Fine Structure of the Female Intoshia variabili(Alexandrov & Sljusarev) (Mesozoa: Orthonectida)

The morphology and fine structure of female Intoshia variabili, new combination for Rhopalura variabiliAlexandrov & Sljusarev, 1992, were studied with transmission electron microscopy. The body surface is covered with a 3-layered cuticula, under which is a layer of ciliated + non-ciliated cells arranged in alternating rings around the body. Ciliated cells have lateral extensions that intercalate with the non-ciliated cells. The kinetosome of each cilium has two longitudinally oriented cross-striated rootlets. The outer surface of the ciliated cells is covered with small tubercles, and the cytoplasm of these cells contains granules, vacuoles, mitochondria, fibrillar structures and lamellary bodies. A band of dense fibrils passes through the upper part of each ring of cells, going from one cell junction to another, encircling the entire body. Between the layer of ciliated + non–ciliated cells and the oocytes, elongated contractile cells from 4–5 longitudinal columns and 1 ring, the latter at the level of ciliated rings 7–9. The contractile cells contain thick and thin longitudinally oriented fibrils. The oocytes contain a large nucleus, numerous mitochondria, electron–dense granules and 1–2 spherical structures. An anteriorly situated, ciliated goblet–like receptor, not described for any other orthonectids, consists of three closely apposed cells, the upper part of which contains densely packed cilia. The genital pore opens through a non–ciliated cell and is surrounded by several cells with granules.     

THE FINE STRUCTURE OF EPENDYMA IN THE BRAIN OF THE RAT

The ciliated ependyma of the rat brain consists of a sheet of epithelial cells, the luminal surface of which is reflected over ciliary shafts and numerous evaginations of irregular dimensions. The relatively straight lateral portions of the plasmalemma of contiguous cells are fused at discrete sites to form five-layered junctions or zonulae occludentes which obliterate the intercellular space. These fusions occur usually at some distance below the free surface either independently or in continuity with a second intercellular junction, the zonula adhaerens. The luminal junction is usually formed by a zonula adhaerens or, occasionally, by a zonula occludens. The finely granular and filamentous cytoplasm contains supranuclear dense bodies, some of which are probably lysosomes and dense whorls of perinuclear filaments which send fascicles toward the lateral plasmalemma. The apical regions of the cytoplasm contain the basal body complexes of neighboring cilia. These complexes include a striated basal foot and short, non-striated rootlets emanating from the wall of each basal body. The rootlets end in a zone of granules about the proximal region of the basal body, adjacent to which may lie a striated mass of variable shape. All components of the basal body complex of adjacent cilia are independent of each other.     

Budding and Metamorphosis in Acineta tuberosa. An Electron Microscopic Study on Morphogenesis in Suctoria

SYNOPSIS. All stages (except conjugation) of the life cycle in Acineta tuberosa (Suctoria) were studied in the electron microscope. Budding starts with the invagination of a small pellicular area near the opening of the vacuole and bearing rows of barren basal bodies. Some of these basal bodies are incorporated into the swarmer-anlage and give rise to 8 rows of cilia, while the other basal bodies remain barren on the mother cell's side of the brood pouch. They will provide their own genetic continuity and later give rise to the swarmer of the next generation. Kineties are not spaced on the growing swarmer-anlage until their cilia are fully grown. Further details on the stalk-forming crganelle and general features of the development of the larvae are given.
Metamorphosis starts with the formation of basal disc and stalk. The stalk-forming material is released thru scopular pores into a tube-like mold formed by the scopula after having been pulled inward. Later, scopula pores spread from the scopular area proper and move upward. On their way to the apical part of the animal the cuticle is secreted. The differentiation of tentacles is described and new observations regarding a possible mode of haptocyst formation are given.
The results are discussed with respect to the morphogenetic potentialities of cortical structures in general.     

