Retinal pigment epithelial fine structure in the bobtail goanna (Tiliqua rugosa)

The retinal pigment epithelium (RPE), the choriocapillaris and Bruch's membrane (complexus basalis) have been studied by light and electron microscopy in the bobtail goanna (Tiliqua rugosa) an Australian diurnal lizard. The RPE consists of a single layer of cuboidal cells which display very deep and tortuous basal (choroidal) infoldings as well as numerous apical (vitreal) processes which interdigitate with the photoreceptor cells. The lateral cell borders are relatively smooth and joined by basally located tight junctions. Internally smooth endoplasmic reticulum is abundant while rough endoplasmic reticulum is not. The RPE cell nucleus is large and vesicular and basally located in the light-adapted state. Polysomes, mitochondria and myeloid bodies are present and widely distributed. Melanosomes are plentiful in the apical region of the epithelial cells in light-adaptation. Bruch's membrane is pentalaminate with the basal lamina of the choriocapillaris being exceptionally thick. The choriocapillaris is a single layer of large-caliber capillaries with thin but only moderately fenestrated endothelium. Numerous dense granules are always present within these endothelial cells.     

Retinal epithelial fine structure in the vervet monkey (Cercopithecus aethiops)

The morphology of the retinal pigment epithelium (RPE) and closely associated Bruch's membrane (complexus basalis) and choriocapillaris have been investigated by electron microscopy in the vervet monkey (Cercopithecus aethiops). The RPE is composed of a single layer of cuboidal cells joined laterally by apically-located junctional complexes. Basally (sclerally) these cells display numerous infoldings while apically (vitreally) abundant processes enclose and interdigitate with rod outer segments. Internally the large vesicular nucleus is centrally located and smooth endoplasmic reticulum, mitochondria and lysosome-like bodies, are plentiful. Rough endoplasmic reticulum, polysomes and melanosomes while present are not abundant. Phagosomes of outer segment discs are noted in various stages of uptake and degradation. The choriocapillaris is highly fenestrated over large areas. Bruch's membrane shows the typical pentalaminate structure noted in other mammalian species without a tapetum lucidum.     

Retinal epithelial fine structure in the grey seal (Halichoerus grypus)

The morphology of the retinal epithelium and the closely associated choriocapillaris and Bruch's membrane (complexus basalis) has been investigated in the eye of the grey seal (Halichoerus grypus) by electron microscopy. The retinal epithelium is composed of a single layer of cuboidal cells joined laterally by apically-located junctional complexes. Basally (sclerally) these cells display numerous infoldings while apically (vitreally) abundant processes enclose rod outer segments. Internally the large vesicular nucleus is centrally located. Smooth endoplasmic reticulum, mitochondria and lysosome-like bodies are abundant. Rough endoplasmic reticulum and polysomes while present are not plentiful. Phagosomes of outer-segment discs are only occasionally noted in the light-adapted state. As the entire fundus is overlain with a choroidally located tapetum cellulosum, only at the extreme periphery is an occasional melanosome present in these epithelial cells. The choriocapillaris endothelium is highly fenestrated and the profiles of these capillaries are deeply indented into the epithelial layer. Bruch's membrane (complexus basalis) is reduced to a trilaminate structure rather than the typical pentalaminate membrane seen in most mammalian species and when associated with capillary profiles is further reduced to a single thick basal lamina.     

Retinal epithelial fine structure in the southern fiddler ray (Trygonorhina fasciata).

The morphology of the retinal epithelium (RPE), choriocapillaris and Bruch's membrane (complexus basalis) has been investigated by light and electron microscopy in an elasmobranch, the southern fiddler ray or guitarfish (Trygonorhina fasciata). The RPE consists of a single layer of cuboidal cells which display basal (scleral) infoldings as well as numerous apical (vitreal) finger-like processes which interdigitate with the photoreceptor outer segments. The lateral cell borders are relatively smooth and are joined in the mid-region by a series of tight junctions. Internally the RPE nucleus is large, vesicular and centrally located. Smooth endoplasmic reticulum (SER) is abundant while rough endoplasmic reticulum (RER) is scarce. Polysomes are however widespread and mitochondria are plentiful. Two unusual organelles are also noted. One consists of a membrane bound array of tubules while the other is a membrane bound structure consisting of a granular matrix with again an internal tubular array. This species possesses a choroidally located tapetum lucidum in the superior fundus and over this tapetal area, melanosomes are absent from the RPE cells. In non-tapetal locations a few melanosomes are present that do not appear to undergo photomechanical movements. Bruch's membrane is a pentalaminate structure with an almost continuous central elastic layer (lamina densa). The choriocapillaris forms a single layer of capillaries with a thin but only minimally fenestrated endothelium facing Bruch's membrane.     

