A role for microglia in the maintenance of photoreceptors in retinal transplants lacking pigment epithelium

Summary Studies on intact retina have pointed to a necessary role for retinal pigment epithelium in the maintenance of photoreceptor outer segments and for regeneration of visual pigment. However, it has been shown that when embryonic retinae are separated from the pigment epithelium and transplanted into the brain of neonatal rats, the transplanted photoreceptors develop outer segments and the retina responds to light in the apparent absence of pigment epithelial cells. We confirm that there are no retinal pigment epithelium cells associated with transplanted retinae in the present series of experiments and show that a row of cells, composed predominantly of microglia of host origin, border the graft. These cells can be seen to contain engulfed outer segments when they are apposed to the outer retina, suggesting that the microglia have assumed, at the least, the phagocytic function normally associated with retinal pigment epithelium. Microglial cells and their processes are also found within the transplant, but these cells are typically devoid of phagosornes, indicating an absence of phagocytic activity. The close physical association of these resting microglia with the transplant may facilitate their role in antigen presentation under specific conditions of immune provocation.     

Retinal fine structure in the European eel Anguilla anguilla. V. Pigment epithelium of the sexually immature silver eel stage

The fine structure of the retinal pigment epithelium (RPE), Bruch's membrane and choriocapillaris has been investigated in the sexually immature silver eel and compared with previous observations made at the glass and yellow eel stages of the lengthy life cycle of the European eel. As in the previous stages the retinal pigment epithelium of the sexually immature silver eel consists of a single layer of cuboidal to squamous cells joined laterally by apically located cell junctions. The basal (scleral) and lateral borders of these cells are relatively smooth while apical (vitreal) processes enclosing photoreceptor outer segments are plentiful. Internally the epithelial cells display a vesicular pleomorphic nucleus, abundant mitochondria and melanosomes as well as myeloid bodies, phagosomes and polysomes. Smooth endoplasmic reticulum (SER) is abundant while rough endoplasmic reticulum (RER) is uncommon. Lipid droplets are also scarce. Wandering phagocytes are a regular feature amongst the retinal epithelial cells. Bruch's membrane is trilaminate and remains thin as in the previous stages investigated. The choriocapillaris is a single discontinuous layer of capillaries showing fenestrations on the side facing Bruch's membrane. A stratum argenteum remains in the choroid but separated from the retinal epithelium by pigment cells of the choroid.     

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.     

Photoreceptor morphogenesis in the human retina: A scanning electron microscopic study

There are a number of scanning electron microscopic (SEM) studies on retinal photoreceptors of vertebrates. However, most of these are concerned with the adult retina, and only a very few deal with developing photoreceptors. In man, SEM studies have not been carried out on photoreceptor morphogenesis during fetal or postnatal stages. Hence, the present study was undertaken to examine the sequential morphological changes in developing photoreceptors during different gestational ages in the human retina. Retinas of human fetuses of gestational ages of 10–25 weeks and from autopsy of a 5-month-old infant were processed for SEM. The observations show some new information on the morphogenesis of photoreceptors. At 10–11 weeks, the outer and inner neuroblastic zones are well developed and separated from each other by the layer of Chievitz. By 15–16 weeks, the photoreceptor precursors appear as spherical inner segments on the scleral surface of the outer neuroblastic zone. Cilia develop as small protrusions from the apical ends of the inner segments. Photoreceptor inner segments become arranged in mosaic pattern by 18–19 weeks. In the mosaic, large cone inner segments (putative blue cones) stand out prominently from the remaining small cone inner segments (prospective red/ green cones). The rod inner segments are identifiable and show cilia. Between 19–20 and 24–25 weeks, the cone inner segments elongate and change in shape from spherical to oval. At 24–25 weeks, the outer segments develop from the distal ends of rod cilia. At this period, the inner segments of rods and cones are interconnected by protoplasmic projections. Although the precursors of both rods and cones appear to be in a similar state of development at 14–15 weeks gestation, the rods undergo morphological maturation earlier than do the cones. Photoreceptor development in the anterior retina lags behind that of the posterior retina by about 10 weeks. At 5 months after birth, the posterior retina possesses fully developed photoreceptors that are comparable to those of the adult. However, the photoreceptors in the ora serrata resemble those in the posterior retina of 24–25 weeks gestation. Anat. Rec. 252:133–139, 1998. © 1998 Wiley-Liss, Inc.     