DEVELOPMENT OF MUCILAGINOUS SURFACES IN EUGLENOIDS. I. STALK MORPHOLOGY OF COLACIUM MUCRONATUM1

The envelope and stalk of Colacium mucronatum Bourr. & Chad, were examined in living cells with light microscopy and in fixed preparations with scanning electron microscopy using critically point dried (CPD) and freeze dried (FD) preparations. The envelope of palmelloid cells is formed over the entire cell surface by many individual strands attached at right angles to areas of articulation of the pellicular strips. Strands were observed to anastomose on the posterior tip of otherwise naked cells. Stalks of living cells in India ink preparations had an optically dark inner core with a lighter outer sheath. In FD stalks a definite inner core was not evident, whereas CPD stalks had an outer surface composed of thick strands which may be the collapsed and aggregated strands of the FD stalks. In both there was also an amorphous matrix. The stalk forms from the aggregation of many strands from the anterior cell tip back to a point encompassing the cell surface anterior to a cross section of the tip 9 μm diam. The outer surface of the stalk comes from the pellicular surface joining that area and the core from the cell tip in the area of the canal opening. Any possible participation of the inner canal surface in stalk formation could not be determined because of the great density of the mucilage at the cell-tip/stalk junction.     

Phosphorus distribution in perichromatin granules and surrounding nucleoplasm as visualized by electron spectroscopic imaging

Summary— The in situ distribution of phosphorus in perichromatin granules (PCGs), and in the surrounding nucleoplasm was investigated in rat liver cells by means of electron spectroscopic imaging of unstained preparations. A 2–3 nm fibril containing high concentration of phosphorus was found to be the main substructural feature of the PCGs revealed in the maps of phosphorus. This fibril is folded within the PCG with no apparent order. Fibrils of similar diameter and phosphorus content were also found in both the halo surrounding the PCG and dispersed in the nucleoplasm. Some of such fibrils are in continuity with those occurring within PCGs. Sometimes these fibrils are grouped forming a stalk connecting the PCG to chromatin. Some stalked PCGs are U-shaped or kidneyshaped, resembling Balbiani ring granules in the process of formation as observed in Chironomus salivary gland cell nuclei. The external fibrils are interpreted as perichromatin fibrils considered to be precursors of PCGs.     

THE CYTOLOGY OF THE NORMAL PARATHYROID GLANDS OF MAN AND VIRGINIA DEER : A Light and Electron Microscopic Study with Morphologic Evidence of Secretory Activity

The normal parathyroids of six humans and a Virginia deer were studied by light and electron microscopy. The parenchyma of the deer parathyroid is composed of uniform chief cells, which contained 100 to 400 mµ electron-opaque, membrane-limited granules, presumed to be secretory granules, in addition to the usual cytoplasmic organelles. Desmosomes are present between adjacent cells, and rare cilia are observed protruding from the chief cells into the intercellular space. The human parathyroids contain chief cells in two phases—active and inactive—as well as oxyphil cells. Active chief cells have a large Golgi apparatus, sparse glycogen, numerous secretory granules, and rare cilia. Inactive chief cells contain a small Golgi apparatus, abundant glycogen, and few secretory granules. Both forms have the usual cytoplasmic organelles and, between adjacent cells, desmosomes. Oxyphil cell cytoplasm is composed of tightly packed mitochondria and glycogen granules, with rare secretory granules. Cells with cytoplasmic characteristics intermediate between chief and oxyphil cells, possibly representing transitional cells, have been observed. Secretory granules of both man and deer are composed of 100 to 200 A particles and short rods, and the granules develop from prosecretory granules in the Golgi region of the cell. The human secretory granules are smaller and more variable in shape than those of the deer. The granules are iron and chrome alum hematoxylin-positive, argyrophilic, and aldehyde fuchsin-positive, permitting light microscopic identification. They are also found in the capillary endothelial cells of the parathyroid and in its surrounding connective tissue. The secretory granules of the parathyroid cells can thus be followed from their formation in the Golgi apparatus almost to their extrusion into the blood stream.     

Das Epithel der Tuba uterina des Neugeborenen Elektronenmikroskopische Befunde

Summary An electron microscopical study of the epithelium of the uterine tube was carried out in the newborn. Among the epithelial cells at least two morphologically well defined types can be distinguished: ciliated and non-ciliated cells.The ultrastructure of the cilia and related structures corresponds to what has been described by other authors in ciliated cells of various organs and of different species. Near the basal bodies of the cilia there is a concentration of vesicular mitochondria, which is thought to be evidence of a high metabolic activity in this region of the cell. Large opaque granules in the supranuclear zone of the ciliated cells are, it is suggested, paraplasmatic inclusions, perhaps supporting material for the ciliokinetic processes. There was no evidence of a secretory function of the ciliated cells.Among the non-ciliated cells, which in general show a straight lined luminal border with few microvilli, there are some cells containing dense granules, which are distributed throughout the cytoplasm and concentrated in the luminar side of the cell. The apical parts of these cells are protruding and sometimes digitated or branched; they contain accumulated granular materials and are separated from the rest of the cell after the formation of an intracellular plasmalemma. A similar detachment was found in an other cell type, but here the protruded apical parts of the cells are edematous and do not contain any visible secretory materials. It is uncertain if the detached cytoplasmic substances form a part of a specific secretory product; there are no secretory granules within the cytoplasm. On the contrary, the detachment of cytoplasmic parts may only accompany the excessive proliferation of cells which takes place during this period of growth.     

