contact the cone pedicles, and rod spherules forming gap junctions. The dendrites of both types of horizontal cell (hI and hII) connect only to cone pedicles and form lateral elements of triads at the ribbon synaptic complex. The HI axon terminals end as lateral elements at rod spherules while the axons of HII horizontal cells connect with cones in a manner similar to their dendrites. Interplexiform cells (ipc) do not contact either rod or cone synaptic endings. Rod bipolar cell (rb) dendrites end as central elements at the ribbon synaptic complex of rod spherules. The dendrites of flat midget (fm), flat top (fb), and giant bistratified bipolar (gb) cells all form basal junctions with cone pedicles. Ending as central elements of triads at cone pedicles are the dendrites of invaginating midget (im), diffuse invaginating cone (ib), and blue-cone (bb) bipolar cells. Biplexiform ganglion cells (bgc) connect to rods as central elements opposite the synaptic ribbon in the spherules. As compared to an earlier summary diagram of the outer plexiform layer (Kolb, 1970), the primate retina is now known not to be as simply organized as was once thought. Although our knowledge of the types of neurons contributing processes to this first synaptic layer, and the nature of their connections with other neurons has been broadened, especially within the past few years, this summary diagram is not intended to represent the complete or final "picture." Undoubtedly, future investigations along the lines of research outlined here will provide additional details to this wiring diagram so that we may better understand the processing of visual information by neurons in the retina.

Inhibitory influence of unstimulated rods in the human retina: evidence provided by examining cone flicker

In the parafoveal retina of human observers, cone-mediated sensitivity to flicker decreases as rods become progressively more dark-adapted. This effect is greatest when a rod response to flicker is precluded. These results indicate that rods tonically inhibit cone pathways in the dark.

The signal-to-noise characteristics of rod-cone interaction

1. The influence of rods on cone-mediated vision was assessed in eight human observers. To this end, increment threshold functions were obtained by determining thresholds of a cone-detected test flash (25 ms duration, 655 nm wave-length, 13' diameter) as a function of the illuminance of larger, 500 ms duration, rod-detected masking flashes. The type of photoreceptor influenced by each stimulus was carefully checked by means of a series of control procedures involving action spectra and selective rod adaptation.2. When the rod mask was 512 nm in wave-length, 40' in diameter, and less than one scotopic td in illuminance, increment threshold functions show that [Formula: see text], where I(Cth) is cone test threshold, I(R) is rod mask illuminance, and D is a dark noise term similar to that used by Barlow (1956). Further increases in I(R) have no additional influences on cone test threshold until threshold is influenced by the combined action of the mask on both rods and cones. If I(R) is expressed in terms of scotopic flux rather than illuminance, the functional relationship obtained with all rod masks </= 40' diameter and </= 580 nm wave-length is identical.3. Over the range of illuminance where a square-root relationship is obtained, probability of seeing functions show that a signal-to-noise mechanism limits the detectability of the cone test flash. These findings suggests a quantitative model in which cones produce a signal in a detector which is proportional to the illuminance of the cone test flash. Within a neural locus designated E (excitatory spatial summator), a response is produced which over at least a 40' diameter area, is proportional to the scotopic flux of the rod mask. E, however, feeds into a gain box, S, which saturates at illuminance levels at least 3 log(10) units less than usual estimates of rod saturation. Other than saturation, S behaves in a linear fashion.4. As diameter increases beyond 60', rod masks of equal scotopic illuminance have progressively less influence on cone test threshold; rod masks > 2 degrees have negligible influence on cone test threshold. We propose that I (inhibitory spatial summator), a neural locus which responds to scotopic flux provided over a very large area, attenuates the activity of E. The combined action of E and I is designated a rod channel. The response of cones and the rod channel summate at a detector. Within the detector, cone signals are distinguished from rod-related activity and intrinsic dark noise on the basis of signal-to-noise discriminations.5. The neural substrate for this rod channel most probably involves the combined action of several neurones which synapse within the inner plexiform layer of the retina. The relationship of this rod channel to other perceptual phenomena is discussed.