Expression profile of heat shock protein 108 during retinal development in the chick

In the developing chick retina, heat shock protein 108 (HSP108), which exhibits transferrin binding activity, has been demonstrated at the mRNA level, while transferrin shows two expression peaks. Here, we investigated the expression profile of HSP108 in the developing chick retina at the protein level. The localization of HSP108 in embryonic days 15 (E15), E18, and postnatal day 2 (P2) chick retina was examined immunohistochemically using monoclonal antibody 9G10 specific for chick HSP108, while the expression levels of HSP108 in developing chick retina from E12 to P2 and adult were measured by Western blot analysis. HSP108 was expressed in the ganglion cell layer, inner nuclear layer, outer plexiform layer, outer nuclear layer, inner segments of photoreceptors and retinal pigment epithelium. Two peaks of HSP108 expression were found at around E13 and E18, respectively. Since the two HSP108 peaks appeared to be correlated with the transferrin expression peaks during retinal development, HSP108 may be associated with iron metabolism during the development of the retina.     

Closer look at lactose-mediated support of retinal morphogenesis

We have previously shown in intact isolated eye rudiments from Xenopus laevis that lactose, but not mannose, permits the formation of organized photoreceptor outer segments in the absence of the retinal pigment epithelium (RPE). The purpose of this study was to determine, using electron microscopic analysis, the key ultrastructural differences between healthy retinas, lactose-protected retinas, and retinas that developed aberrantly to reveal which subcellular structures were exclusively present in healthy retinas. Filamentous actin was also localized in retinas to determine its distribution under the various conditions. In healthy retinas, calycal processes extending approximately three-fourths of the length of the outer segment surrounded highly organized photoreceptor outer segments. Adherens junctions were localized between adjacent photoreceptors and Müller cells at the outer limiting membrane. In addition, Müller cells possessed apical processes that extended for a short distance beyond the adherens junctions. These fine cytoarchitectural details were missing in retinas that completed differentiation in the absence of the RPE; both calycal and apical processes were no longer present and adherens junctions were sparsely intermittent. Müller cells appeared atrophic. Similarly, mannose promoted none of the fine cytoarchitectural details of the retina. Lactose, however, supported the formation of the proper subcellular cytoarchitecture of both photoreceptor and Müller cells. These results suggest that these subcellular structures may be fundamental for the proper assembly and stability of organized outer segments and are necessary to allow for normal cytogenesis of the outer retina.     

Daily morphological variations in the viscacha (Lagostomus maximus maximus) retina. Probable local modulatory action of melatonin

Given that the local melatonin levels exhibit rhythmic daily changes in the retina of the viscacha, we considered it important to study the likely daily variations in morphology and specific 2-[(125)I]-iodomelatonin binding in retinas from this rodent and to correlate these putative changes with local indole levels. Adult animals of both sexes were captured in their habitat and were kept under a natural photoperiod. For light and electron microscopic studies the viscachas were sacrificed by decapitation at 08:00, 16:00, and 24:00 hr. A computer-assisted image analysis system was used to measure the thickness of the complete retina, the photoreceptor layer, the rod outer and inner segments, and the outer nuclear layer. The daily variation in 2-[(125)I]-iodomelatonin binding sites was followed during a 24-hr light-dark cycle, the animals being sacrificed at six time points. The parameters studied showed significant variations throughout the 24-hr period. Maximal specific binding, lysosomal content in the pigment epithelium, and photoreceptor layer outer segment thicknesses were observed at 24:00 hr. Close contact between photoreceptor membranes and microvilli of the pigment epithelium was observed at 08:00 and 16:00 hr. Moreover, the minimal outer segment thickness at 16:00 hr was accompanied by a scarcity of dense bodies, such as lysosomes, a maximum dispersion of melanin pigment granules, and a minimum density of radioligand binding sites. Therefore, in the retina of the viscacha, we suggest that the interaction between melatonin and specific sites could be one of the factors or causes that participate in the regulation of the daily morphological changes observed in viscacha.     

