[Myeloid bodies of the pigmented epithelium of the retina in vertebrates]

The amphibian Kupffer cells build and demolish melanosomes: an ultrastructural point of view

This ultrastructural research was carried out to investigate the nature of the liver pigment cells of anuran and caudate amphibians, the pattern of melanosome ontogenesis, and the demolition processes of old melanosomes. We demonstrate that these liver pigment cells are able to internalize zymosan particles and latex beads; therefore, being professional phagocytes, they, as liver resident macrophages, can be classified as Kupffer cells (KCs). They show "melanosomogenesis centers" in which several maturation stages of premelanosomes are visible; the premelanosomes are formed by two principal components: a filamentous structure that will constitute the "inner" area of the melanosome and a vesicular component, budding from the Trans Golgi Network and that carries enzymes, which will constitute the "cortical area" of the melanosome. Thus the KCs, thanks to the presence of the "melanosomogenesis centers," are also melanosome producing cells. They are also able to demolish melanosomes by heterophagocytosis and, probably, also by autophagocytosis. In conclusion, we propose a classification of vertebrate pigment cells.

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)

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)

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)

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.

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.

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.

Membrane formations in the pineal cells of the teleost Gambusia affinis

Highly organized membrane formations in the cytoplasm of photoreceptor and supporting cells of the pineal organ of Gambusia affinis are described. These membranous structures show different characteristics depending on the cell type where they are located. Some of the morphological characteristics of these membranous formations are similar to those of myeloid bodies, but their presence in the photoreceptor cells raises some doubt about their being myeloid bodies. The three-dimensional interpretation of these structures is discussed.

Structural changes in retinal pigmented epithelium of Rivulus marmoratus Poey embryos during development

In Rivulus marmoratus development of retinal pigmented epithelium (RPE) parallels that of retinal photoreceptors. Although structurally functional by mid-incubation the full complex structure is not achieved even when the yolk-sac is absorbed (3-days post-hatched). Melanogenesis is evident at 0.2 incubation with premelanosomes present up to three days after hatching. The distribution of junctional complexes, basal membrane foldings and coated-pits throughout development is noted. Myeloid bodies, already present at mid-incubation, appear initially as single lenticular-shaped structures which later may form whorls, or coalesce around oil droplets, glycogen clusters or phagosomes thereby giving rise to myeloid patterns characteristic of a mature RPE. The functional significance of these changes is discussed.

Myeloid bodies associated with lipid droplets in pinealocytes of blind, blind-bulbectomized, blind-underfed or blind cold-exposed rats

The ultrastructure of pinealocytes was studied in rats subjected to manipulations known as enhancers of pineal actions: blindness (B), blindness + olfactory bulbectomy (BObx), blindness + underfeeding (BU), blindness + cold exposure (BC). A large number of myeloid bodies (MBs) were found especially in the light pinealocytes of BObx, BU and BC rats but not in intact controls. These MBs were, in most cases, associated with lipid droplets. The presence of a large number of MBs in rats in which pineal-dependent gonadal impairment has been demonstrated, strongly supports the hypothesis that MBs play a role in pineal photoneuroendocrine activity.

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)

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.

Morphology of the pineal complex of the anadromous sea lamprey, Petromyzon marinus L

The pineal complex of the anadromous sea lamprey, Petromyzon marinus L., has been examined by light and electron microscopy. It consists of two subunits: a dorsomedial pineal organ, and a ventral, left-lateral parapineal organ, with both remaining cytologically unaltered throughout the life cycle. However, during metamorphosis there is an increase in the size and a rostral migration of the parapineal organ and a dorsolateral displacement of the pineal nerve tract. The pineal organ is composed of an end bulb, an atrium, and a nerve tract. Two varieties of photoreceptors, termed type I and type II cells, as well as supporting and ganglion cells are present. Supporting cells are ubiquitous, but there are regional variations in the distribution and abundance of the other cell types. Type I cells are a well-differentiated photoreceptor. Conversely, type II cells exhibit a poorly developed photosensory apparatus but possess some features commensurate with an endocrine activity. The parapineal organ is composed of an end bulb, a ganglion region, and a nerve tract. The end bulb is dominated by type II photoreceptor and supporting cells, whereas type I and ganglion cells are sparse. The parapineal ganglion and nerve tract include neuropil, polymorphic neurons, and ependymal cells. It is concluded that the pineal organ of P. marinus is a structurally well-developed photosensory and photoneuroendocrine organ that is probably capable of transducing photic stimuli into nervous and endocrine messages to the brain and other organs. In contrast, the parapineal of this lamprey species is a poorly developed organ of regressed or rudimentary structure.

