Biomedical subjects

D H Dickson

Publications and source records attributed to D H Dickson.

At least 19 recordsLinked to original sources

PubMed1996

The differing effects of regional and general anaesthesia on cerebral metabolism during carotid endarterectomy.

OBJECTIVES: To examine the effects of either regional (RA) or general (GA) anaesthesia upon parameters of cerebral metabolism (near infrared spectroscopy, continuous jugular venous oximetry) during carotid endarterectomy. DESIGN: Prospective, non-randomised, observational study. MATERIALS: Sixty-five consecutive patients (33 RA; 32 GA) undergoing carotid endarterectomy. METHODS: (i) Near infrared spectroscopy: measurement of concentrations of cerebral oxyhaemoglobin (HbO2), deoxyhaemoglobin (HHb) and oxidised cytochrome oxidase (caa3). (ii) Continuous jugular venous oximetry: O2 saturation of jugular venous blood (SJvO2). (iii) Stump pressure in internal carotid artery. RESULTS: A reduction in SJvO2 (RA: 13% (95% CI-3 to 29%) GA: 9% (95% CI-2 to 20%), p < 0.08) and a fall in caa3 levels (RA vs. GA: 25/31 vs. 19/31, p = 0.2) was more likely in patients given a RA following application of the carotid clamps. When HbO2 and caa3 did fall however spontaneous recovery occurred more often (RA vs. GA; caa3: 18/25 vs. 5/19, p < 0.005; HbO2: 30/31 vs. 4/28, p < 0.001). CONCLUSIONS: Although GA may offer a degree of cerebral protection by reducing cerebral metabolic rate (lower falls in SJvO2 and caa3) RA preserved cerebral autoregulation as judged by the spontaneous recovery in caa3 and HbO2 levels.

Aged↗

PubMed1994

Diurnal change and prolonged dark effect on myeloid bodies in the retinal pigment epithelium of the leopard frog.

Myeloid bodies (MBs) are multi-lamellar membrane structures which are widely distributed in the retinal pigment epithelium (RPE) of lower vertebrates. The functional significance of these organelles is not known, but they are believed to be associated with lipid processing in the retina. To determine the nature of the daily changes which occur in MBs, and to ascertain the best sampling time for MB isolation as a prelude to the biochemical characterization of these organelles, this study has assessed the occurrence and area density of MBs in the leopard frog (Rana pipiens) over a daily light-dark cycle. Eye tissues from light-entrained (12L:12D) frogs were sampled at 1, 4, 7, 10 hr after both lights-on and lights-off (n = 5). In addition, to determine whether MB formation is a circadian or light-driven event, the effect of a period of prolonged darkness on MB formation was also examined by maintaining frogs in the dark for an additional 1, 4, 7, and 10 hr at the end of a normal dark phase (n = 5). The number and area density as a percentage of total RPE cell area (area %) for phagosomes (PGs), oil droplets (ODs) and MBs were determined morphometrically using light and electron microscopy. Data were analyzed by a nested analysis of variance. Results indicated that: 1) PG numbers were elevated significantly following lights-on and were reduced almost to nil in the late dark period.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Diurnal variation in myeloid bodies of the chick retinal pigment epithelium.

Myeloid bodies (MBs) are distinct lamellar regions of the normally branched tubular smooth endoplasmic reticulum of retinal pigment epithelial (RPE) cells. These organelles are present in the retinas of many lower vertebrates. Previous investigations have reported a relationship between outer segment disk shedding and phagocytosis by the RPE, and the formation of MBs. The current morphometric study was undertaken to establish if a temporal relationship existed between MB occurrence and the phagocytosis of shed photoreceptor outer segment tips in the chick retina. We report on the occurrence of phagosomes, MBs, and for the first time, MB precursors (templates) in the RPE over a 24-hr diurnal cycle. Phagosome numbers were observed to be highest within 2 hr following lights on, and again following lights off, while MB precursors were most prevalent at two time points, immediately prior to the times for rod and cone outer segment shedding at lights on and lights off respectively. Traditional MBs of the small (0.06 to 0.239 microns2) and medium (0.24 to 0.99 microns2) variety increased both in size and number from 01:00 hr of a 24 hr time period, with smaller and fewer MBs being present late in the dark part of the diurnal cycle, and larger and more numerous MBs in the later part of the light portion and into the early part of the dark portion of the cycle.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Effect of photoreceptor outer segment disk shedding on myeloid body formation in the retinal pigment epithelium of the leopard frog.