The visual cells of the skate retina: structure, histochemistry, and disc-shedding properties

Earlier studies have shown that visual function in skate is subserved solely by the rod mechanism and that the retina of this elasmobranch contains only rod photoreceptors. Nevertheless, the skate retina is capable of responding to levels of illumination that extend well into the photopic range, and we have detected in histological sections (usually from younger animals) small, proximally displaced, conelike photoreceptors which possibly represent another class of visual cell. However, ultrastructural and histochemical studies showed that the membranous discs of the outer segments of these cells were isolated from the plasma membrane, and that their synaptic terminals appeared immature and unlike those usually associated with cone receptors. In addition, the pattern of incorporation of 3H-fucose, as revealed by radioautography, was similar for both the rods and the smaller visual cells; i.e., the label was concentrated along the basal discs of the outer segment. When we examined the disc-shedding behavior of the visual cells in skates entrained for 2 weeks or longer to a 12-hour light:12-hour dark cycle, enhanced phagocytic activity was seen only following light onset; there was no significant increase following light offset. On the available evidence, it seems reasonable to conclude that the small visual cells are rods that have recently differentiated, and are growing and being incorporated into the photoreceptor layer of the retina.

The spatial frequency sensitivity of bipolar cells

Retinal bipolar cells constitute the output stage of the outer layer of the retina. There are several constraints on the ability of the bipolar cell array to respond to the different spatial frequency components of the visual image, including (i) electrical coupling in the dendritic tree receiving receptor input; (iii) the "lateral inhibition" mediated by horizontal cells. Using simple mathematical models, we derive analytical expressions for the spatial frequency response of the bipolar cell array for the case in which horizontal cells are presynaptic to bipolar cells (feedforward model) and also for the case in which horizontal cells are presynaptic to receptors (feedback model). The results illustrate the importance of the three factors mentioned in determining the bipolar cells' properties. The optimal spatial frequency for stimulating the bipolar cell array, and the range of spatial frequencies transmitted onward to the inner plexiform layer, are thus related to the anatomical and electrical properties of the cells in the outer plexiform layer.

A physiological and morphological study of the horizontal cell types of the rabbit retina

A perfused, isolated retina-eyecup preparation of the rabbit was utilized to correlate the physiology and morphology of horizontal cells. Neurons were physiologically characterized by intracellular recording techniques and subsequently stained with intracellular iontophoretically injected horseradish peroxidase for morphological identification. Three types of rabbit horizontal cell recordings have been differentiated, based on variations in response waveform, amplitude-intensity properties, and area summation characteristics. These three types have been unequivocally associated with the axonless A-type horizontal cells and the somatic and axon terminal endings (each displaying its own distinct physiology) of B-type horizontal cells first described in studies using Golgi-impregnation techniques (Fisher and Boycott, '74). In addition, the sizes of A-type horizontal cells were found to be directly related to their retinal eccentricities from the optic desk. However a unique subclass of A-type cells has been discovered (elongated or Ae type) which displayed the largest dendritic field of any cells studied here, yet had the smallest eccentricities--within 1.4 mm of the optic disk. Moreover, elongated A-type cells exhibited long asymmetrical dendritic fields which were oriented parallel with the visual streak. The unique asymmetry and orientation of these cells suggests that they may have orientation-biased receptive field properties. Physiological evidence for an orientation-biased horizontal cell is presented in support of this notion.

