Cone Pedicles Research Articles

Colour receptors, and their synaptic connexions, in the retina of a cyprinid fish

Morphologically speaking, there are five kinds of cone cells in the retina of the rudd ( Scardinius erythrophthalmus ). But two of them, the principal elements of the double cones and the free principal cones, are probably functionally equivalent, while another, sparse, population of small ( oblique ) cones (which disappear in older fish), is unlikely to make a significant contribution to visual spectral sensitivity. Thus, principal and accessory cones (usually paired with one another), and single cones seem to be the three receptors which underlie the fish’s trichromacy. Photographic densitometry of individual cone cells was used to provide evidence that accessory cones contain a green-absorbing photopigment and the single cones a blue one. Other arguments are given in support of those identifications, and they also strongly suggest that principal cones contain the red-absorbing pigment. Golgi-impregnated bipolar cells were examined electron-microscopically to determine the specific patterns of synaptic connexion they make with these different, anatomically identifiable, colour cones and with the retinal rods. Three principal arrangements were distinguished (see figure 69, page 190). (1) Rod bipolar cells comprise two distinct morphological types, both of which connect exclusively to principal (red) cones as well as to the rods within the outlines of their dendritic fields. (2) Selective cone bipolar cells, more delicate neurons with considerably wider dendritic fields, connect (according to type) to one or other of the different colour cone populations. Examples analysed were specific for the accessory (green) or for the single (blue) cones; no bipolar cells were found connected only to red cones. (3) Mixed cone bipolars have the smallest dendritic fields, and connect to combinations of cones (for example, red and green, or green and blue, but not red and blue). They also have synaptic input (usually relatively sparse) from the rods. Cells were encountered connecting to all three cone types, but they were only partially analysed, and are not described at length. The light microscopic morphology of these bipolar cell types consistently reflects the detailed pattern of connexion each makes with the different receptor populations (just as the morphology of the cones reflects the spectral properties of their photopigment). But while their synaptic connectivity is generally highly specific for cone type, they do occasionally make anomalous connexions with the ‘wrong’ receptors. There is a high degree of divergence (page 85) at the receptor-bipolar synapses, and the different kinds of cones each characteristically connect to different numbers of bipolar cells. Principal (red) cones, which are the most numerous, individually connect to more bipolars than cones of other types, whose characteristic synaptic divergence is likewise related to the frequency with which they occur in the retina. However, rods, which are much more numerous than cones, do not conform with this generalization. The selectivity with which the synaptic terminals of the different cones are connected together by their invaginating basal processes was also examined. These processes link neighbouring synaptic terminals of differently coloured cones: specifically, principal (red) cone basal processes invaginate accessory (green) cone pedicles, and vice versa. Single (blue) cone basal processes connect only to accessory cone pedicles, but that synaptic relation is not reciprocated. These synapses between the cones have important bearing upon interpretation of the bipolar cell connectivity patterns. In their light, the interaction between colour channels which the convergence of different cones onto the mixed cone bipolar dendrites mediates, seems to re-iterate a process already undertaken more peripherally. Likewise, whereas the anatomy of the selective cone bipolars appears designed to convey activity from the individual cone populations, the responses of the receptors they sample must already be influenced by activity in other colour channels.

Organization of the outer synaptic layer in the retina of the larval tiger salamander.

The organization of the outer synaptic layer in the salamander retina was studied electronmicroscopically in serial sections of tissue prepared by conventional techniques or stained by the method of Golgi. Rod cell pedicles make ribbon junctions on cone cell processes, and rod cell processes invaginate cone pedicles without otherwise making any specialized contact with them. Horizontal cells make ribbon and distal junctions with the photoreceptor cell pedicles; a single horizontal cell may contact both rods and cones. Bipolar cells were observed to make either ribbon or basal junctions with the photoreceptor cell pedicles; in addition, certain processes believed to belong to bipolar cells make both ribbon and basal junctions with the same or different pedicles. A single bipolar cell may make contact with both rods and cones. Horizontal cells synapse on bipolar cell dendrites and on certain unidentified processes which in turn are also presynaptic to bipolar cells. Ascending branches of these processes invaginate deeply the rod and cone pedicles without otherwise engaging them in any junction. Horizontal cell processes are linked by two kinds of junctions: close membrane appositions, and contacts analogous to the distal junctions between horizontal cells and rod pedicles.

The connections between bipolar cells and photoreceptors in the retina of the domestic cat.