Surface fine structure of the subfornical organ in the Japanese quail,Coturnix coturnix japonica

The surface ultrastructure of the subfornical organ (SFO) was investigated in the Japanese quail. The SFO consists of a body and a stalk. The body of the SFO can be divided into rostral and caudal parts. On the rostral part, each ependymal cell possesses a short central solitary cilium; clustered cilia are also occasionally seen. Microvilli are abundant. On the caudal part, cells with a solitary cilium are fewer in number, and clustered cilia are rarely found. Microvilli are not as abundant as on the rostral part. In addition, large bulbous protrusions, tufts of small protrusions, deep funnel-shaped hollows, small pinocytotic invaginations and possible cerebrospinal fluid-contacting axons are sporadically observed on the surface of various regions of the body. Each ependymal cell of the stalk has a wide apical surface. A central solitary cilium, microvilli and other structures are observed more rarely on the stalk than on the body, while clustered cilia are not seen on the stalk. These structures are compared with those of the mammalian SFO and further discussed in relation to the possible dipsogenic receptor function for angiotensin II.     

Morphology of chemosensory organs required for feeding in the leech Hirudo medicinalis

Sensilla that line the upper edge of the lip in the leech Hirudo medicinalis and that contain chemoreceptors required for feeding were examined in the scanning and transmission electron microscopes. The sensilla include two size-classes of ciliated button-like mounds--one about 35 microns in diameter and another about 10 microns in diameter. The larger sensilla are at the center of unpigmented patches of skin which are visible in the light microscope, while the smaller sensilla have not been previously described as distinct structures. Electron microscopy, though not light microscopy, shows that the lip sensilla differ markedly from the segmental sensilla of the leech, which have been shown to mediate mechanoreception and photoreception. In particular, the chemosensory lip sensilla contain multiciliated cells with cilia of a uniform length, whereas the segmental sensilla contain uniciliated cells with long, whip-like cilia, as well as multiciliated cells with short, stiff cilia. Thus, the two types of sensilla differ morphologically as well as functionally. In addition to the ciliated sensilla along the upper lip, structures consisting of a short, club-like process surrounded by granular material were observed inside the mouth. These structures may also be chemosensory organs.     

An ultrastructural study of byssus stem formation in Mytilus californianus

In Mytilus californianus, root lamellae of the byssus stem are formed by two morphologically distinct exocrine cell types. Type 1 cells contain large ellipsoid granules which are ultrastructurally identical to those of the collagen gland associated with byssus thread formation: these granules are secreted only at the base of the stem generator. Type 2 cells contain small cylindroid granules which are secreted only from the lateral surfaces of generator septa. The resultant matrix is biphasic because the two secretions are incompletely mixed. Lamellar sheets of matrix are propelled outward by the action of cilia and are molded into a cylinder at the neck region of the stem. However, the stem retains a lamellar pattern. Byssus threads are attached to the stem by flattened rings formed from thread material which is secreted into the cervical crevice surrounding the neck. The microanatomy of the stem forming region is described and a new term, “stem generator,” is proposed for this organ.     

Histochemical and ultrastructural features of the epidermis of the land planarian Bipalium adventitium

The epidermis of the land planarian Bipalium adventitium was examined by light and electron microscopy. In all regions, the epidermis consists of a simple columnar ciliated epithelium associated with a prominent basement membrane. The epithelial cells, possessing abundant microvilli and poorly developed terminal webs, are conjoined laterally at their apical ends by septate junctions. The epidermis of the creeping sole is distinguished from that of adjoining regions by a “insunken” condition of the epithelial cells, a greater number of cilia per cell, and an absence of glandular secretions other than mucus. The insunken cells of the sole possess large glycogen disposits and attributes of metabolically active cells. Unusual intranuclear inclusions of unknown significance are also found in many of the epidermal cells in all regions. The basement membrane lacks distinct layering and consists of fine fibrils displaying a beaded appearance but no obvious cross-banding. Histochemical tests indicate that the fibrils are collagenous. In addition to mucus, secretory material found in nonsole regions includes lamellated granules and rhabdites, both stained intensely by acidic dyes. Rhabdites and the basement membrane also contain disulfide-enriched proteins. In scanning electron micrographs, the sole appears as a faint, longitudinally oriented band extending along the entire length of the animal. In all regions except the sensory border of the head, the microvilli are generally obscured by the densely arranged cilia. The sensory border consists of a row of toothlike papillae and grooves covered almost exclusively by microvilli, small club-shaped structures, and larger spherical protrusions.     