Physiological and morphological identification of two types of on-center bipolar cells in the carp retina

Two types of on-center bipolar cells, rod- and cone-dominant bipolars have been identified in the dark-adapted retina of the carp by means of intracellular recordings and Lucifer-yellow dye injection. They differ physiologically and morphologically in the following respects: 1) responses of rod-dominant cells to bright lights are characterized by a transient depolarization followed by a smaller sustained depolarization, while those of cone-dominant cells are approximately rectangular; 2) the cone-dominant cells are about 1.5 log units less sensitive to light than the rod-dominant cells; 3) the latency of the response is shorter in the cone-dominant cells than in the rod-dominant cells; 4) the mean diameters of the cone-dominant cell receptive field (0.7 mm) and dendritic field (90 micrometers) are larger than those of the rod-dominant cell receptive field (0.5 mm) and dendritic field (56 micrometers); 5) the mean diameter of the cone-dominant cell soma (8 micrometers) is smaller than that of the rod-dominant cell soma (13 micrometers); and 6) the terminations of the cone-dominant cell axons form a ramification (67 micrometers mean diameter) in contrast to a big terminal swelling of the rod-dominant cell axons (37 micrometers mean diameters). At least two ionic mechanisms are responsible for generating the depolarizing response of on-center bipolar cells, one having a reversal at a positive potential and the other at a negative potential. Responses with a negative reversal potential only are obtained from some of cone-dominant cells and responses with a positive reversal potential only are obtained from some other cone-dominant cells and the rod-dominant cells. There are a large number of bipolar cells that respond by both ionic mechanisms, although the ratio between them varied considerably in different cells.

Membrane specializations in the outer plexiform layer of the turtle retina

The internal organization of the plasma membrane at specialized contacts in the outer plexiform layer of the turtle, Pseudemys scripta elegans, was analyzed with the aid of the freeze-fracturing technique. In the invaginating synapse of cone pedicles the plasma membrane of the photoreceptor ending contains an aggregate of P-face particles, images of synaptic vesicle exocytosis, and rows of forming coated vesicles which are arranged in sequence from apex to base of the synaptic ridge. Thus, freeze-fracturing provides positive evidence that the synaptic ridge represents the active zone at the surface of the photoreceptor endings. Horizontal cell processes of dyads and triads have an aggregate of P-face particles opposite the apex of the ridge, but lack images of vesicle exocytosis. Deep-etching and rotary-shadowing demonstrate that an array of minute protrusions decorates the true outer surface of the horizontal cell membrane at the site of the intramembrane particle aggregate. The membrane of the invaginating bipolar dendrite is unspecialized. At basal junctions, the cone pedicle membrane has a sparse complement of P-face particles, but images of vesicle exocytosis are absent. The adjoining bipolar membrane is characterized by a prominent aggregate of E-face particles, often arranged in an orthogonal lattice. The freeze-fracture profile therefore suggests the existence of (1) a sign-conserving cone-to-horizontal cell synapse; (2) a sign-inverting synapse between cones and invaginating bipolar dendrites; and (3) a sign-conserving synapse between cones and bipolar dendrites at basal junctions. No freeze-fracture evidence was found for a horizontal-to-cone or horizontal-to-bipolar cell synapse within the synaptic invaginations.

Effects of colchicine on gap junction formation during retinal neurogenesis

Colchicine, injected into the amniotic cavity of 3 to 10 day-old chick embryos, has the following effects on the developing retina: 1. Cells in arrested metaphase accumulate in the ventricular portion of the matrix region. If colchicine is applied at the end of the first week of incubation, the kinetic migration of the cell nuclei is inhibited. 2. The cells lose their typical slender, bipolar shape to become ovoid to spherical. These morphological changes are more obvious when colchicine acts during the first week of incubation. 3. The formation of temporary gap junctions between the matrix cells of the retina occurs during the first week of development. When colchicine is administered around the 3rd day of incubation, the assembly of intramembranous particles (IMPs) during gap junctions formation is incomplete. These results suggest that an intact microtubular system within the submembranous cytoskeleton is essential for the assembly of these intercellular contacts.

Rod-cone interaction in the distal human retina

During the rod-isolated phase of dark adaption, b-wave implicit time of the human cone electroretinogram increased exponentially with a time constant corresponding to that for the regeneration of rhodopsin. In the presence of different photopically equated short-wave backgrounds, cone b-wave implicit time varied inversely with the scotopic brightness of the background. Taking into account the origin of the b wave, these measurements support the idea of a rod effect on cone function in the distal human retina.