Sequential ultrastructural changes of the pancreas in zinc toxicosis in ducklings.

The sequential ultrastructural alterations of the pancreas in zinc toxicosis were examined in ducklings fed 2500 ppm Zn (as ZnSO4) for 56 days. From days 3 to 17, acinar cells had cytoplasmic vacuoles that contained electron-dense, zymogen-like material and increased autophagocytosis. Other changes were swollen mitochondria and dilatation, vesiculation, degranulation and intracisternal sequestration of rough endoplasmic reticulum. Apoptosis was the predominant form of cell deletion. By day 10, acinar cellular atrophy and interstitial fibrosis were noted. Islets appeared normal. After day 19, the pancreas consisted of ductlike structures embedded in fibrous connective tissue with a minimal inflammatory cell response. These ductlike structures were lined by attenuated to cuboidal, atrophic acinar cells. Many cells contained granular, electron-dense cytoplasmic debris that served as a marker of previous cell damage. This ultrastructural study provides support for a previously proposed theory that ductlike structures (tubular complexes) arise by atrophy and dedifferentiation of acinar cells.     

The subcellular localization of rat photoreceptor-specific antigens

The subcellular localization of three photoreceptor antigens (RET-P1, rhodopsin and RET-P2) has been studied by electron microscopic immunocytochemistry of rat retinas. Localization was also examined by determining the amount of RET-P1 and RET-P2 antigen in various subcellular fractions. RET-P1 and RET-P2 antigens were further characterized by immunoblotting of crude retina membrane proteins which had been separated by one-dimensional gel electrophoresis. RET-P1 antigen has been detected with a monoclonal antibody that reacts with the perikarya, inner segments, and outer segments of adult rat photoreceptors by peroxidase immunolabelling of fixed tissue sections. Analysis at the electron microscopic level has shown that RET-P1 antigen is located on the external face of the inner and outer segment plasma membrane. A monoclonal antibody against purified bovine rhodopsin (RHO-C7) labels the outer segments of rat retinas by peroxidase immunocytochemistry. Ultrastructural antibody localization indicates that this particular determinant of rhodopsin is exposed on the external face of the plasma membrane of outer segments and may also be expressed on the surface of the inner segments. RET-P2 antibody labels only the outer segments of adult rat photoreceptors by peroxidase immunocytochemistry. The light microscopic labelling of RET-P2 antibody in the presence, but not in the absence, of detergent suggests that it is an intracellular antigen. The results of both ultrastructural labelling and biochemical fractionation are consistent with the localization of RET-P2 antigen on the internal face of the plasma membrane and/or the cytoplasmic face of the disc membranes. RET-P2 antigen was found to be a protein (or glycoprotein) of apparent molecular weight 38 000 +/- 3000.     

Light dependence of osmium reactivity in mouse photoreceptor cells

Mouse photoreceptor cells exhibit local accumulations of osmium deposits after prolonged osmic staining at slightly elevated temperatures. Deposits were evident along the membranes of outer segment lamellae, Golgi cisternae and vesicles, nuclear envelopes, and synaptic vesicles. Other membranes within the photoreceptor cells were unreactive. No osmium reactivity was seen in other cells of the retina except for osmiophilic outer segment material which had been phagocytized by the pigment epithelium. In the outer segments, inner segments, and synaptic regions of the photoreceptor cells, the amount of osmium reactivity was increased by light stimulation and decreased following extended dark adaptation. The possible significance of the localized osmium reactivity is discussed.     