Collagen biogenesis and assembly into fibrils as shown by ultrastructural and 3H-proline radioautographic studies on the fibroblasts of the rat food pad

To examine whether collagen is assembled into fibrils within or outside fibroblasts, the connective tissue of the rat foot pad was investigated by electron microscopy and by radioautography at times varying from 4 min to 3 days after an intravenous injection of 3H-proline. The fibroblasts of the rat food pad are long polarized cells with the nucleus at one end, the Golgi apparatus in the center, and a region with long processes at the other end. This region contains secretory granules and is considered to be the secretory pole of the cell. In the Golgi apparatus the stacks of saccules are separated from rough endoplasmic reticulum (rER) by groups of "intermediate vesicles" including similarly structured tubules which may be over 300 nm long and are referred to as "intermediate tubules." The Golgi saccules exhibit distended portions which differ at the various levels of the stack. On the cis side, the distentions tend to be spherical and contain fine looping threads; in the middle of the stack, they are cylindrical and present distinct straight threads; whereas on the trans side, they are again cylindrical, but the straight threads are grouped in parallel aggregates. Between these cylindrical distentions and the secretory granules, there are transitional forms within which thread aggregates are packaged more and more tightly. Finally, the fibroblasts are associated with two types of collagen fibrils: extracellular ones arranged into large groups between the cells and intracellular ones located within long intracytoplasmic channels. Quantitative radioautography after 3H-proline injection reveals that the number of silver grains per unit area reaches a peak over the rER at 4-10 min, Golgi apparatus at 40 min, secretory granules at 60 min, and extracellular collagen fibrils at 3 h. At no time are intracellular collagen fibrils labeled. Qualitative observations further indicate that spherical Golgi distentions are mainly labeled at 40 min, and cylindrical distentions, at 60 min. In addition, from 20 min to 3 hr, some lysosomal elements are labeled. The biogenetic pathway leading to the formation of collagen fibrils is interpreted as follows.(ABSTRACT TRUNCATED AT 400 WORDS)     

Ultrastructure of the median eminence of the rat

Summary The ependymal cells bordering the median eminence to the third ventricle are characterised by many microvillus-like projections and bulbous cell processes of the luminal plasma membrane. The latter contain many vesicles 500–1,000 Å in diameter. Cilia with 9+2 fibrillar pattern are seen occasionally. Adhesive devices in the from of zonula adhaerens and zonula occludens are found in the apical part of the intercellular junction. Unmyelinated nerve fibres with a mean diameter of 1 and containing many electron dense granules of 830–1,330 Å are often seen between the ependymal cells.Two types of glial cells are found in the median eminence. One is characterised by a nucleus with dense blods of chromatin and dense cytoplasm, and it is associated chiefly with the nerve fibres in the region of the hypothalamo-hypophysial tract. The other type of glial cell is characterised by fine, uniformly distributed chromatin in the nucleus and a relatively pale cytoplasm and branched processes which terminate perivascularly in the base of the median eminence.Myelinated nerve fibres are seen only in the region of the hypothalamo-hypophysial tract. Only a part of them contain electron dense granules 1,330–2,330 Å in diameter.Three types of unmyelinated nerve fibres can be distinguished in the median eminence according to the size of the electron dense granules they contain: 1. Nerve fibres containing granules 1,330–2,330 Å in diameter. They are seen primarily in the hypothalamo-hypophysial tract, but also in the zona externa; 2. those containing granules with a mean diameter of 1,330 Å; and 3. those containing granules with a mean diameter of 1,000 Å. The last two types are both encountered in the hypothalamo-hypophysial tract, the zona externa and the perivascular region of the base of the median eminence. Under high magnification, the membrane of the granules show evidence of a trilaminar structure and the content of the granules with a low electron density appeares to consist of small microvesicles or globular components. Besides granules, these nerve fibres contain vesicles mostly 420 Å in diameter whose relative number increases towards the perivascular nerve endings. 53 per cent of the inclusions in the hypothalamo-hypophysial tract are granules and 47 per cent vesicles, while the corresponding percentages for the zona externa are 40 and 60 and for the perivascular nerve endings 20 and 80.The mean width of the pericapillary space is 1 , but it varies greatly. It containes many collagen fibrils and fibroblasts. The capillary endothelium is frequently fenestrated and contains many vesicles of various sizes.Two types of granules-containing cells are found in the pars tuberalis depending on the size of the electron dense granules: 1. cells containing granules with a mean diameter of 1,330 Å: and 2. cells containing granules with a mean diameter of 2,000 Å. In addition, there are occasional follicular cavities filled with amorphous material, microvilli and cilia of 9+2 fibrillar pattern.Aided by a grant from the Sigrid Jusélius Stifteise.     