An intracellular electrophysiological study of the ontogeny of functional synapses in the rabbit retina. I. Receptors, horizontal, and bipolar cells

An isolated perfused retina-eyecup of the rabbit neonate was used to study the functional maturation of synaptic interactions in the outer plexiform layer. Intracellular recordings from receptors, horizontal cells, hyperpolarizing and depolarizing bipolar cells were obtained in the adult and at different stages of maturation. Three types of horizontal cells could be distinguished based on the relative amount of rod and cone input, response wave form, receptive field diameter, and amplitude-intensity relationships. Two of the horizontal cell types were encountered with sufficient frequency such that maturation of these response characteristics could be followed over the developmental period of this study (8 days-adult). The growth rate of the amplitude-intensity relationships was different for the two commonly encountered types of horizontal cells; the small diameter receptive field type achieved an adultlike amplitude-intensity range at an earlier age than the large field type. Immature bipolar cell responses were initially monophasic potentials with transient-sustained components appearing at a later stage. Center surround antagonism of bipolar cells developed after center-mediated responses were first observed. This suggests some secondary modifications during synaptogenesis are responsible for the late maturation of center surround antagonism. The morphological appearance of synaptic contacts during different stages of synaptogenesis is discussed in reference to the different phases of functional, synaptic interactions of this study. It appears that some synapses in the vertebrate retina are functional at a time when synaptic structure is incomplete, based on ultrastructural observation. Maturation of receptive field diameter and amplitude-intensity function is discussed in relationship to possible presynaptic and postsynaptic mechanisms which may influence this growth.

A freeze-fracture study of photoreceptor presynaptic membranes during ribbon synapse formation

The outer plexiform layer (OPL) of the developing chick retina from 11 embryonic days to 11/2 weeks posthatching has been studied by freeze-fracture to characterize changes in the membrane structure of photoreceptor terminals during synaptogenesis. At early stages, the undifferentiated photoreceptor synaptic base is characterized by a sparse distribution of intramembrane articles on the inner leaflet (P-face). Later, as the synaptic base begins to differentiate by extending filopodia into the OPL, numerous small aggregates of large particles appear between and on filopodial surfaces. Many of the aggregates occupy crater-like expressions, which are seen in cross-fractures through the underlying cytoplasm to be associated with vesicular invaginations of the presynaptic membrane. Corresponding thin sections through these regions at this time reveal immature arciform densities and coated vesicles fusing with the presynaptic membrane adjacent to these densities. At later stages, many of the particle aggregates on the photoreceptor membrane appear to have coalesced into longer arrays overlying ridges surrounded by numerous vesicle fusion sites. These intramembrane changes correlate with the formation of the mature arciform density-synaptic ribbon specialization in the photoreceptor presynaptic terminal and with physiological maturation of the chick retina.

Intramembrane events accompanying junction formation in a liver cell line

To study intramembrane events leading to the establishment of intercellular junctions between epithelial cells in vitro, we examined monolayer cultures of a rat liver cell line by an in situ freeze-fracturing technique (Pauli et al., J. Cell Biol., 72:763, 1977). Our observations indicate that an early step of junction formation between liver cells consists of the differentiation of a particle-poor membrane stretch showing a honeycomb pattern of shallow P-face depressions or E-face bulges ("formation band"). This change in membrane organization precedes and accompanies the subsequent aggregation of junctional particles. The latter process results in the formation of irregular particle islands with peripheral branchings which tend to encompass the depressions in the membrane. The linear branchings grow and interconnect in a network of beaded strands, which gradually transform into smooth tight junctional fibrils, as previously described in fetal liver in vivo (Montesano et al., J. Cell Biol., 67:310, 1975), while the particle islands assume the typical configuration of mature gap junctions. Formation bands are particularly prominent between liver cells growth in the presence of hydrocortisone (5 microgram/ml) in the culture medium.

