Biomedical subjects

M S Eckmiller

Publications and source records attributed to M S Eckmiller.

9 recordsLinked to original sources

PubMed1998

Association of kinesin with microtubules in diverse cytoskeletal systems in the outer segments of rods and cones.

The membranous outer segments of vertebrate photoreceptors are supported by cytoskeletons consisting of microtubules and associated proteins, which occur as the ciliary axoneme in rods and cones, and as a separate cytoskeletal system at the incisures of rod outer segments. We performed an immunocytochemical study of the cytoskeleton in photoreceptors isolated from amphibian retinas and found that immunoreactivity to the heavy chain of the motor protein kinesin was closely associated with the microtubules in each of these outer segment cytoskeletal systems. In the outer segments of cones, kinesin heavy chain immunoreactivity was confined to a streak at the axoneme that extended to the outer segment tip. In the outer segments of rods, kinesin heavy chain immunoreactivity was found as both a short streak at the axoneme and a series of long parallel lines that coincided with the microtubules at rod outer segment incisures. Our findings constitute the first report of kinesin in the axoneme of cones and at the incisures of rods. Closely associated with microtubules, kinesin in photoreceptor outer segment axonemes and at rod outer segment incisures can transport materials longitudinally along the microtubules and/or connect these with each other and/or with other components. Because these cytoskeletal systems differ in fundamental ways, kinesin can play different roles in each case, e.g., kinesin at rod outer segment incisures can have structural and functional roles that are unique to rods. These findings may have clinical relevance because similar cytoskeletal systems are expected to occur in the outer segments of human photoreceptors; thus, a disturbance involving kinesin in the cytoskeletal systems at photoreceptor axonemes and/or at rod outer segment incisures could interfere with the normal structure and function of photoreceptors and contribute to human photoreceptor degenerations.

Animals↗

PubMed1996

Renewal of the ciliary axoneme in cone outer segments of the retina of Xenopus laevis.

The ciliary axoneme in photoreceptors from the retina of Xenopus laevis was examined by immunofluorescent staining of tubulin throughout the light/dark cycle. The immunofluorescent axoneme extended along only part of the length of rod outer segments but the entire length of cone outer segments. Both the cone axoneme and outer segment elongated during the day and shortened (presumably by shedding) during the night. Fragments of immunofluorescent axonemes were found within packets of outer segment material breaking off from cone tips. These findings show that the ciliary axoneme in the Xenopus retina is replaced during the renewal of outer segments in cones. No evidence has been found for renewal of the axoneme in rods, and thus the stability of the ciliary axoneme may differ in rod and cone photoreceptors.

Animals↗

PubMed1993

Shifting distribution of autoradiographic label in cone outer segments and its implications for renewal.

To clarify how newly-synthesized membrane materials become distributed within the outer segments (OS) of cone photoreceptors during renewal, the retinas of Xenopus laevis were processed for autoradiography at short times after intraocular injection of radioactive glycoprotein precursors (3H-leucine or fucose). Autoradiograms were examined by light microscopy and the labelling density (number of silver grains divided by surface area) was analyzed as a function of COS height. At the first time (2 hours post-injection) that significant label was observed over COS, it was present at all COS heights but not homogeneously distributed. At this time the labelling density in the most basal compartment was significantly greater than that in the rest of the COS, and in some COS the silver grains seemed to form lines of label parallel to the lamellae. At later times, lines were not observed and the labelling density in the most basal compartment had decreased such that at 17.5 hours post-injection the labelling density in the basal and distal regions of COS was similar. At still later times the labelling density in the most basal compartment was significantly less than that in the rest of the COS. Thus, the initial labelling over COS was not diffuse, and during the first 23 hours after injection the location of highest labelling within COS shifted from the base towards the tip. These observations indicate that new membrane does not quickly diffuse so as to become homogeneously distributed throughout the entire COS, nor does it flow exclusively into new lamellae forming at the COS base. The findings are entirely consistent with, and thereby support, an alternative model, namely that new membrane entering COS flows into partial membrane folds that are known from ultrastructural studies of COS to occur both at the extreme base (basal evaginations) and distributed throughout more distal heights (distal invaginations). The diffuse distribution of label observed over COS at later times is used to derive a revised kinetics for COS renewal, which may have implications for cone sensitivity.