To test the hypothesis that myeloid body (MB) formation results from the shedding of retinal photoreceptor outer segments and the consequent degradation of lipids derived from outer segment disk membranes, the effect of massive outer segment shedding and the disruption of such outer segment shedding on MB formation were examined in the leopard frog. Light entrained frogs were first placed in constant light (700 lux) for 48 hours to inhibit shedding, followed by a 1.5 hours dark priming either in vivo or in vitro, and then returned to light for an additional 4 hours which results in massive outer segment shedding. To serve as a control, the effect of shedding disruption on MB formation was assessed in vivo using light manipulation to inhibit shedding, or mechanical removal of the neurosensory retina in vitro. The results indicate that although the phagosome numbers were clearly elevated in the samples taken from either in vivo or in vitro eye-cup preparations where outer segment shedding had been stimulated, there was no significant concomitant increase in Mbs number over controls kept under constant light for 48 hr or constant light 48 hr plus 1.5 hr in dark, where MBs represent approximately 5% of the total RPE cell area. In contrast, when shedding was interrupted either by removal of the neural retina immediately after in vitro eye-cups were returned to light or by maintaining frogs in dark without light stimulation, the RPE cells contained very few phagosome, yet in both conditions RPE cells showed a two-fold increase in MB area over the shedding-stimulated controls (p = 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Biogenesis of myeloid bodies in regenerating newt (Notophthalmus viridescens) retinal pigment epithelium.

Myeloid bodies are believed to be differentiated areas of smooth endoplasmic reticulum membranes, and they are found within the retinal pigment epithelium in a number of lower vertebrates. Previous studies demonstrated a correlation between phagocytosis of outer segment disc membranes and myeloid body numbers in the retinal pigment epithelium of the newt. To test the hypothesis that myeloid bodies are directly involved in outer segment lipid metabolism and to further characterize the origin and functional significance of these organelles, we examined the effects on myeloid bodies of eliminating the source of outer segment membrane lipids (neural retina removal) and of the subsequent return of outer segments (retinal regeneration) in the newt Notophthalmus viridescens. Light- and electron-microscopic analysis demonstrated that myeloid bodies disappeared from the pigment epithelium within six days of neural retina removal. By week 6 of regeneration, rudimentary photoreceptor outer segments were present but myeloid bodies were still absent. However, at this time, the smooth endoplasmic reticulum in some areas of the retinal pigment epithelial cells had become flattened, giving rise to small (0.5 micron long), two-to-four layer-thick lamellar units, which are myeloid body precursors. Small myeloid bodies were first observed one week later at week 7 of retinal regeneration. This study revealed that newt myeloid bodies are specialized areas of smooth endoplasmic reticulum. It also showed that a contact between functional photoreceptors and the retinal pigment epithelium is essential to the presence of myeloid bodies in the epithelial cells.

Animals↗

PubMed1992

Phospholipid composition of myeloid bodies from chick retinal pigment epithelium.

The phospholipid composition of a myeloid body (MB) enriched subcellular fraction of chick retinal pigment epithelium (RPE) was determined in order to further characterize the origin and functional significance of these lamellar membrane organelles. The major MB phospholipids found were phosphatidylcholine and phosphatidylethanolamine which represented 43% and 34% of the total MB lipids respectively. Sphingomyelin and phosphatidylinositol comprised the remaining detectable phospholipids. The fatty acyl chain composition of all detected phospholipids showed that the long-chain polyunsaturated fatty acids [arachidonic (20:4 n-6), docosapentaenoic (22:5 n-3) and docosahexaenoic (22:6 n-3)] account for greater than 45% of the fatty acids in MB membranes. This high proportion of long-chain polyunsaturated fatty acids in MBs is particularly striking when compared to the long-chain fatty acid composition of the photoreceptor outer segments from this predominantly cone retina which contains less than 25% long-chain polyunsaturated fatty acids. The results from this study clearly demonstrate that MB lipids represent a significantly enriched pool of long-chain polyunsaturated fatty acids.