Retinal pigment epithelial fine structure in the red-backed salamander (Plethodon cinereus).

The retinal pigment epithelium (RPE) of the red-backed salamander (Plethodon cinerus) consists of a single layer of large squamous shaped cells. The RPE cells are but minimally infolded basally (sclerally) but show many large apical (vitreal) processes interdigitating with the rod outer segments. These epithelial cells are joined laterally by prominent tight junctions located in the mid region of the cells. Internally smooth endoplasmic reticulum is very plentiful while rough endoplasmic reticulum is not. Polysomes, small dense mitochondria and small round to oval melanosomes are plentiful. Golgi zones and lysosome-like bodies are also present as are phagosomes of outer segment material and myeloid bodies. The RPE cell nucleus is large and vesicular. It is felt that the melanosomes undergo retinomotor movements but as only light-adapted specimens were examined it is not known how extensive are these movements. Bruch's membrane or complexus basalis shows the typical pentalaminate structure noted for most vertebrates. The choriocapillaris is a single layer of large anastomosing capillaries which are minimally fenestrated facing Bruch's membrane.     

Fine structure of the retinal epithelium and tapetum lucidum of the ranch mink Mustela vison

The morphology of the retinal pigment epithelium (RPE), Bruch's membrane (complexus basalis), choriocapillaris and tapetum lucidum has been studied in the eye of the ranch mink (Mustela vison) by light and electron microscopy. The RPE is composed of a single layer of cells joined laterally by apically located junctional complexes. Basally (sclerally) these cells display numerous infoldings whereas apically (vitreally) two types of processes are associated with rod and cone outer segments. Smooth endoplasmic reticulum and mitochondria are abundant in these cells whereas rough endoplasmic reticulum and polysomes, although present, are not plentiful. An occasional wandering phagocyte is noted at the RPE-photoreceptor interface. In the posterosuperior part of the fundus, a degenerative tapetum lucidum is present. The presence of only a few layers of tapetal cells containing but little reflective material and the haphazard arrangement of this material makes it very unlikely that this area functions as an effective tapetum lucidum. The RPE over the aberrant tapetum, however, shows the morphology that is seen when a functioning tapetum cellulosum is present, namely the absence of melanosomes and an indented choriocapillaris. Bruch's membrane in non-tapetal areas is pentalaminate but, over the tapetum and where it is associated with capillary profiles, it is reduced to a single, thickened basal lamina. The choriocapillary endothelium is highly fenestrated and in nontapetal areas these capillaries are not indented into the epithelial layer.     

Electron microscopic observations on the retinal pigment epithelium and choroid in the domestic pig (Sus scrofa)

Summary The morphology of the retinal pigment epithelium (RPE) and adjacent choroid has been investigated by electron microscopy in the domestic pig. The RPE consists of a single layer of cells which are columnar posteriorly but become cuboidal and even squamous moving peripherally in the fundus. The cells of the RPE layer regardless of location display basal (scleral) infoldings and apical (vitreal) processes and are joined laterally by junctional complexes. Throughout the retina the epithelial cells are rich in smooth endoplasmic reticulum and mitochondria but less so in rough endoplasmic reticulum and polysomes. The epithelial nucleus is vesicular and basally located. In the superior fundus an area of the RPE is very lightly pigmented and richer in lysosomes than is this layer in the inferior and peripheral fundus. The choroid overlying this area is also lightly pigmented and contains much collagen in a lamellar arrangement. This region may represent a vestigial tapetum fibrosum. Bruch's membrane is slightly thicker posteriorly but is everywhere seen to have a pentalaminate substructure. The choriocapillaris is a single layer of large capillaries which show numerous fenestrations facing the RPE. In the superior fundus the choriocapillaris is also highly fenestrated facing the choroid.     