Animals↗

PubMed1990

Distal invaginations and the renewal of cone outer segments in anuran and monkey retinas.

Although it is now clear that the outer segments of mature vertebrate cones are regularly renewed, it is not known how a cone outer segment can maintain a tapered shape if its narrower tip is periodically lost by shedding. This problem was addressed by morphological examination of photoreceptors in retinas of anurans (Xenopus laevis) and monkeys (Macaca fascicularis). Light microscopy revealed a marked daily change in the shape of cone outer segments in X. laevis: at light offset they were long and conical, at light onset they had shed their narrow tips, were sharply truncated, and 40% shorter. Electron microscopy revealed previously undescribed fine-structural features in these mature cone outer segments, most notably the presence of many partial membrane infoldings within their distal lamellae. The growth of each of these "distal invaginations" apparently split 1 pre-existing distal lamella into 2 daughter lamellae of reduced width. The formation of distal invaginations at various heights within a cone outer segment would thus make it longer and narrower. Similar ultrastructural features were also found in cone outer segments of monkey retinas. These findings suggest that during outer segment renewal the tapered shape of mature cone outer segments is maintained via a remodelling process that accompanies the formation of distal invaginations.

Animals↗

PubMed1989

Outer segment growth and periciliary vesicle turnover in developing photoreceptors of Xenopus laevis.

It has been proposed that periciliary vesicles in the photoreceptor inner segment represent newly synthesized membrane en route to the outer segment, and that membrane is delivered to the outer segment via fusion of these vesicles with the plasma membrane at the base of the connecting cilium and sclerad flow of the ciliary membrane. The present research was undertaken to test the periciliary vesicle hypothesis and clarify the dynamics of membrane flow in vertebrate photoreceptors. Light- and electron-microscopic measurements on developing photoreceptors in the retina of Xenopus laevis were used to determine the amount of membrane in outer segments and in periciliary vesicles. No significant diurnal variations were found in outer segment growth rate or size of the periciliary vesicle population. In all rods and in cones at the end of the experiment, the area of periciliary vesicle membrane was proportional to the rate at which membrane was added to the outer segment. Thus, the turnover time for the periciliary vesicle population was similar in rods and cones, supporting the periciliary vesicle hypothesis. Quantification of periciliary vesicle membrane in inner segments provides a method for determining the rate at which membrane is added to outer segments, heretofore not possible for cones.

Animals↗

PubMed1987

Cone outer segment morphogenesis: taper change and distal invaginations.

Because cone outer segments (COS) are now known to be continually renewed, I reexamined COS morphogenesis in retinas of Xenopus tadpoles (prepared by standard histologic techniques and viewed by light and electron microscopy) to clarify how COS incorporate new membrane. I observed that developing COS underwent an unexpected shape change: they were always conical, but their taper (width divided by length) continually decreased. Ultrastructural examination revealed that many of the membrane foldings within distal COS were partial or incomplete, not extending across the full COS width but ending at variable distances from the ciliary side. Because these partial folds represented infoldings of the plasma membrane of an existing lamella, and they occurred at all COS levels except the base, I have termed them distal invaginations (DI). The completion of each DI increased COS length by one lamella but caused no noticeable change in local COS width; thus the formation of many DI throughout the distal COS presumably resulted in the observed decrease in overall COS taper. Based on these findings, I suggest that DI indicate growing membrane fronts and may represent sites where newly synthesized membrane is incorporated into COS. Because DI occur in developing and adult COS of various vertebrate species, I propose that DI formation plays an important role in the generation of COS taper during development and the remodeling of COS taper in mature cones after tip shedding.

Aging↗

PubMed1983

Light-induced photoreceptor shedding in teleost retina blocked by dibutyryl cyclic AMP.