Cell types and synaptic organization of the medullary electromotor nucleus in a constant frequency weakly electric fish, Sternarchus albifrons

The medullary electromotor nucleus (EMN) of Sternarchus albifrons was studied at the light and electron microscopic levels. The EMN consists of a dense meshwork of myelinated axons and glial elements with interposed large neurons; it is provided with an abundant supply of capillaries. Two types of essentially adrendritic nerve cells were distinguished on the basis of size: giant neurons (approx. 70 micrometers in diameter) and large neurons (approx. 30 micrometers in diameter). Their population ratio is 1:4. Only giant cells are labelled following the injection of retrograde tracer into the spinal cord; they are therefore identified with the so-called "relay cells" of other gymnotids. Tracer experiments further suggest that the descending axons of these relay cells give off collateral branches throughout the elongated spinal electromotor nucleus. In contrast, the large cells remain unlabelled and therefore lack spinal projections; they most likely correspond to "pacemaker cells." The perikaryal surface, including axon hillock and proximal part of initial segment of both types of EMN cells, is contacted by clusters of synaptic terminals and astrocytic processes. Two main varieties of synaptic terminals occur: (1) large endings and (2) ordinary end feet with standard size (S-type) and variable size (Sv-type) clear, spherical vesicles. The junction between large endings and EMN cells is characterized by the combination of gap junctions and surrounding intermediate junctions whose freeze-fracture characteristics were morphometrically analyzed. The large endings were formed by nodes of Ranvier as well as by fiber terminations, and synchronization within the EMN may be achieved by presynaptic fibers. Some of the contacts occur directly on the initial segment, which could allow activity to bypass the soma. It is concluded that the elctromotor system of Sternarchus is comprised of a rapid conduction pathway where medullary pacemaker and relay cells as well as spinal electromotor neurons are coupled by synapses with gap junctions. In contrast to the spinal electromotor neurons, the medullary EMN cells receive synapses with morphological characteristics of chemical transmission, and the S-type and SV-type terminals may possibly correspond to Gray's Type I and Type II synapses, respectively. These synapses may be involved in modulation of the electric organ discharge frequency.

The single-photon signal in rod bipolar cells of the dogfish retina

1. Responses of depolarizing bipolar cells to dim light flashes were recorded with intracellular micro-electrodes in the dark-adapted retina of the dogfish, Scyliorhinus canicula. 2. Fluctuations in the responses were analysed by a method of matched filtering in order to improve the signal-to-noise ratio. 3. Both the mean and variance of the response amplitude increased linearly with light intensity for intensities not exceeding a mean of 1 photon absorbed per 50 rods. 4. On the assumption that the most significant source of the fluctuation is the quantal absorption of light by the rod outer segments, a single photoisomerization leads to a post-synaptic event of mean size 250 micronV. 5. The mean number of rods in the pool sending signals to a bipolar cell is estimated as 1600. Individual rod pools are 90-330 micrometer in diameter on the retinal surface. 6. It is estimated that the conductance of 1 divided by 400 of the total number of light-modulated ionic channels in the bipolar cell is increased by a single photon acting within its rod pool. 7. In the absence of a light stimulus, the residual noise in the output of a matched filter can be interpreted as due mainly to spontaneous isomerization of rhodopsin in the rods and behaves as the 'dark light' postulated to limit detection at absolute threshold.

The septum of the lateral axon of the earthworm: a thin section and freeze-fracture study