AbstractAn invaginating bipolar cell that has dendritic terminals forming the central elements of the cone triads is described for the retina of the cat. This type of bipolar contacts a minimum of four or five and a maximum of nine or ten cones. There is no evidence for a bipolar cell which contacts only one cone, i.e., a midget bipolar cell as in simians. There are flat bipolar cells that make superficial contacts with the bases of the cone pedicles and are postsynaptic to between 8 and 14 cones. One cone can be in contact with both an invaginating and a flat bipolar cell. There is evidence suggestive of two kinds of flat bipolars. A comparison is made between the bipolar connections in simians and the cat. The comparison is summarized in figures 29 and 30.

Retinal structure in the smooth dogfish, Mustelus canis: light microscopy of photoreceptor and horizontal cells.

AbstractPhotoreceptor and horizontal cells in retinas of the smooth dogfish were impregnated by the rapid Golgi method. Cones as well as rods are present; there are hundreds of rods per cone. The rod synaptic endings are small spherules which send out basal processes (telodendria). The cone synaptic endings are large pedicles which also extend telodendria.Horizontal cells of three distinctive varieties are segregated in vertically separate layers. Horizontal cells of the first (external) row are thick and cuboidal. They send out processes which enter into the invaginations in rod spherules. These cells and their processes cover a circular to elliptical field measuring 75–125 by 125–200 μm. They probably contact every rod in this field. Horizontal cells of the second (intermediate) row are flattened and stellate. Their processes also enter into the rod spherules. These cells cover a circular to elliptical field slightly larger than that covered by cells of the first row. Although they appear not to contact every rod in their field, the fields of adjacent cells of this type overlap. Every rod, therefore, probably contacts one horizontal cell of the second row as well as one of the first row. Horizontal cells of the third (internal) row are stellate, with a few long cylindrical horizontal processes. Finer vertical processes go from these to contact cone pedicles. Probably each cone contacts one such process, and each third‐row horizontal cell contacts on the order of ten cones. Although somewhat uncertain, the size, shape, and degree of overlap of the fields covered by third‐row cells are probably not very different from those of first‐ and second‐row cells. The vertical sequence of horizontal cells which contact rods or cones in dogfish is inverted from that in teleosts.

Ultrastructural survey of the retina of Pseudemys scripta elegans.

An electron microscopic survey wasmade on the avascular retina of the turtle, Pseudemys scripta elegans. Retinal components were identified and characterized according to their ultrastructural morphology. No rods were identified; single and double cones predominated. Single cones and principal members of double cones also contained a large oil droplet in the inner segment. Cone pedicles contained several synaptic ribbons and numerous vesicles in a dense cytoplasmic matrix. Single and serial chemical synapses and few ribbon synapses were demonstrated in the inner plexiform layer. Muller cell processes containing dense concentrations of β-glycogen were seen extending across the retina. A possible pathway for the transport of glucose across the retina was proposed.

Synaptic organization of cone cells in the turtle retina

The intercellular junctions of cone pedicles in the turtle retina were studied electronmicroscopically in tissue prepared by conventional techniques or impregnated by the method of Golgi. Dendritic branchlets of bipolar cells make specialized contacts with the basal surface of the pedicles. Processes ending laterally to wedge-shaped projections (synaptic ridges) of the pedicles probably belong always to horizontal cells, and make proximal and distal (to the synaptic ridges) junctions with the pedicles. Processes ending opposite the apex of the synaptic ridges are engaged also in two kinds of specialized contacts, termed apical and distal junctions. Basal processes of the cone pedicles make specialized contacts with adjacent pedicles, and with unidentified processes at the outer plexiform layer; their endings abut upon horizontal cell processes lodged within the pedicles of other cone cells.

Organization of the outer plexiform layer of the primate retina: Electron microscopy of Golgi-impregnated Cells