In retinas of lower vertebrates, at least two retinal phenomena appear to be closely tied to the diurnal light-dark cycle: photoreceptor renewal and retinomotor movements. The authors have previously reported that treatments that elevate retinal cyclic AMP levels induce dark-adaptive retinomotor movements. In the present study, the authors have tested whether cyclic nucleotides might also inhibit the burst of rod outer segment shedding expected to occur shortly following light onset. Green sunfish (Lepomis cyanellus) entrained to a 12hL:12hD schedule were given intraocular injections 1 hr before the time of light onset and killed 1 hr after light onset. Epon sections of retinas were used for RPE phagosome counts and for measurements of photoreceptor and RPE retinomotor positions. It is reported that injection of the cyclic AMP analog dibutyryl cyclic AMP before light onset (1) completely blocked the light-induced burst of photoreceptor shedding seen at dawn in these fish; and (2) inhibited light-adaptive retinomotor movements in the pigment epithelium but not in photoreceptors.

Animals↗

PubMed1981

Localized depigmentation of the retinal pigment epithelium and macrophage invasion of the retina in the bullfrog.

An unusual condition of the inferior retinal pigment epithelium (RPE) and neural retina has been observed in essentially all the large bullfrogs examined (Rana catesbeiana, 13 to 20 cm body length, supplied from the U.S. West Coast and Midwest). By ophthalmoscopy the inferior fundus exhibited numerous white spots and lines, which were found by light microscopy to be overlain by smaller black dots and lines. Closer examination revealed that the light areas were regions of depigmented RPE and that the black dots and lines were melanosome-laden macrophages within the adjacent retina. Further examination by light microscopy and electron microscopy allowed the formulation of the following sequence. (1) Monocytes in the choroidal capillaries crossed Bruch's membrane and passed vitreally between adjacent RPE cells. (2) In the subretinal space monocytes transformed into phagocytic macrophages, which became engorged with melanin granules and other RPE inclusions, whereas nearby RPE cells became much thinner and very depigmented. (3) The pigment-laden macrophages then moved vitreally into the avascular neural retina. Although in most areas only the RPE appeared affected by macrophage invasion, occasional localized photoreceptor disruption occurred. The severity of the lesion varied with frog size, being pronounced in large frogs, moderate in medium-sized animals, and absent in small frogs. Because the pigmentary changes were localized to the inferior part of the eye (which receives the most light from the sun overhead) of large bullfrogs (which are likely old), this phenomenon may be due to a change in RPE melanin granules resulting from the cumulative effect of light exposure.

Animals↗

PubMedDate not supplied

Microtubules in a rod-specific cytoskeleton associated with outer segment incisures.

In many vertebrate retinas the outer segments of rod photoreceptors have multiple incisures, that is, there are numerous indentations in the highly curved membrane forming the edge of their disks and in the plasma membrane enclosing the entire stack of disks. Immunofluorescent localization of tubulin in amphibian photoreceptors yielded a novel series of thin, parallel, fluorescent lines in rod outer segments that extended their full length and coincided with their multiple incisures. Electron-microscopic examination of amphibian retinas revealed the structures responsible for this fluorescence: longitudinally oriented microtubules were associated with incisures at heights throughout rod outer segments. These microtubules were located between the disk rims and the overlying plasma membrane, in the small cytoplasmic compartment at the mouth of incisures; the microtubules and membranes were separated from each other by distances that were uniform, as though interconnected by filaments described in other studies. Thus, in amphibian rod outer segments the incisures mark the site of a cytoskeletal system containing longitudinal microtubules distinct from those of the ciliary axoneme, linked by filaments to the adjacent membranes. This cytoskeleton is expected to be important for the normal structure, function, and renewal of rod outer segments. In amphibian cone outer segments, which do not have incisures, the only anti-tubulin immunofluorescence and the only microtubules were at the axoneme. These findings may help elucidate the diverse properties of rods and cones in many vertebrate retinas and could prove relevant for human retinal degenerations.

Animals↗