Septa occur between the axonal segments in the lateral giant septate axon of the nerve cord of the earthworm. This septum is demonstrated here to be permeable to fluorescein and to exhibit a negligible time delay for impulse transmission. Periodic anastomoses between the two lateral axons of the nerve cord are revealed by fluorescein. The permeability of the septum is correlated with the demonstration that nexuses occur along the septum. In thin sections, the nexuses may appear as long septilaminar or pentalaminar membrane appositions, but most frequently appear as a series of short or punctate membrane appositions. In freeze-fracture replicas, the nexuses appear as particles 10-12 nm in diameter on the PF face and as pits on the EF face. The particles and pits are arranged in plaques, in anastomosing strands, or most frequently in small plaques with strands of particles or pits emerging from the periphery. In addition to the nexuses, a junction characterized by the presence of 31 nm diameter hemispherical densities on the cytoplasmic surfaces of the septal membranes is revealed in thin sections. The densities are paired on the adjacent septal membranes, and most frequently are shown by optical diffraction to be arranged on the membrane surfaces in hexagonal or rhomboidal lattices with a centre-to-centre spacing of 34.8 nm. In freeze-fracture replicas, an array of particles and pits with a similar lattice symmetry and spacing to the arrays of hemispherical densities is demonstrated.

Anisotropic receptive field structure of cat horizontal cells

Horizontal (H-)cells were recorded intracellularly in the retinae of optically intact feline eyes in vivo. A small light spot orbiting slowly around the receptive field centers was used to quantify the fine structure and diameter of the receptive fields. Receptive field diameters measured in this way were larger than those measured with centered spots of increasing diameter. All H-units studied showed clearly anisotropic receptive field structures. These results are summarized in polar plots representing the local response generating sites with their corresponding "response plus transport" latencies. It is shown that the anisotropic receptive field properties are not incompatible with the approximately homogenous spatial distribution of H-cell somata reported by Wässle and Rieman (1978) for the axonless type of horizontal cell. Finally it is concluded that each H-cell might be involved in many different locally specialized signal processing activities.

Increase of gap junctions between pancreatic B-cells during stimulation of insulin secretion

The development of gap junctions between pancreatic B-cells was quantitatively assessed in freeze-fracture replicas of isolated rat islets under different conditions of insulin secretion. The results show that in resting B-cells, gap junctions are small and scarce but that these junctions increase when insulin secretion is stimulated. Both a short (90 min) stimulation by glucose in vitro and a prolonged (2.5 d) stimulation by glibenclamide in vivo raise the number of gap junctions; in addition, the glibenclamide stimulation causes an increase in the size of individual gap junctions. As a consequence, the total area occupied by gap junctions on the B-cell membrane and the ratio of this area to the cell volume were found significantly increased in the latter condition. The slight increase of these values observed after the glucose stimulation did not reach significance. These data indicate a change of gap junctions during the secretory activity of the pancreatic B-cells. The possibility that the coupling of the cells is affected by the treatment is discussed.

Electrical coupling between cones in turtle retina

1. The electrical coupling between cones of known spectral sensitivity in the peripheral part of the turtle's retina was studied by passing current through a micro-electrode inserted into one cone and recording with a second micro-electrode inserted into a neighbouring cone. 2. Spatial sensitivity profiles were determined by recording flash responses to a long narrow strip of light which was moved across the impaled cones in orthogonal directions. These measurements gave both the length constant lambda of electrical spread in the cone network and the separation of the two cones. 3. The cone separation determined from the spatial profiles agreed closely with that measured directly by injecting a fluorescent dye into two cones. 4. The length constant lambda varied from 18 to 39 micron with a mean of 25 micron for red-sensitive cones and 26 micron for green-sensitive cones. 5. The majority of cone pairs studied were electrically coupled provided they had the same spectral sensitivity and were separated by less than 60 micron: thirty-two out of thirty-six red-red pairs, two out of two green-green pairs, none out of eight red-green pairs: no blue cones were observed. 6. The strength of electrical coupling was expressed as a mutual resistance defined as the voltage in one cell divided by the current flowing into the other. Mutual resistances decreased from a maximum value of about 30 M omega at separations close to zero to 0.2 M omega, the lower limit of detectable coupling at separations of about 60 micron. Mutual resistances were always positive and were independent of which cell was directly polarized. The coupling seemed to be ohmic and any rectification or non-linearity probably arose in the cone membranes rather than in the coupling resistances. 7. The results were analysed in terms of the Lamb & Simon (1977) theories of square and hexagonal lattices, which approximate to the continuous sheet model except in the case of the cone to which current is applied. 8. The total membrane resistance of a single cone was estimated as 100--300 M omega and the connecting resistances as 100 M omega for a square array and 170 M omega for a hexagonal array. The input resistance of a cone in the network was 25--50 M omega. Lower values were often obtained but may be due to injury by the micro-electrodes. 9. The time constant of an isolated cone was estimated as about 20 msec and the capacity as about 100 pF. 10. Discrepancies between experimental findings and theoretical predictions of the hexagonal or square array models were tentatively attributed to an overestimate of lambda resulting from light scattering.