Endocytosis in the rat retinal pigment epithelium

Endocytosis in the retinal pigment epithelium (RPE) of rats was studied using horseradish peroxidase, microperoxidase and ferritin tracers. Tracer uptake was mediated by coated pits and coated vesicles. Coated pits formed at two discrete regions at the RPE plasma membrane: that portion of basal membrane directly opposing Bruch's membrane, and at the bases of the apical lamellae and villi. Two populations of coated vesicles were identified and distinguished by size, location and function. Large coated vesicles (91.8 +/- 14.7 nm in diameter) were located near the cell surface and incorporated tracer. Small coated vesicles (64.5 +/- 15.7 nm diameter) located more deeply within the cell were not tracer-labeled, and were often fused with the endoplasmic reticulum or the Golgi apparatus. Observations of the endocytic pathway in rat RPE cells are presented. Tracer was also found in organelles of the lysosomal system, e.g. the multivesicular body, but was not identified in the smooth endoplasmic reticulum or Golgi apparatus.     

Fine structure of the retinal epithelium and tapetum lucidum of the opossum (didelphis virginiana)

The fine structure of the retinal epithelium has been studied by electron microscopy in the opossum (Didelphis virginiana). The retinal epithelium, over most of the retina, is typical of that in other vertebrates and consists of a single layer of heavily pigmented, cuboidal cells. These cells display extensive basal (scleral) infoldings and numerous apical (vitreal) processes which enclose photoreceptor outer segments. A semicircular area of the retinal epithelium in the superior fundus is further specialized as a tapetum lucidum. The reflecting material consists of a large quantity of lipoidal spheres scattered throughout the epithelial cells. Centrally in the tapetal area very few or no melanosomes are found, indicating a non-occlusible tapetum. Peripherally in the tapetum, the epithelial cells contain both reflecting material and melanosomes. As in the non-tapetal area, the epithelial cells of the tapetum display large amounts of smooth endoplasmic reticulum and numerous mitochondria. Bruch's membrane everywhere displays the usual pentalaminate structure described for most vertebrates. The choriocapillaris is also typical, in that numerous fenestrations are present in the endothelium bordering Bruch's membrane.     

Ocular pathology in the minimally depigmented subline of the vitiliginous Smyth chicken

Choroidal melanocytes and the retinal pigmented epithelium (RPE) were studied morphologically and histochemically in the Smyth chicken, an avian model for human vitiligo. The sequence of cytological events occurring in the ocular tissue of minimally depigmented Smyth birds was determined. Abnormalities of melanocytes and the associated inflammation was least severe in peripheral areas of the choroid and most pronounced in the back of the eye at the base of the optic nerve head. In the peripheral choroid, morphologically normal melanocytes and an occasional mononuclear leukocyte were observed. However, some of these morphologically normal melanocytes histochemically demonstrated atypical tyrosinase activity at the trans area of the Golgi apparatus. Toward the back of the eye, the melanocytes first appeared swollen and had retracting dendrites. Ultrastructurally these melanocytes demonstrated an increase in extramelanosomal cytoplasm. Later, melanocytes became spherical and had membrane bound, autophagosome-like compartments of pigment granules. As the melanocyte injury progressed, macrophages invaded the tissue and phagocytized melanocytic dendrites. These were followed by numerous plasma cells. Eventually, the back of the eye contained no pigment and was infiltrated with numerous mononuclear inflammatory cells. The retinal pigment epithelium also demonstrated a gradient in the degree of destruction, related to its topography. These cytological features consisted of the retraction of apical RPE processes, the disappearance of the basal plasma membrane infoldings, and the replacement of Bruch's membrane by collagen-like fibrils. These results demonstrate that the uveitis which develops in vitiligo appears to be a consequence of an inherent choroidal melanocyte defect.     