Animals↗

PubMed1992

Myeloid body development in the chick retinal pigment epithelium.

Previous investigations have reported a correlation between outer segment disk phagocytosis by the retinal pigment epithelium (RPE), and the formation of myeloid bodies, which are lamellar specializations of the smooth endoplasmic reticulum of the RPE. To further test the hypotheses that MBs are directly related to the process of outer segment shedding, we have undertaken to study the development of the retinal pigment epithelium in the chick utilizing a morphometric ultrastructural approach with specific attention to the timing of outer segment disk shedding and the formation of myeloid bodies. This study has demonstrated a temporal relationship between the initiation of photoreceptor outer segment disk shedding in the chick embryo and the occurrence of myeloid bodies. We have shown that photoreceptor outer segment disk shedding and phagocytosis by the RPE has been initiated in a substantial proportion of the retina by day 18 of development. Myeloid body formation does not begin until two days later at embryonic day 20, and is preceded by the development within the RPE of flattened areas of the normally tubular SER (templates), which we believe represent the precursors of myeloid body formation. Temporal development of outer segment phagocytosis, lamellar SER (template) occurrence and myeloid body formation are all statistically significant at the 95% level.

Animals↗

PubMed1990

Posthatch day/night differences in synaptic ribbon populations of the chick pineal.

Pineal synaptic ribbon (SR) populations of the early posthatch white leghorn chick were counted to determine if they demonstrate a rhythm that is in accordance with the light/dark cycle. SRs were counted between day 7 and day 10 and on day 14 of posthatch development, with samples at midlight, middark (14L:10D), and constant darkness. SR populations did not exhibit significant changes on days 7 and 8 under cycled lighting conditions nor on days 9 and 10 under constant darkness. A second experiment demonstrated that the dark:light ratio of SR populations of day 14 chicks, under cycled lighting, was 3.4:1.0, indicating SR rhythmicity by that stage of development. In that a preliminary experiment had demonstrated a 4.2:1.0 dark:light ratio in SR populations in a predominantly day-10 population of chicks, we believe that SR rhythmicity begins on, or near, day 10 of posthatch development. To determine if the invasion of sympathetic fibers from the superior cervical ganglion (SCG) correlates with the initiation of SR light/dark population differences, we employed tyrosine hydroxylase immunofluorescence to reveal the distribution of catecholaminergic fibers in chick pineal follicles. Follicular innervation doubled over the day 7 to day 14 period, during which time light/dark differences in SR populations were established. There is a correlation, in time, between the invasion of the pineal by the sympathetic fibers and the initiation of SR light/dark differences. The circadian rhythm of pineal N-acetyltransferase (NAT) activity, the rate-limiting enzyme in the melatonin pathway, is established earlier (day 2) than the light/dark differences in SR populations (day 10). It is possible that SR rhythmicity is influenced by the ingrowth of the pineal sympathetic innervation, and that SRs respond to an extrapineal oscillator rather than the independent oscillators of the chick pineal responsible for the rhythm of NAT activity and melatonin synthesis.

Age Factors↗

PubMed1987

Day-night differences in the number and structure of synaptic ribbons in chick pineal.