Retinal synaptic arrays: continuing development in the adult goldfish

We report a light- and electron-microscopic examination of the inner plexiform layer of the central retina of young (c. 1 year) and old (3-4 year) goldfish. There were no new neurons added to this region during the growth period. Nonetheless, there were substantially more synapses (per cell, per mm2, or per degree 2) in the older retinas. This result is discussed in the contexts of retina function and neural development.

Tight junction development between cultured hepatoma cells: possible stages in assembly and enhancement with dexamethasone

Freeze-fracture and thin-section methods were used to study tight junction formation between confluent H4-II-E hepatoma cells that were plated in monolayer culture in media with and without dexamethasone, a synthetic glucocorticoid. Three presumptive stages in the genesis of tight junctions were suggested by these studies: 1) "formation zones" (smooth P-fracture face ridges deficient in intramembranous particles), apparently matched across a partially reduced extracellular space, develop between adjacent cells; 2) linear strands and aggregates of 9--11 nm particles collect along the ridges of the formation zones. The extracellular space was always reduced when these structures were found matched with pits in gentle E-face depressions; 3) the linear arrays of particles on the ridges associate within the membranes to form the fibrils characteristic of mature tight junctions. The formation zones resemble tight junctions in terms of size, complexity and the patterns of membrane ridges. Although some of the beaded particle specializations may actually be gap junctions, it is unlikely that all can be interpreted in this way. No other membrane structures were detected that could represent developmental stages of tight junctions. Dexamethasone (at 2 x 10(-6)M) apparently stimulated formation of tight junctions. Treated cultures had a greater number of formation zones and mature tight junctions, although no differences in qualitative features of the junctions were noted.

Gap junctions coupling photoreceptor axons in the first optic ganglion of the fly

The first optic ganglion of the fly, the lamina ganglionaris, was investigated with the transmission electron microscope for the purpose of demonstrating possible electronic junctions. Within a cartridge, the six short receptor cell axons R1--R6 are extensively coupled by symmetrical gap junctions. This is mainly seen in the distal third of the first synaptic region where none or only a few lateral branches of the centrally lying L-fibres (L1, L2) penetrate the ring of six short receptor fibre endings. Gap junctions as found between R1--R6 are distinguished morphologically from chemically-mediated synapses by the closely apposed cell membranes. They exhibit only a 2--4 nm extracellular cleft. Unlike the chemical synapse the gap junction in the neuropile of the fly appears structurally symmetrical. No such gap junctions are found either between R-fibres and glial cells, interneurons and glial cells, between glial cells and between interneurones themselves, nor between the parallel long receptor axons R7/8, which bypass the lamina outside the cartridge. In accordance with electrophysiological data, it can now be argued that the six short receptor axons R1--R6 are electrically interconnected by symmetrical gap junctions.

Receptor contacts of horizontal cells in the retina of the domestic cat

The terminal aggregations of A- and B-type horizontal cells, stained by the Golgi-Colonnier method, have been analysed. The pattern of the aggregations is regular and is shown to be in register with the cone mosaic. Both tyes of horizontal cell are in contact with at least 80% of the cones above their dendritic fields. Therefore, the different horizontal cell classes cannot be selective for a special kind of cone but must have at least 60% of the cone input in common. Each A-type horizontal cell makes contacts with between 120 and 170 cones, and each B-type horizontal cell with 60-90 cones. An individual A-type horizontal cell occupies an average of 20% of the lateral elements of the triads in a cone pedicle, but an individual B-type cell fills only some 13%. Each and every cone is connected with several of both types of horizontal cell. An estimation of the number of rods converging onto a single axon terminal system showed that it could be as many as 3000.