Directional protein secretion by the retinal pigment epithelium: roles in retinal health and the development of age‐related macular degeneration

The structural and functional integrity of the retinal pigment epithelium (RPE) is fundamental for maintaining the function of the neuroretina. These specialized cells form a polarized monolayer that acts as the retinal–blood barrier, separating two distinct environments with highly specialized functions: photoreceptors of the neuroretina at the apical side and Bruch's membrane/highly vascularized choriocapillaris at the basal side. The polarized nature of the RPE is essential for the health of these two regions, not only in nutrient and waste transport but also in the synthesis and directional secretion of proteins required in maintaining retinal homoeostasis and function. Although multiple malfunctions within the RPE cells have been associated with development of age‐related macular degeneration (AMD), the leading cause of legal blindness, clear causative processes have not yet been conclusively characterized at the molecular and cellular level. This article focuses on the involvement of directionally secreted RPE proteins in normal functioning of the retina and on the potential association of incorrect RPE protein secretion with development of AMD. Understanding the importance of RPE polarity and the correct secretion of essential structural and regulatory components emerge as critical factors for the development of novel therapeutic strategies targeting AMD.     

Basement membrane stimulates the polarized distribution of integrins but not the Na,K-ATPase in the retinal pigment epithelium.

The basement membrane stimulates the differentiation and polarity of simple transporting epithelia. We demonstrated for the retinal pigment epithelium (RPE) of chicken embryos that polarity develops gradually. Although the RPE and an immature basement membrane are established on embryonic day 4 (E4), the distribution of the Na,K-ATPase and a family of basement membrane receptors containing the beta 1 subunit of integrin is nonpolarized. The percentage of polarized cells increases gradually until cells in all regions of the epithelium are polarized on E11. During this time, the basement membrane increases in size and complexity to form Bruch's membrane. To study the ability of the basement membrane to stimulate the polarized distribution of the beta 1 integrins or the Na,K-ATPase, RPE was harvested from E7, E9, or E14 embryos and cultured on Bruch's membrane isolated (in association with the choroid) from E14 embryos. As a control, the RPE was plated on the side of the choroid lacking a Bruch's membrane. The distribution of the beta 1 integrins and the Na,K-ATPase was determined by indirect immunofluorescence. Bruch's membrane stimulated the polarized distribution of the beta 1 integrins regardless of the developmental age of the RPE even though E7 RPE is nonpolarized in vivo. To examine the role of individual matrix components, RPE was plated on matrix-coated filters. The polarized distribution of the beta 1 integrins was stimulated by laminin, collagen IV, and Matrigel but not by fibronectin. Interestingly, laminin and collagen IV are present in the basement membrane on E4 when RPE is not polarized in vivo. Under no circumstances was the distribution of the Na,K-ATPase polarized. These data indicate that the basement membrane influences the distribution of a subset of plasma membrane proteins but that other factors are required for full polarity.     

The fine structure of the retina in the Japanese Quail (Coturnix Coturnix Japonica). I. Pigment Epithelium and its Vascular Barrier

The uItrastructure oft he pigment epithelium and its vascular barrier was examined in the Japanese quail by electron microscopy. Most endothelial pores in the choriocapillaris appear bridged by double diaphragms. The pigment epithelium is characterized by numerous slender basal infoldings, myeloid bodies and phagosomes. Myeloid bodies communicate with the nuclear envelope and profiles of both the rough and smooth endoplasmic reticulum.Phagosome formation appears to be accomplished by concomitant activity of the rod itself (curling of apical lamellae) and the apical villi of the pigment epithelium. Within the pigment epithelium cytoplasm the phagosomes undergo degeneration and are associated with increased numbers of lysosomal-like granules.     