Retinal development in the lamprey (Petromyzon marinus L.): premetamorphic ammocoete eye

Development of the retina of the ammocoete begins early in embryogenesis, with the formation of the optic vesicle, but development of the rudimentary eye is suspended and remains arrested during larval life. Prior to the onset of metamorphosis, the retina of the ammocoete is completely undifferentiated, with the exception of a small area (Zone II) surrounding the optic nerve head, where all of the adult retinal layers are found. The photoreceptors in this area have developed to include synaptic contacts as well as inner and outer segments. The pigment epithelium in this area, too, has differentiated to include well-formed melanin granules, myeloid bodies and endoplasmic reticulum and is closely associated with the receptor cell outer segments. With the approach of metamorphosis, differentiation of the remainder of the retina (Zone I) begins, taking place in a radial fashion from the optic nerve head. Differentiating pigment epithelial cells adjacent to the differentiated retinal zone begin to accumulate melanin granules. In the neural retina, junctional complexes are established in the form of an external limiting membrane, and connecting cilia project into the optic ventricle. Photoreceptor differentiation begins with the formation of a mitochondria-filled ellipsoid within the inner segment. Development and differentiation of the ammocoete retina is unique to vertebrates in that only a small area of differentiated retina is present during the larval stage. The remainder of the retina differentiates and becomes functional during metamorphosis.

Studies on the eyes of anchovies Anchoa mitchilli and A. hepsetus (Engraulidae) with particular reference to the pigment epithelium

A study has been made of the eyes of the anchovies Anchoa mitchilli and A. hepsetus (Engraulidae) with particular reference to the pigment epithelium. Eyes of dark (d.a.) and light (l.a.) adapted fish were investigated by optical and electron microscopy. In light adapted eyes the pigment epithelium is radially extended with the vitread region of the cells inserted amongst the cones and lying in vertical rows above them. They are separated from adjacent rows of pigment epithelial cells by masses of rods arranged in similar vertical sheets. The vitread end of the cell, where it fits between the cones, is cuneate in shape. In l.a. eyes lateral lobes develop from the cuneate portion of the cell, extend into the spaces between the cone rows, and serve to shield the lateral margins of the cones. On dark adaptation of the eye the lateral lobes are withdrawn as the cell shortens radially. The cytoplasm of the pigment epithelium contains numerous myeloid bodies composed of stacks of smooth endoplasmic reticulum distributed throughout the cell. In l.a. eyes masses of less well ordered e.r. cisternae and tubules are also common, especially in the vitread region among the tapetal material and melanosomes. Mitochondria, some large vacuoles, lysosomes containing ingested portions of rod outer segments, and also occasionally melanosomes and tapetal material occur in the basal region of the cell in the vicinity of the nucleus. Structurally differentiated ground substance and ribosomes are poorly represented throughout all regions. Spherical and cylindrical melanosomes and tapetal material composed of guanine are plentiful within the cell. In the l.a. state melanosomes occur in the vitread region, at the level of the cones, but in the d.a. state they become concentrated in the basal region forming a dark scleral zone throughout the tissue. The tapetal material consists of approximately isodiametric crystallites, truncate needles and thin almost square platelets. All forms are strongly birefringent. The crystallites are distributed throughout the l.a. cell but in the d.a. condition they become concentrated mainly in the middle region forming a prominent whitish zone throughout the tissue vitread to the melanosome layer. In l.a. eyes the needles are arranged axially and are distributed mainly around the periphery of the cell in the vitread region. On dark adaptation of the eye some of the needles are dispersed amongst the crystallites in the middle region. The platelets are grouped into two stacks which remain essentially unaltered in organization and location during photomechanical movements of the eye. The stacks are grouped into a V in the vitread cuneate region with the platelets abutting the cone units and lying in the same vertical plane as the cone lamellae. Each stack is composed of several regularly spaced rows with each row made up of two files of platelets. The average thickness of platelets measured from vertical sections through stacks is about 82 nm and the distance between platelets of adjacent rows is about 112 nm. A cisterna of endoplasmic reticulum runs between each row and is connected to the membrane sacs of adjacent platelets by tubular connections. Non-membrane crossbridging material also links the cisterna and platelet sacs. The consequence of the organization of the platelet stacks as a system for the reflexion of light into the cones by constructive interference is discussed.