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

Summary 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 m 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.     

Ultrastructure of the chromatophores of Palaemon affinis Heilprin (crustacea: decapoda). The structural basis of pigment migration

The relationships between pigment granules and the prominent smooth endoplasmic reticulum in the chromatophores of the shrimp, Palaemon affinis Heilprin, were investigated by transmission electron microscopy. Different types of pigment granules within the chromatophores were found to exhibit a close structural continuity with the cisternal membranes. The membranes of membrane-bound pigment granules were seen to be continuous with those of the ER cisternae, while pigment granules lacking membranes appear to adhere to the external cisternal surfaces. The reticulum, which seems to form a network enmeshing the pigment granules, is proposed to be part of a continuum linking these granules with their translocating force.     

THE STRUCTURE AND FORMATION OF PROTEIN GRANULES IN THE FAT BODY OF AN INSECT

In the larva of the butterfly Calpodes ethlius, the fat body begins to store protein in the form of granules at about 30 to 35 hours before pupation, at a time when the endocuticle is being resorbed. At least two sorts of granule can be distinguished. The first granules to arise are those within vesicles of the Golgi complex. These may increase in size by incorporating material from microvesicles at their surface and by coalescence with one another. Later, at about 10 hours before pupation, another sort of granule arises by the isolation of regions of the endoplasmic reticulum (ER) within paired membranes derived from Golgi vesicles. Several of these ER isolation bodies coalesce, with fusion of their outer isolating membranes. The ribosomes and membranes may then disappear and the granules become indistinguishable from the protein granules formed from Golgi vesicles, or the ribosomes may remain and be embedded in dense crystalline protein, forming a storage body for both protein and RNA. Mitochondria are isolated within paired membranes in the same way as regions of the ER. The isolated mitochondria also coalesce in a similar manner. When the inner membranes are lost, the structure of a group of isolation bodies is indistinguishable from that of a cytolysome. Isolation within paired membranes, as described here, may be of general importance in segregating regions of massive lysis or massive sequestration.     

Fine Structure of the Retinal Pigment Epithelium of the River Lamprey (Lampetra fluviatilis,Cyclostomi)

The retinal pigment epithelium of Lampetra fluviatilis was studied by electron microscopy. The epithelial cells differ in many details from those of gnathostomes. The lateral cell membranes are difficult to distinguish. The smooth endoplasmic reticulum is well developed; in some animals undulated membrane complexes, comprising systems of tightly fused double membrane plates, are related to the endoplasmic reticulum. Myeloid bodies are common and well developed, but pigment granules are comparatively sparse. The intercellular space between pigment epithelium and photoreceptors is rather wide. There are only a few inclusion bodies with membranous contents. The importance of the pigment epithelium in the retinal metabolic exchange is discussed in view of the fine structure of the cells. Compared with that of hagfishes, the lamprey retina is well developed. However, any comparison must be made against the background of a diphyletic development of the two groups.     

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.     

Cytomembranes in first cleavage xenopus embryos

Summary The ultrastructure and interrelationships of the Golgi body, endoplasmic reticulum and lipid droplets have been studied in the first cleavage Xenopus embryos. Lipid droplets, usually spherical or sometimes multilobed, did not have a discernible limiting membrane, although some had an incomplete electron dense partition. The Golgi bodies and endoplasmic reticulum were seen continuous with lipid droplets and the profiles indicated a probable formation of these membranes from lipid droplet material. Rough endoplasmic reticulum (ER) mainly consisted of paired tubular cisternae and vesicles containing filamentous material that gave a fringed appearance. The relationships of paired cisternae with the Golgi body suggested a transformation of ER membranes into the Golgi body membranes. In addition, paired ER cisternae showed a close apposition with the limiting membrane of the yolk platelet. Lone ER cisternae that contained moderately electron dense material instead of filaments were also present and showed numerous associated vesicles near the Golgi body. The Golgi body showed several morphological forms including a single fenestrated cisterna, two to four flat or cup-shaped cisternae, or up to seven cisternae, some of which were dilated and similar to fringed ER in appearance. These forms could be different developmental stages of the organelle. Coated vesicles were seen continuous with the cisternae of the Golgi body. A probable route for the assembly of the cell surface material has been proposed.This work was supported by a grant from the Medical Research Council of Canada to one of us (E.J.S.).     

Direct Visualization of a Transmembrane Channel Associated with Ribosomes

Examination of directly frozen rough endoplasmic reticulum (ER) of retinal pigment epithelial cells by freeze-fracture and freeze-substitution revealed distinct paired transmembrane proteins associated with membrane ribosomes. Ribosomal subunits on intact ER membrane are directly visualized for the first time, providing a global view of the structure of the ribosome and the corresponding structures on the ER membrane. The ribosomal intersubunit cleft appears to be continuous with a cleft between paired transmembrane proteins that extends into the lumen of the ER. This continuous cleft may be the path taken by nascent polypeptides.     