Regulated expression of apolipoprotein E by human retinal pigment epithelial cells

In early age-related macular degeneration (AMD), lipid-containing deposits (drusen) accumulate in Bruch's membrane underlying the retinal pigment epithelium (RPE). Recent studies indicate that apolipoprotein E (apoE) may play a role in lipid trafficking in AMD. Compared with the apoE3 allele, the apoE4 and apoE2 alleles are associated with decreased and increased risk for AMD, respectively; drusen contain high levels of apoE, and apoE null mice develop lipid deposits in Bruch's membrane similar to those observed in AMD. Primary cultures of human RPE cells expressing the apoE3 allele were grown on Transwell culture plates. Western blotting, ELISA assay, and mass spectrometry confirmed that apoE3 was secreted into the apical and basal chambers and that secretion was upregulated by thyroid hormone, 9-cis-retinoic acid, and 22(R)-hydroxycholesterol. In addition, basally secreted apoE associated with exogenously added HDL. These results indicate that apoE secretion can be regulated by specific hormones and that apoE associates with HDL. The findings are consistent with a role for apoE in lipid trafficking through Bruch's membrane and may be relevant to AMD.     

Vitronectin is a constituent of ocular drusen and the vitronectin gene is expressed in human retinal pigmented epithelial cells.

Age-related macular degeneration (AMD) leads to dysfunction and degeneration of retinal photoreceptor cells. This disease is characterized, in part, by the development of extracellular deposits called drusen. The presence of drusen is correlated with the development of AMD, although little is known about drusen composition or biogenesis. Drusen form within Bruch's membrane, a stratified extracellular matrix situated between the retinal pigmented epithelium and choriocapillaris. Because of this association, we sought to determine whether drusen contain known extracellular matrix constituents. Antibodies directed against a battery of extracellular matrix molecules were screened on drusen-containing sections from human donor eyes, including donors with clinically documented AMD. Antibodies directed against vitronectin, a plasma protein and extracellular matrix component, exhibit intense and consistent reactivity with drusen; antibodies to the conformationally distinct, heparin binding form of human vitronectin are similarly immunoreactive. No differences in vitronectin immunoreactivity between hard and soft drusen, or between macular and extramacular regions, have been observed. RT-PCR analyses revealed that vitronectin mRNA is expressed in the retinal pigmented epithelium (RPE)-choroidal complex and cultured RPE cells. These data document that vitronectin is a major constituent of human ocular drusen and that vitronectin mRNA is synthesized locally. Based on these data, we propose that vitronectin may participate in the pathogenesis of AMD.     

Electron Microscopy of the Tapetum Lucidum of the Cat

The fine structure of the tapetum of the cat eye has been investigated by electron microscopy. The tapetum is made up of modified choroidal cells, seen as polygonal plates grouped around penetrating blood vessels which terminate in the anastomosing capillary network of the choriocapillaris. The tapetal cells are rectangular in cross-section, set in regular brick-like rows, and attain a depth of some thirty-five cell layers in the central region. This number is gradually reduced peripherally, and is replaced at the margin of the tapetum by normal choroidal tissue. The individual cells are packed with long slender rods 0.1 µ by 4 to 5 µ. The rods are packed in groups and with their long axes oriented roughly parallel to the plane of the retinal surface. Each cell contains several such groups. Cells at the periphery or in the outer layers of the tapetum are frequently seen to contain both tapetal rods and melanin granules, the latter typical of the choroidal melanocytes. Also melanocyte granules may have intermediate shapes. These observations plus the similar density of the two inclusions lead to the belief that the tapetal rods may be melanin derivatives. A fibrous connective tissue layer lies between the tapetum and the retina. The subretinal capillary network, the choriocapillaris, rests on this layer and is covered by the basement membrane of the retinal epithelium. The cytoplasm of the retinal epithelium exhibits marked absorptive modifications where it comes in contact with the vessels of the choriocapillaris. This fibrous layer and the basement membrane of the retinal epithelium apparently comprise the structural elements of Bruch's membrane.     