Communicating junctions of the human sensory retina. A freeze-fracture study

Using the freeze-fracture technique communicating junctions could be identified between the cells of the adult human sensory retina. Communicating junctions with particles arranged in linear or circular rows as well as in small aggregates were found between adjacent photoreceptor endings. Communicating junctions with particles forming plaques were localized in the inner plexiform layer. They were isolated between unidentified cell processes or lay close to active sites of chemical synapses, thus suggesting the occurrence of "mixed" (chemical and electrical) synapses in this layer. Furthermore, communicating junctions were detected between portions of Müller's cells.

Responses to light of solitary rod photoreceptors isolated from tiger salamander retina

Single, isolated rod photoreceptors were obtained by enzymatic dissociation of the tiger salamander (Ambystoma tigrinum) retina. These solitary cells retained the morphological features of rods of the intact retina and could be maintained in culture for several days. When impaled with micropipettes for electrophysiological recording, dark-adapted solitary rods had during darkness a resting potential of approximately -45 mV and a steady-state slope resistance of 500 Momega at rest. The current-voltage relationship showed both inward- and outward-going rectification. The responses to light of solitary rods were similar to those recorded from rods in the intact retina stimulated with large-diameter spots of light. The reversal potential of the light response of solitary rods was near 0 mV when measured in either the inner or outer segment.

A freeze-fracture study of exocytosis and reflexive gap junctions in human ovarian decidual cells

Fine-structural features of ovarian decidual cells and their mode of secretion were examined by means of freeze-fracture microscopy. Unique cortical peduncular processes contained secretory vesicles within the expanded peduncle tip, the membrane-leaflets of which exhibited a particle-poor E face adjacent to the vesicle lumen and a P face containing a greater particle number. Exocytosis from attached peduncles involved release of vesicular profiles 40-55 nm in diameter; small particles 8.5-11.5 nm in diameter were also observed at degranulation sites. In fractures revealing the E face of the plasmalemma, cytoplasmic portals at the bases of peduncular stalks were distinguishable from endocytic vesicles. The frequent occurrence of reflexive gap junctions associated with peduncles was shown by freeze-fracture. However, there appeared to be no consistent spatial relationship between gap junctions, secretory peduncles, or sites of exocytosis. Freeze-fracture analysis of the topography of reflexive gap junctional profiles revealed that such gap junctions share basic similarities with intercellular gap jum particle-free aisles. The finding in the present study of reflexive gap junctions occurring between peduncles and the cell soma, as well as between peduncles, suggests that the original definitiof the same cell should be broadened to include any gap junctional specialization formed between portions of the plasma membrane of one cell.

Intercellular junctional complexes of the rat seminiferous tubules: a freeze-fracture study

This paper provides a description of the intercellular junctions in the rat testis, as observed using the freeze fracture technique. These intercellular junctions were categorized into four general types: Sertoli cell tight junctions, myoid cell tight junctions, gap junctions, and "heterogeneous junctions." The Sertoli cell tight junctions had a mean depth of 3.1 micron in the basal to apical direction, and contained 25 to 50 (average: 36) parallel rows of particulate sealing elements. The myoid cell tight junction was neither continuous nor extensive, but focal in nature. Interestingly the sealing elements of this junction, like those of the Sertoli cell tight junction, were quite particulate in nature. Typical gap junctions were observed between Sertoli cells where they were intercalated between the parallel rows of the Sertoli cell tight junction. The most interesting observation was the identification of gap junction-like structures, in various stages of formation, on germ cell membrane fracture faces, both in the basal and adluminal compartments. Lastly, the unusual "heterogeneous junction" was observed on large membrane fracture faces in close proximity to cells in the adluminal compartment, presumably between Sertoli cells. These junctions appeared to consist of both tight and gap junction elements.