The fine structure of the pigment epithelium and photoreceptor cells of the newt,Triturus viridescens dorsalis (Rafinesque)

The morphology of the retinal pigment epithelium and photoreceptor cells has been studied in the common newt Triturus viridescens dorsalis by light, conventional transmission and scanning electron microscopy. The pigment epithelium is formed by a single layer of low rectangular cells, separated by a multilayered membrane (Bruch's membrane) from the vessels of the choriocapillaris. The scleral border of the pigment epithelium is highly infolded and each epithelial cell contains smooth endoplasmic reticulum, myeloid bodies, mitochondria, lysosomes, phagosomes and an oval nucleus. Inner, pigment laden, epithelial processes surround the photoreceptor outer and inner segments. The three retinal photoreceptor types, rods, single cones and double cones, differ in both external and internal appearance. The newt, rod, outer segments appear denser than the cones in both light and electron micrographs, due to a greater number of rod lamellae per unit distance of outer segment and to the presence of electron dense intralamellar bands. The rod outer segments possess deep incisures in the lamellae while the cone lamellae lack incisures. Both rod and cone outer segments are supported by a peripheral array of dendritic processes containing longitudinal filaments which originate in the inner segment. The inner segment mitochondria, forming the rod ellipsoid, arelong and narrow while those in the cone are spherical to oval in shape. The inner segments of all three receptor cell types also contain a glycogen-filled paraboloid and a myoid region, just outside the nucleus, rich in both rough and smooth endoplasmic reticulum. The elongate, cylindrical nuclei differ in density. The rod nuclei are denser than those of the cones, contain clumped chromatin and usually extend further vitreally. Similarly, the cytoplasm of the rod synaptic terminal is denser than its cone counterpart and contains synaptic vesicles almost twice as large as those of the cones. Photoreceptor synapses in rods and cones are established by both superficial and invaginated contacts with bipolar or horizontal cells.     

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.     

Unusual intracytoplasmic lamellar bodies in a malignant gonadal stromal tumor

Characteristic intracytoplasmic lamellar bodies were found in a malignant gonadal stromal tumor. These bodies consisted of the stacks of up to 200 tubular cisternae arranged in parallel. Each cisterna had a circular section in tangential view and a diameter of about 85 nm. The cisternae on the outermost side of these lamellar bodies tended to be dilated and adorned with ribosomes. The ends of cisternae were often contiguous with rough-surfaced endoplasmic reticulum. The latter feature is also seen in annulate lamellae, but periodically spaced annuli or discontinuities characteristic of annulate lamellae were never observed. Furthermore, fine ribosomal granules resembling a rosary were recognizable along the whole circumference of the outer surface of each cisterna. The unique structure we describe is a cytoplasmic organelle which, like annulate lamellae, is closely associated with the endoplasmic reticulum and is presumed to be related to the genesis of rough-surfaced endoplasmic reticulum in tumor cells.     

Fine structure of the retinal pigment epithelium of the mallard duck (Anas platyrhynchos)

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 investigated by light and electron microscopy in the mallard (Anas platyrhynchos). In this species the RPE consists of a single layer of cuboidal cells which display numerous very deep basal (scleral) infoldings and extensive apical (vitreal) processes which enclose photoreceptor outer segments. The RPE cells are joined laterally by prominent basally-located tight junctions. Internally smooth endoplasmic reticulum is the most abundant cell organelle with only small amounts of rough endoplasmic reticulum present. Polysomes are abundant as are basally-located mitochondria which often displayed a ring-shaped profile. The cell nucleus is large and vesicular. Melanosomes are plentiful only within the apical processes of the RPE cells in the light-adapted state. Myeloid bodies are large and numerous and very often have ribosomes on their outer surface. Bruch's membrane (complexus basalis) shows a pentalaminate structure but with only a poorly represented central elastic lamina. Profiles of the choriocapillaris are relatively small and the endothelium of these capillaries while extremely thin facing the retinal epithelium is but minimally fenestrated.     

The fine structure of the adipose cell of the leech Glossiphonia complanata

The two distinct types of cytoplasm seen with the light microscope in the adipose cell of the leech Glossiphonia complanata have been identified in the electron microscope image of this cell. One of these, the basophil cytoplasm, contains many well oriented, paired membranes which are much more clearly evident when calcium ions are added to the fixative. The membranes sometimes appear as concentric arrays of lamellae and are thought to represent sections through a phospholipide-containing body. The paired membranes and the concentric lamellae have granules attached to them and resemble in size and structure the membranes of the endoplasmic reticulum encountered in many mammalian cells. Small dense cytoplasmic particles are present throughout the cell; they may be ferritin molecules, derived from the breakdown of haemoglobin taken in as food. On the basis of a previous histochemical study and the present electron microscope investigation, it is suggested that these paired membranes are similar to the organized type of mammalian ER and the results seem to confirm the belief that these membranes are composed of layers of phospholipoprotein together with attached particles of ribonucleoprotein.     