The objective of this study was to evaluate the effect of lighting (day-night) changes on pinealocyte synaptic ribbon shape (conformation) and numbers. Three-dimensional reconstruction analysis of pinealocyte basal processes revealed that 30% (6/20) of all ribbons from dark-adapted animals were either curved or split. Synaptic ribbons from light-adapted animals did not show this variant morphology; all were linear structures. An analysis of each section from each series, containing curved or split ribbons, revealed that 44% (16/36) of all ribbon profiles would yield inflated counts if used in random morphometric sampling protocols. Therefore, split and curved (variant) ribbon morphologies could result in an overestimation of synaptic ribbon populations of approximately 13% (0.44 X 30%) of dark-adapted samples. In spite of this potential sampling error, the fourfold increase in the number of synaptic ribbons observed during the dark phase of a light-dark cycle remains highly significant (P less than 0.0001).

Animals↗

PubMed1986

Morphometric analysis of circadian variations in the retinal photoreceptor synaptic terminals of the adult and fetal guinea pig.

In order to test whether the alterations in photoreceptor synaptic terminal size and shape reported in lower vertebrates occur in a mammalian visual system, adult and fetal guinea pig retinas were exposed to an LD 12:12 lighting cycle, as well as to long-term light (LL) and long-term dark (DD) regimes. Representative random samples from all retinal quadrants, obtained at various times during these lighting regimes, were processed for electron microscopy. The synaptic terminals of all three photoreceptor cell types in this retina (alpha and paranuclear rods, and cones) were analyzed with computer-assisted morphometrics for changes in their area, perimeter, synaptic vesicle density, and the degree of plasmalemmal infolding. The data showed all three types of adult receptor terminals to have increased area and vesicle density, as well as decreased membrane infolding, during the light period, while both types of rods showed increased perimeter measurements in the dark. Results from adults maintained under extended lighting conditions (LL and DD) showed no difference when compared with sample times during a typical LD 12:12 lighting regimen where clear statistical differences existed. Data from fetal retinas showed no significant sustainable pattern in any of the measured variables. These quantitative findings have led to the conclusion that while alterations in perimeter measurements may be explained by using the vesicle recycling hypothesis, observed changes in terminal size and shape may be controlled by a light-initiated or light-enhanced mechanism and effected through an annular configuration of cross-striated fibrils found within these photoreceptor synaptic terminals.

Adaptation, Physiological↗

PubMed1985

Effects of temperature and bright light on myeloid bodies in the retinal pigment epithelium of the newt, Notophthalmus viridescens.

Myeloid bodies (MBs) are specialized regions of endoplasmic reticulum which occur in the retinal pigment epithelium of a number of vertebrate species. In the newt, Notophthalmus viridescens, the effects of temperature and brief exposure to bright flashed-light on myeloid bodies have been studied. Morphometric analysis has shown that in animals sampled at 06.30 h, myeloid body sectional area remained unchanged in animals maintained in the cold (1 degree C), compared with control animals at 15 degrees C, whereas phagosome area was significantly increased. At higher temperatures (30 degrees C), myeloid body area was observed to decline from control values, while phagosome area was substantially increased. During the first 2 h of the light phase of a normal (15 degrees C) 12:12 LD lighting cycle, myeloid-body sectional area dropped significantly from values recorded in the latter part of the dark phase. This reduction of MB area at the normal time of "lights-on" was greatly reduced when animals experienced an extended period of darkness. When animals experienced a bright flashed-light at the normal time of "lights-on", followed by a period of extended darkness, reduction in MB area was less pronounced when compared to cycled control animals. These results are discussed in the context of the hypothesis (Yorke and Dickson 1984) that MBs represent a temporary storage site for lipids entering the pigment epithelium after phagocytosis of shed outer segment tips, prior to their permanent storage in lipid droplets.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1985

Lamellar to tubular conformational changes in the endoplasmic reticulum of the retinal pigment epithelium of the newt, Notophthalmus viridescens.