Etude histologique,histochimique et ultrastructurale de la pars intercerebralis chez Locusta migratoria L. (Orthoptere)

Summary A histological, histochemical and ultrastrucutral study of the pars intercerebralis (PI) has been made in Locusta migratoria. The acellular neural lamella is made up of an elastic tissue and collagen fibrils. The cells of the perilemma contain numerous lysosome structures and lipid granules.Three different types of neurosecretory cells (NSC A, B and C) have been distinguished in the PI associated with giant neurons.The cells termed A and B seem not to have an activity cycle during the two last larval instars. At the moment of sexual maturity the NSC A show an important accumulation of neurosecretory material and their number increases at the expense of the NSC B. The NSC A, which are characterized by a highly developped endoplasmic reticulum, contain numerous secretory granules which appear to be individualized in the Golgi complex in three different ways. The NSC B, with a reduced endoplasmic reticulum and an almost quiescent Golgi complex, contain abundant lysosome structures and more seldom some neurosecretory granules. In fact, the study of the fine structure shows different intermediate types, linking in a continuous way typical A cells and typical B cells. NSC A and NSC B might correspond to two opposed stages of secretory activity of one single cell type: the A cell representing the activity stage and the B cell the quiescent stage.NSC C show an accumulation of their neurosecretory products in relation to metamorphosis and sexual maturity. Ultrastructural evidence confirms their neurosecretory activity.A mode of regulating neurosecretion in NSC A and B by internal catabolism of the secretion and formation of lysosome like structures is discussed in the present paper.The giant neurons, which are surrounded by a glial envelope (trophospongium), contain several dense granules originated from Golgi complex.     

Actin pegs and ultrastructure of presumed sensory receptors of Beroë (Ctenophora)

Summary We have investigated the actin content and ultrastructure of two kinds of presumed sensory projections on the lip epidermis of beroid ctenophores. Transmission electron microscopy showed that conical pegs contain a large bundle of densely packed, parallel microfilaments. Rhodamine-phalloidin brightly stained the pegs, confirming that they contain filamentous actin. Epidermal cells with actin pegs also bear a single long cilium with an onion-root structure, previously described as arising from a different type of cell. The actin peg and onion-root cilium project side-by-side, defining a polarized axis of the cell which is shared by neighboring cells. The onion-root body is surrounded by a flattened membranes sac which lies immediately below the plasma membrane. The perimeter of the membrane sac is encircled by aggregates of dense material. An extra layer of dense material is found along the side of the membrane sac facing the peg; this material often makes direct contact with the adjacent actin filament bundle. Cells with actin pegs and onion-root cilia synapse onto adjacent neurites and secretory gland cells, indicating that one or both types of projections are sensory elements. Since the feeding responses of beroids are reported to depend on chemical and tactile stimuli to the lips, the cells bearing pegs and cilia may function as both mechanoreceptors and chemoreceptors, that is, as double sensory receptors.     