Immunohistochemical localization of complement regulatory proteins in the human retina

Age-related macular degeneration (AMD) is a complex disease. Genetic studies have found strong associations between AMD and variants of several complement pathway-associated genes. The regulation of the complement cascade seems to be critical in the pathogenesis of AMD. In 45 human donor eyes immunohistochemistry was performed using antibodies directed against major regulators of the complement system: complement factor H (CFH), decay accelerating factor (DAF/CD55), complement receptor 1 (CR1/CD35), and membrane cofactor protein (MCP/CD46). All eyes were classified in AMD and controls. 11 eyes were graded as early AMD. 34 eyes were controls. In all eyes staining was found in intercapillary pillars of choroid adjacent to Bruch's membrane for CFH, at the basal surface of RPE cells for MCP, and at the apical side of the retinal pigment epithelium for CR1. DAF immunoreactivity was increased along the inner segments of rod and cone photoreceptor cells at the level of the external limiting membrane Labeling of soft drusen was found for CFH and CR1. In addition, DAF and CR1 showed staining of ganglion cells in all eyes. CFH and particularly MCP showed decreased or absent staining in eyes with early AMD adjacent to Bruch's membrane. The overlapping expression of regulators at the level of Bruch's membrane and the retinal pigment epithelium shows the importance of this site for control of the complement system. Decreased and therefore unbalanced expression of regulators, as shown in this study for CFH and MCP, may ultimately lead to AMD.     

Occurrence of unusual heterogeneous lipid-containing granule-storing cells in the main excretory duct epithelium of the male mouse submandibular gland

Summary The fine structure of the main excretory duct epithelium of the male mouse submandibular glands was investigated by scanning and transmission electron microscopy. Three principal cell-types were observed: type I and II, and basal cells. This epithelium was characterized by the presence of intercellular canaliculi. Type-I cells were the most numerous. They had an abundance of mitochondria, well-developed Golgi apparatus, a few electron-lucent lipid-containing granules and poorly developed basal infoldings. These cells were also characterized by many glycogen granules throughout the cytoplasm and abundant smooth endoplasmic reticulum in the apical cytoplasm. Type-II cells were the second most numerous. Their most characteristic feature was the presence of abundant heterogeneous lipid-containing granules having acid phosphatase activity at the periphery. They were concentrated in the infra- and supranuclear cytoplasm. The granules may be derived from mitochondrial transformation and seem to be a special kind of secondary autolysosome. Type-II cells also contained abundant mitochondria throughout the cytoplasm, much smooth endoplasmic reticulum in the apical cytoplasm, a well developed Golgi apparatus adjacent to the heterogeneous lipid-containing granules and no basal infoldings. Basal cells were situated adjacent to the basal lamina. They had a large nucleus and the cytoplasm was filled with glycogen granules.     

Fine structure of the transitional zone of the rat seminiferous tubule

An electron microscopic study was made on the structure of the testicular transitional zone (TZ) in the adult rat. The TZ proper consists of modified Sertoli cellss, with only a few spermatogonia and macrophages, surrounding distally a very narrow lumen. The TZ Sertoli cells have nuclei with a somewhat coarser matrix and more peripheral heterochromatin than Sertoli cell nuclei of the nearby seminiferous tubules, and the electron density of the cytoplasm varies from cell to cell. Smooth endoplasmic reticulum is abundant, but usually there are also scattered ribosomal rosettes and an occasional profile of rough endoplasmic reticulum. Microtubules are very numerous in the columnar portion of the cell, and laminar structures seemingly joining the cell surfaces are sometimes seen. Lipid droplets and lysosmal structures are frequent cellular components in proximal TZ Sertoli cells. Empty intracellular vacuoles are abundant, sometimes arranged around areas of smooth endoplasmic reticulum. Occasionally, membrane-limited fine granules and vacuoles are seen within Sertoli cells and also in the TZ lumen, suggesting a possible secretory activity by these cells. The apical processes of the Sertoli cells form large vacuolar structures, and in the basal parts of the epithelium vacuoles with capillary-like appearance are frequently seen. Phagocytosis of germinal cells by the Sertoli cells occurs in the proximal region of the TZ. Round waste bodies in contact with the Sertoli cell apices protruding into the tubulus rectus, are also common. The tunica propria of the TZ is thickened and somewhat wrinkled, and in the proximal region the myoid cell layer loses its continuity and is replaced by fibroblasts. The epithelium of the tubulus rectus adjacent to the TZ consists of several overlapping epithelial cells. The typical junctional complexes between TZ Sertoli cells appear to be impermeable to the lanthanum tracer.