The retinal pigment epithelium (RPE) of the newt (Notophthalmus viridescens) was examined ultrastructurally under both in-vivo and in-vitro conditions. Five distinct conformations of smooth endoplasmic reticulum (SER), two lamellar and three tubular, were observed. The two lamellar conformations included myeloid bodies, which have previously been described (Yorke and Dickson 1984), and fenestrated SER. The latter appeared as layers of flattened or curved cisternae which were penetrated by fenestrations. Fenestrated SER became indistinguishable from the highly branched and convoluted random-tubular SER through the formation of an intermediate configuration ("tubular sheets"). The remaining tubular SER conformations appeared to arise from random-tubular SER through a progressive reduction in branching and a straightening of individual tubules. Fascicular SER was represented by the hexagonal organization of straight, unbranched tubules into bundles (fascicles). Spiral SER consisted of a similar hexagonal arrangement, but the unbranched tubules spiralled about one another. Neighbouring tubules in areas of spiral SER were also joined together by pairs of electron-dense bars. Although lamellar (especially myeloid bodies) and random-tubular configurations of the SER were common features in vivo, fascicular and spiral SER were primarily conformations encountered in vitro. Conditions favouring bilayer lipid phases also appear to facilitate the formation of both myeloid bodies and fascicular SER. These conditions included increased duration of incubation, low (less than 20 degrees C) incubation temperatures, and Ca2+-free incubations with EGTA. Random-tubular SEB was most prevalent in media supplemented with fetal calf serum and also after warmer (30 degrees C) incubation temperatures.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1985

A cytochemical study of myeloid bodies in the retinal pigment epithelium of the newt Notophthalmus viridescens.

It has been suggested (Yorke and Dickson 1984) that myeloid bodies (MBs) in the retinal pigment epithelium (RPE) of the newt, Notophthalmus viridescens, may represent areas of endoplasmic reticulum where lipids, such as 11-cis retinal derived from phagocytized outer segment tips, accumulate prior to esterification. Experiments in which an artificial ester substrate was added during in-vitro incubations have shown that esterase activity is represented in all areas of the newt RPE endoplasmic reticulum, including sites adjacent to all MBs. In related tests in which the localization of enzyme activity was restricted to areas of the cell where there had been accumulations of naturally-occuring (endogenous) esters, the products of ester hydrolysis were restricted to profiles of endoplasmic reticulum associated with lipid droplets, and with the interior of about 20% of those MBs that appeared completely circular in sections. This enzyme activity was not associated with other MB configurations. Results from endogenous-ester hydrolysis were identical to those obtained after staining with ZIO. This ZIO-reactive was not affected by pre-incubation with agents that blocked or protected sulphydryl groups, and ZIO-reactive sites associated with MBs did not form complexes with digitonin. These observations suggest that MBs are a site of lipid-ester formation, but that they do not represent unique intracellular areas for this activity.

Animals↗

PubMed1985

Photoreceptor synaptic ribbons: three-dimensional shape, orientation and diurnal (non) variation.

A variety of different lighting regimes have been reported to cause alterations in the length and number of synaptic ribbons within photoreceptor synaptic terminals. Whether these alterations occur in mammalian visual systems during a diurnal lighting cycle was tested using computer-assisted morphometrics. Adult guinea pigs were entrained to a 12:12/ligh:dark regime and subsequently killed at various times throughout the cycle. Representative samples from all quadrants of the eyes were processed for electron microscopy and the synaptic ribbons within the synaptic terminals of the three different types of photoreceptors in this retina (alpha and paranuclear rods, and cones) were analyzed with the aid of an image analysis computer (Zeiss IBAS) for their length and absolute number per terminal. Contrary to previously published reports on other species, the synaptic ribbons in all three terminal types exhibited no statistically significant change in either number per terminal or in length, throughout the 24 hr cycle. Computer-based three-dimensional reconstructions from serial thin sections revealed horseshoe-shaped synaptic ribbons with pleats in numerous planes conforming to the contours of the invaginating postsynaptic elements.

Animals↗

PubMed1984

Diurnal variations in myeloid bodies of the newt retinal pigment epithelium.