The Fine Structure of the Adipose Cell of the Leech Glossiphonia complanata

The two distinct types of cytoplasm seen with the light microscope in the adipose cell of the leech Glossiphonia complanata have been identified in the electron microscope image of this cell. One of these, the basophil cytoplasm, contains many well oriented, paired membranes which are much more clearly evident when calcium ions are added to the fixative. The membranes sometimes appear as concentric arrays of lamellae and are thought to represent sections through a phospholipide-containing body. The paired membranes and the concentric lamellae have granules attached to them and resemble in size and structure the membranes of the endoplasmic reticulum encountered in many mammalian cells. Small dense cytoplasmic particles are present throughout the cell; they may be ferritin molecules, derived from the breakdown of haemoglobin taken in as food. On the basis of a previous histochemical study and the present electron microscope investigation, it is suggested that these paired membranes are similar to the organized type of mammalian ER and the results seem to confirm the belief that these membranes are composed of layers of phospholipoprotein together with attached particles of ribonucleoprotein.     

Ultrastructural observations on anther tapetum development of freeze-fixed Ledebouria socialis Roth (Hyacinthaceae)

The tapetal ultrastructure of high-pressure-frozen, freeze-substituted Ledebouria socialis Roth (Hyacinthaceae) is described from the tetrad stage up to microspore mitosis. Cytoplasmic degeneration of the tapeturn occurs after microspore mitosis. During the tetrad stage and the early free-microspore stage the tapetum cells appear to be meristematic; after callose dissolution they show an intense exocytosis of polysaccharides into the anther locule. Later, the tapetum cells are characterized by abundant endoplasmic reticulum (ER). Highly osmiophilic pollenkitt precursor substances accumulate within distinct, partly irregular shaped cytoplasmic domains (“osmiophilic bodies”), which are intimately associated with the ER. It remains to be verified whether or not these bodies are derived from the ER. Because of their preservation and staining patterns the contents of these bodies are tentatively interpreted as flavonoids, one of the main pollenkitt pigments in angiosperms. Apart from these pigment bodies, there exist four other kinds of lipophilic inclusion within the anther (cells). The general aspects of lipid preservation in freeze-substituted samples are discussed. Staining with hot alcoholic phosphotungstic acid yielded good contrast of the ER and other membranes, which are often difficult to visualize in freeze-substituted, resin-embedded samples.     

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 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.     

Myeloid bodies in the pigment epithelium of a teleost embryo, the viviparous Poecilia reticulata

Appearance of myeloid bodies (MB) in the retinal pigment epithelium (RPE) precedes photoreceptor outer segment development in Poecilia reticulata embryos reared under a 12 hrs LD cycle, in constant darkness (DD) and constant light (LL). When first formed, MB are predominantly continuous with rough endoplasmic reticulum (RER). The same is observed in the peripheral growth zone of the developed eye, whereas in differentiated parts, MB are continuous with the smooth endoplasmic reticulum (SER). At onset of photomechanical movements, wavy MB predominate in light-adapted LD embryos, are exclusively present in LL and are located in the RPE processes. SER abounds. Straight MB predominate in dark-adapted LD embryos, are exclusively present in DD and contain electrondense material between lamellae. Diurnal appearance of electrondense material may be coupled with transfer of retinol, mediated by various transport proteins.     

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.     

Pigment biogenesis in freshwater shrimp ventral nerve chord chromatophores

Summary The possible biogenesis of two pigment granule types present in the monochromatic, brown chromatosomes enveloping the ventral nerve chord of the freshwater palaemonid shrimps Macrobrachium acanthurus, M. heterochirus and M. olfersii is examined by transmission electron microscopy in thin section and freeze fracture replicas. Prominent, membrane limited granules are suggested to have their origin in a complex, juxtanuclear, smooth endoplasmic reticulum labyrinth, continuous with the nuclear envelope. Amembranous, lipocarotenoid granules possibly derive from the external surface of the smooth endoplasmic reticulum. Nuclear envelope and SER membranes contain numerous 11 nm diameter intramembranous particles while pigment granule membranes exhibit fewer particles. A dictyosomal origin for the lipocarotenoid granules is discounted. Granulogenesis is suggested to be a continuous process in crustacean chromatophores.     

The parathyroid gland of the woodchuck (Marmota monax): a study of seasonal variations in the chief cells

The ultrastructure of the parathyroid chief cell in the woodchuck, Marmota monax, was studied during the four seasons of the year. Spring chief cells have stacks of granular endoplasmic reticulum, prominent multiple Golgi zones and many clumped mitochondria. Summer cells resemble those seen in the spring but the mitochondria are associated with stacks of granular endoplasmic reticulum. Multiple areas of stacked granular endoplasmic reticulum characterize the fall chief cells. Their Golgi zones are large and are associated with many dense core secretory granules. Lipoid vacuoles are frequently noted. Winter chief cells have secretory granules and phagolysosomes (dense bodies). Some of these cells contain stacked arrays of granular endoplasmic reticulum associated with mitochondria, others have only short segments. The above morphological findings are discussed in relation to those in other hibernators, the parafollicular (C) cell, and to the cyclic seasonal activities of the woodchuck.