Retinal epithelial fine structure in the brush-tailed possum (Trichosurus vulpecula)

As part of a comparative morphological study, the fine structure of the retinal pigment epithelium (RPE), the choriocapillaris and Bruch's membrane (complexus basalis) has been studied by electron microscopy in the brush-tailed possum (Trichosurus vulpecula), an Australian marsupial. In this species, the RPE consists of a single layer of squamous cells which show numerous shallow basal (scleral) infoldings as well as extensive apical (vitreal) processes enclosing rod outer segments. The epithelial cells are joined laterally by prominent, apically-located tight junctions. Internally smooth endoplasmic reticulum is the most abundant cell organelle with only small amounts of rough endoplasmic reticulum present. Basally-located mitochondria and apically-located melanosomes are abundant while polysomes and the occasional Golgi zone are also noted. The epithelial cell nucleus is large and vesicular. Bruch's membrane shows the typical pentalaminate structure noted for mammalian species. The choriocapillaris is a single layer of anastomosing capillaries which are heavily fenestrated facing the retinal epithelium.     

Histochemical localization of zinc and copper in rat ocular tissues

Zinc and copper were histochemically localized in rat ocular tissues. Zinc was demonstrated by the dithizone method, and copper by the rubeanic acid and rhodanine methods. The retinal photoreceptor's outer segment showed the presence of zinc, but no zinc reaction was seen in other parts of the retina or other ocular tissues, even in rats fed excess zinc. It seems that photoreceptor cells contain the highest concentration of zinc. Copper was seen in the corneal epithelium and endothelium, iris, ciliary body, lens epithelium, retinal outer nuclear layer, photoreceptor inner and outer segments, retinal pigment epithelium, choroid, sclera, and optic nerve. These trace elements seems to play roles as components of some metalloenzymes and may have other functions till now unknown.     

Fine structure of the retinal epithelium and tapetum lucidum in the giant danio (Danio malabaricus) (Teleost)

Summary The normal morphology of the retinal epithelium and tapetum lucidum has been studied by transmission electron microscopy in the eye of a teleost; the giant danio (Danio malabaricus). The retinal epithelium forms a single layer of cells joined laterally by cell junctions. Centrally in the retina these cells are tall columnar, while more peripherally they become more cuboidal in shape. Apical processes of these cells enclose photoreceptor inner and outer segments. Few basal (scleral) infoldings are seen in the epithelial cells. Throughout the epithelial layer the cells display smooth endoplasmic reticulum, numerous mitochondria and phagosomes. Within the epithelial cells centrally located in the retina are found numerous lipid spheres which act as a tapetum lucidum. Only at the extreme periphery are these reflective spheres scarce or absent from the epithelial cells. Melanosomes, although found throughout the epithelial layer, are scarce centrally and more numerous peripherally. A population of wandering phagocytes appears to be a normal feature within the retinal epithelial layer of this species. Bruch's membrane shows a trilaminate composition rather than the usual pentalaminate structure seen in most vertebrates. The choriocapillaris endothelium facing Bruch's membrane is very thin but only minimally fenestrated.     

Fine structure of the retinal pigment epithelium in the Port Jackson shark (Heterodontus phillipi)

The structure of the retinal epithelium (RPE), choriocapillaris and Bruch's membrane (complexus basalis) has been studied by light and electron microscopy in the Port Jackson shark (Heterodontus phillipi). In this elasmobranch the RPE consists of a single layer of low cuboidal cells which show basal (scleral) infoldings and apical (vitreal) processes that enclose photoreceptor outer segments. Laterally these epithelial cells are joined by a series of apically located tight junctions. The RPE cells display a large vesicular nucleus, abundant smooth endoplasmic reticulum as well as numerous polysomes and mitochondria. Phagosomes are present, rough endoplasmic reticulum is scarce and myeloid bodies were not observed. Melanosomes are absent over the choroidally located tapetum lucidum, but are not abundant even in extratapetal areas. This paucity of melanosomes probably makes retinomotor movements unimportant. Bruch's membrane or complexus basalis is a pentalaminate structure. The endothelium of the choriocapillaris is thin but minimally fenestrated.     