Myeloid bodies (MBs) occur in the newt (Notophthalmus viridescens) retinal pigment epithelium (RPE) and are similar to areas of specialized endoplasmic reticulum found in a variety of other cell types. The function of these structures is unknown, although a role in lipid metabolism has been strongly suggested. Random samples from conventionally-fixed and sectioned newt RPE, obtained over a 24-hr cycle (LD 12:12), were examined by electron microscopy. Myeloid bodies appear as stacks of flattened endoplasmic reticulum-associated saccules which increase in length and number as the RPE accumulates shed outer segment material, prior to increase in the amount of stored lipid. Associations of MBs with the nuclear envelope can be related to this increased length. Myeloid bodies decrease numerically in the cell as phagosomes are removed from the cytoplasm, but a decrease in mean sectional MB area, seen in the light phase, is counteracted in darkness where individual MBs are larger than those found in the light. The total sectional area of MBs within a cell and their mean length varied depending on the lighting condition; differences were also found between eyes after extended periods of continuous light and dark. Ribosomes were found in association with the surfaces of both flattened and circular MBs, but they were consistently more densely associated with the shorter concave surfaces of curved regions. A new hypothesis for MB function is presented, which is concerned with their role in isolating toxic lipids such as retinoids, which are accumulated during phagocytosis of shed outer segment tips, and which are capable of disrupting membrane-bound systems necessary for their eventual metabolism and safe storage.

Animals↗

PubMed1983

Diurnal variations in the photoreceptor synaptic terminals of the newt retina.

Newt photoreceptor synaptic terminals undergo a variety of morphological changes over a 24-hr (LD 12:12) cycle. During the day, dense-cored synaptic vesicles were found to increase in number and accumulate near the synaptic lamellae; during the dark phase, the dense-cored vesicles decreased in number, while large clear vesicles and profiles of smooth endoplasmic reticulum increased in frequency. The most marked change in photoreceptor synaptic terminal morphology occurred after 10 hr of darkness, at 0730 hr. At this time, photoreceptor synaptic terminal cross-sectional area was found to increase dramatically. Morphometric analysis showed that the number of synaptic vesicles in these terminals remained constant throughout the day, as did the perimeter of photoreceptor terminal profiles. The observed increase in area of synaptic terminals at 0730 hr was found to be due to a decrease in the folding of the terminal plasma membrane. Qualitative observations showed endocytosis to be occurring at a rapid rate at this time as well; and since the number of synaptic vesicles and terminal perimeter did not change, exocytosis of synaptic vesicles was assumed to be occurring at an equally rapid rate. These findings support an extension to the hypothesis of Monaghan and Osborne (1975), suggesting that photoreceptor synaptic vesicles become "supercharged" with transmitter substance in the light.

Animals↗

PubMed1983

Displaced amacrine and ganglion cells in the newt retina.

The inner nuclear layer and ganglion cell layer of the newt retina contain taurine- and GABA-accumulating cells which are located immediately adjacent to the inner plexiform layer. Transport studies with horseradish peroxidase (HRP) indicate the presence of displaced ganglion cells within the inner nuclear layer of the retina. These are more numerous at the periphery of the retina and constitute about 2.5% of the retinal ganglion cells. Autoradiography, combined with HRP-transport studies, indicate that the taurine-accumulating cells in the ganglion cell layer of the newt retina are not 'true' ganglion cells, but may be displaced amacrine cells. Counts of axons in the optic nerve compared with cell bodies in the ganglion cell layer, demonstrate that only 50-60% of the cells in the ganglion cell layer of the newt retina are truly ganglion cells.

Animals↗

PubMed1983

Identification of sulfur groups in the outer plexiform layer of the newt retina.

The staining of newt photoreceptor synaptic vesicles with the zinc iodide-osmium (ZIO) method can be abolished by prior incubation in amphibian Ringer's containing 0.1 M-N-ethyl maleimide which specifically blocks -SH groups. The existence of high concentrations of sulfur groups in the outer plexiform layer of the newt retina has been further demonstrated with mercury orange and a newly synthesized fluorescent thiol probe, DACM. The reason why such high concentrations of -SH groups are present within photoreceptor synaptic vesicles is unclear, however, it is possible that ZIO stains the photoreceptor neurotransmitter itself.

Animals↗