Electron microscopic studies on the pineal organ of the antarctic penguin, (Pygoscelis papua)

The pineal organ of the migratory antarctic penguin, Pygoscelis papua, has a lobular structure. Clusters formed by different types of parenchymal cells are separated by connective tissue septa containing blood vessels. The predominant cell type displays a well-developed Golgi complex, free ribosomes, clear and granular vesicles (secretory granules), and lysosomes. Other cell types found in the gland are supporting and ependymal-like cells. The former contain dense bodies and filament bundles, the latter possess abundant cilia and clusters of ribosomes. Typical photoreceptor elements are lacking. Blood vessels are located within a perivascular space bordered by basal laminae. This perivascular space extends between the basal protrusions of the parenchymal cells. The presence of pinocytotic vesicles, secretory granules and cytoplasmic processes in the vicinity of these spaces suggests active sites of transport and exchange of substances. Intercellular conaliculi-like spaces are surrounded by parenchymal cells rich in microvilli. These cancliculi are continuous with the cavities (invaginations) of secretory and other parenchymal cells.     

Glycogen degradation during migration of presumptive cell types in Dictyostelium discoideum.

During the time course of differentiation in Dictyostelium discoideum, glycogen was found to accumulate from the amoebae stage to the culmination stage of development. Upon sorocarp formation (23 h), glycogen was rapidly degraded. Ultramicrotechniques, utilizing amplification of glycogen by enzymatic cycling, were used to follow glycogen metabolism in pre-stalk and prespore cells during the differentiation cycle. Both cell types accumulated glycogen at nearly the same rate. By the pseudoplasmodium stage of development glycogen had accumulated to 50% of its maximum value, and no differences were found between pre-stalk and pre-spore cells. Glycogen was degraded as pre-stalk cells migrated into the position for stalk construction. At the culmination stage of development stalk cells near the base were devoid of glycogen while pre-stalk cells near the apex of the stalk showed no loss of glycogen. The complete loss of glycogen from stalk cells occurred over a distance occupied by approximately 100 cells, and over a time period of approx. 1 h. Pre-spore cells at the culmination stage showed no loss of glycogen even though separated from stalk cells by only a thin cellulose sheath. The degradation of prespore cell glycogen did not commence until stalk construction was completed and the pre-spore mass had reached the apex of the stalk. Pre-spore cells at the culmination stage contained high levels of glycogen while only 2 h later, total degradation had occurred.     

Cell surface patterning and morphogenesis: biogenesis of a periodic surface array during Caulobacter development

Shape changes, extended processes, and other surface elaborations are associated with cellular differentiation, and the cell membranes involved with these developmental changes often are reshaped without a major alteration in biochemical composition. Caulobacter crescentus produces a hexagonally-packed periodic surface layer that covers the entire cell and further, mimics some of the membrane-mediated changes of higher organisms by forming a membranous stalk during its distinctive life cycle. Growth of the surface layer was examined during the cell cycle by treating synchronously growing cells with surface layer antibody, continuing growth, and then labeling for electron microscopy with a protein A-colloidal gold conjugate. Three regions of distinctive surface array biogenesis were resolved. The periodic surface layer on the main cell body was enlarged by insertion of new material at numerous uniformly distributed points. In contrast, the surface layer on the stalk appeared as entirely new synthesis. In examining growth of the stalk in subsequent generations, we noted that growth of stalk surface persisted at the stalk-cell body junction. The region of cell division also showed a pattern of entirely new surface layer production at late stages in division, similar to the stalk. The immunocytological method also facilitated a careful examination of stalk initiation and growth. Although initiation was under precise temporal and spatial regulation, the rate of stalk elongation was variable from cell to cell and apparently no longer under cell cycle control. The similarity of surface layer biogenesis on the stalk and the site of cell division may be a significant reflection of other events occurring at the cell pole. A model suggested by this and other studies that can account for the temporal pattern of polar morphogenesis is discussed, as is the potential relationship between the geometrically ordered surface array and the formation or maintenance of the stalk.     

In situ hybridization and immuno-electron microscope analyses of the Us11 gene of herpes simplex virus type 1 during transient expression

The distribution of Us11 RNA and of its encoded protein have been investigated at the ultrastructural level in HeLa cells transiently expressing the Us11 gene of herpes simplex virus type 1. In these transfected cells, Us11 protein accumulates at sites identical to those of lytically infected cells, i.e., in nucleoli and in regions of the cytoplasm that contain ribosomes. Us11 RNA and polyadenylated RNA are scattered over the ribosomerich areas of the cytoplasm. They also accumulate in the nucleoplasm on clustered ribonucleoprotein (RNP) fibrils but also in clusters of interchromatin granules, some of them contiguous to nucleoli. However they are never found in nucleoli. These data reveal the involvement of interchromatin granules in some steps of Us11 mRNA maturation and/or transport.