Fine structure of the retinal pigment epithelium in the Port Jackson shark (Heterodontus phillipi)

The structure of the retinal epithelium (RPE), choriocapillaris and Bruch's membrane (complexus basalis) has been studied by light and electron microscopy in the Port Jackson shark (Heterodontus phillipi). In this elasmobranch the RPE consists of a single layer of low cuboidal cells which show basal (scleral) infoldings and apical (vitreal) processes that enclose photoreceptor outer segments. Laterally these epithelial cells are joined by a series of apically located tight junctions. The RPE cells display a large vesicular nucleus, abundant smooth endoplasmic reticulum as well as numerous polysomes and mitochondria. Phagosomes are present, rough endoplasmic reticulum is scarce and myeloid bodies were not observed. Melanosomes are absent over the choroidally located tapetum lucidum, but are not abundant even in extratapetal areas. This paucity of melanosomes probably makes retinomotor movements unimportant. Bruch's membrane or complexus basalis is a pentalaminate structure. The endothelium of the choriocapillaris is thin but minimally fenestrated.     

The pineal gland in wild-type and two zebrafish mutants with retinal defects

Light and transmission electron microscopy were used to characterize the ultrastructural features of the pineal glands of wild-type and two mutant zebrafish strains that have retinal defects. Particular attention was given to the pineal photoreceptors. Photoreceptors in the pineal gland appear quite similar to retinal cone photoreceptors, having many of the same structural characteristics including outer segment disk membranes often confluent with the plasma membrane, calycal processes surrounding the outer segments, and classic connecting cilia. The pineal photoreceptor terminals differ from photoreceptor terminals in the retina in that they have short synaptic ribbons and make dyad synapses which may or may not be invaginated. Pineal photoreceptors in two zebrafish mutants with abnormal retinal photoreceptors were also studied. Pineal photoreceptors in the niezerka (nie) mutant degenerate, as they do in the retina, indicating that pineal and retinal photoreceptors share at least some genes. However, the synaptic terminals of no optokinetic response c (nrc) pineal photoreceptors are normal, suggesting that this mutation is specific to the retina.     

Development of ultrastructural heterogeneity among hepatocytes in the mouse

Ultrastructural stereological analyses of periportal and centrilobular hepatocytes of newborn, 5- and 10-day-old, and adult male ddY mice were carried out to study the postnatal development of the morphologic heterogeneity among hepatocytes. In newborn animals, the periportal and centrilobular cells did not differ in the volume densities of the smooth and rough endoplasmic reticulum; in the volume and numerical densities of the mitochondria, lysosomes, peroxisomes, and lipid droplets; or in the shape (the axial ratio) of the mitochondria. In 5-day-old animals, the volume densities of the mitochondria and rough endoplasmic reticulum were greater in periportal cells than centrilobular cells, and the volume density of the smooth endoplasmic reticulum was greater in centrilobular cells than periportal cells. In 10-day-old animals, a further difference was seen in the numerical density of the mitochondria, which was greater in centribular cells than periportal cells. Adult hepatocytes showed also a difference in the axial ratio of the mitochondria, which was greater in centrilobular than periportal cells; there was no difference in the volume density of the rough endoplasmic reticulum. When the data were expressed as volume and number per hepatocyte, the patterns of sublobular distributions of these organelles differed from the patterns seen in the volume and numerical density data, mainly in adult animals. This difference was caused by the marked increase in hepatocyte volume between 10 days of age and adulthood, especially in centrilobular cells. The results show that, in general, the ultrastructural heterogeneity among hepatocytes, evident in adult animals, is not present in newborn animals but arises during postnatal development, and suggest the occurrence of a lobular gradient in postnatal development of hepatocyte functions.     