Golgi-impregnated retinae of rhesus monkeys have been examined by serial section electron microscopy to establish in a quantitative manner the neural connexions in the outer plexiform layer. The results have shown that there are two types of midget bipolar cell, here called the invaginating midget bipolar and the flat midget bipolar. Both types of midget bipolar are exclusive to a single cone. The invaginating midget bipolar has been found to fit a dendritic terminal process into every invagination in the cone pedicle base. The flat midget bipolar has dendritic terminals that make superficial contact on the cone pedicle base. There are twice as many dendritic terminals and points of contact with the cone pedicle on a flat midget bipolar top as compared with an invaginating midget bipolar top. These observations, together with light microscope counts of the numbers of the two types of midget bipolars, suggest that there are two midget bipolars per cone. The diffuse cone bipolar (the flat bipolar) also makes superficial contacts on the cone pedicle base, and serial sections have shown that a flat bipolar contacts about six cones. Rod bipolars connect exclusively to rods and their dendritic terminals always end as one of the central processes that penetrate the invagination. Horizontal cell dendrites end exclusively in cone pedicles and their axon terminals end in rod spherules. The point of contact with both the types of receptor is as the lateral elements of the invaginations. A single small horizontal cell contacts about seven cones and a large horizontal cell contacts about twelve cones. The numbers of contacts per cone pedicle decrease from the centre to the periphery of the horizontal cell's dendritic field, suggesting there is an overlap of four to six horizontal cells onto a single cone pedicle. The horizontal cell axon terminals are too numerous to assess in absolute numbers but there is only one terminal to a given rod spherule from any particular axon.

Basal junctions at synaptic endings of turtle visual cells.

It has been often assumed that the first synapse of the visual pathway is confined to the region where the visual cell ending is indented by processes of other retinal elements (4, 9, 15), identified as horizontal and bipolar cells (11, 16). In the vicinity of the indented area, the visual cells contain small vesicles similar to those found at other synapses (3, 13, 14), and dense lamellae are conspicuously placed between the branches of the indenting processes (9). This recessed junction has received considerable attention in the past, but recently the existence of other junctional areas between visual cells and second order neurons has been suggested in the primate and frog retinas (6, 7, 11). In those instances junctions were found at the basal surface of cone pedicles, and structures interpreted as filaments extending between both cell membranes were noted in the frog retina (6). During the course of previous work on toad and rat retinas, I have also occasionally noticed the presence of opaque cross-bars in the intercellular gaps surrounding visual cell endings. These structures did not seem to be located within the area of the recessed junction, but the complex architecture of the toad and rat retinas made this interpretation

The fine structure of the radial fibres in the reptile retina

The radial fibre system in the reptilian retina is particularly profuse and shows a number of unusual morphological features which may throw some light on its function. Retinas from Hemidactylus turcicus, Phelsuma inunguis, Hemidactylus flaviviridius, Gecko gecko , and Lacerta muralis were examined, using osmium-fixed and aralditeembedded tissue. It is suggested that the complex specialization of structure in the vicinity of the visual cell surface has developed to facilitate interchange of fluids or metabolites across the visual cell membrane in the absence of any intraretinal vessels. It is also suggested that the complex interlocking of the radial fibre processes, particularly in the nerve fibre layer, is probably a means of preserving precise inter-relationships of neuronal material to minimize relative movement in the interests of visual acuity. It is speculated that the profuse glial specialization found on the vitread surface of the cone pedicle may represent one of the pathways for lateral inhibitory impulses passing between adjacent visual cells, particularly since no evidence of synaptic components was found in the cytoplasm of the so-called horizontal cells which are usually accepted to be the structural pathway for the lateral passage of information in the retina. A morphological discrepancy between the light and the electron microscopic appearances of the radial fibre system is discussed, and an explanation offered. Finally, a new retinal layer is described.

Gluten-free diets.

The present study examined the consequences of eliminating horizontal cells from the outer retina during embryogenesis upon the organization and assembly of the outer plexiform layer (OPL). Retinal horizontal cells exhibit a migration defect in <i>Lim1</i>-conditional knock-out (<i>Lim1</i>-CKO) mice and become mispositioned in the inner retina before birth, redirecting their dendrites into the inner plexiform layer. The resultant (mature) OPL, developing in the absence of horizontal cells, shows a retraction of rod spherules into the outer nuclear layer and a sprouting of rod bipolar cell dendrites to reach ectopic ribbon-protein puncta. Cone pedicles and the dendrites of type 7 cone bipolar cells retain their characteristic stratification and colocalization within the collapsed OPL, although both are atrophic and the spatial distribution of the pedicles is disrupted. Developmental analysis of <i>Lim1</i>-CKO retina reveals that components of the rod and cone pathways initially co-assemble within their normal strata in the OPL, indicating that horizontal cells are not required for the correct targeting of photoreceptor terminals or bipolar cell dendrites. As the rod spherules begin to retract during the second postnatal week, rod bipolar cells initially show no signs of ectopic growth, sprouting only subsequently and continuing to do so well after the eighth postnatal week. These results demonstrate the critical yet distinctive roles for horizontal cells on the rod and cone pathways and highlight a unique and as-yet-unrecognized maintenance function of an inhibitory interneuron that is not required for the initial targeting and co-stratification of other components in the circuit.