Photoreceptor fine structure in the goldeye (Hiodon alosoides)(Teleost)

Summary The retinal photoreceptors of the goldeye (Hiodon alosoides) are arranged in large bundles of 40–50 cells optically isolated from other bundles by the retinal epithelial cells. Within each bundle are found both rods and cones in roughly equal numbers. Rod photoreceptors show marked retinomotor responses to project beyond the photoreceptor bundle in light-adaptation and to lie entirely within the bundle in dark-adaptation. In all stages of the light cycle cone outer segments remain at the apex of the photoreceptor bundle. In light-adaptation, rod inner segments display an apical ellipsoid separated from a basal ellipsoid by the greatly elongated myoid. In dark-adaptation the rod inner segment is much the same diameter throughout its length. In both rods and cones, profiles of rough and smooth endoplasmic reticulum and Golgi zones are present in a supranuclear location. The nuclei of rods display little heterochromatin and are located vitreal to the external limiting membrane in light-adaptation, whereas in dark-adaptation more heterochromatin is noted and the nuclei lie scleral to the external limiting membrane. Cone nuclei display the same changes in chromatin pattern as rods but they show changes in nuclear location opposite that of the rods. Throughout its length, the rod photoreceptor cytoplasm is more electron dense than that of the cone. The synaptic spherule, of rods displays 2–3 invaginated synaptic, sites while the cone pedicle is larger and presents 8–10 invaginated synaptic sites. Both rods and cones also appear to have superficial synaptic sites. Membrane specializations are found along the length of the inner segments where rods and cones are contiguous. These may act as sites of intercellular communication and the whole photoreceptor bundle may therefore be considered, as a macroreceptor.     

Ultrastructural localization of calcium in rat retina with oxalate pyroantimonate and energy-dispersive X-ray detector

Intracellular calcium plays an important role in the intracellular signal transduction as one of the second messengers. In this study, we examined the ultrastructural distribution of calcium in rat retina, using the oxalate pyroantimonate technique and X-ray microanalysis. Large amounts of precipitates were observed inside the disc of outer segments of photoreceptor cells (OS) and the synaptic vesicles of the inner (IPL) and outer plexiform layer (OPL). Precipitates also were observed in the ribosome-rich regions in the cytoplasm and the euchromatinic part in the cell nuclei of the ganglion, amacrine, and bipolar and horizontal cells. However, few precipitates were found in the inner segment of the photoreceptor cells and the retinal pigment epithelium (RPE). X-ray microanalysis with an energydispersive X-ray detector revealed that these precipitates had a peak of antimony and calcium. Therefore, it was suggested that these precipitates were Ca[Sb(OH)₆]₂, the reaction products of the oxalate-pyroantimonate technique. Our findings showed that calcium precipitates are abundant in retinal regions that are related to visual transmission.     

Microtubular aggregates within rough endoplasmic reticulum in myxopapillary ependymoma of the filum terminale

Aggregates of microtubules with-in rough endoplasmic reticulum were found in many neoplastic cells of three cases of myxopapillary ependymoma of the filum terminale studied ultrastructurally. The cytoplasm and cellular processes of some neoplastic cells were distended by the aggregates. In general the involved rough endoplasmic reticulum contained three to six microtubules, but some enclosed more than 10 microtubules. The enclosed microtubules were straight parallel hollow cylindrical structures with fuzzy coats. They had an inner diameter of 12 to 15 nm, an outer diameter of 30 to 35 nm, and a center-to-center distance of 50 to 62 nm. The involved rough endoplasmic reticulum often showed various degrees of loss of surface ribosomes and some appeared totally degranulated. Vacuolar degeneration of involved rough endoplasmic reticulum with fragmentation and disintegration of the enclosed microtubules was frequent. Direct relationship of the enclosed microtubules to the cytoplasmic microtubules and ciliary formation was not found. This unusual microtubular aggregate has not been described in other types of ependymoma, or in other brain tumors. They may represent a characteristic ultrastructural feature of myxopapillary ependymoma.