Tubular invaginations with caveolae and coated pits in the sinus endothelial cells of the rat spleen

The fine structure of plasmalemmal tubular invaginations with caveolae and coated pits in the sinus endothelial cells of the rat spleen has been demonstrated by scanning and transmission electron microscopy. In addition, the three-dimensional structure of the tubular invagination has been revealed by computer-aided reconstruction. The tubular invaginations of the plasma membrane plunged into the cytoplasm everywhere from the apical, lateral, and basal surfaces of the plasma membrane. The invaginations were tubular and branched away, and their plasma membranes were reinvaginated to form numerous caveolae and occasional coated pits. Numerous caveolae were found in clusters that looked similar to a bunch of grapes and the coated pits were present at the base of the clusters. The caveolae and coated pits derived from the tubular invaginations were almost ultrastructurally identical to those derived from the surface plasma membrane. From examination of the fractured surfaces of the endothelial cells treated with the aldehyde prefix osmium-dimethyl sulfoxide-osmium method and of ultrathin sections of those infiltrated by lanthanum nitrate, the tubular invaginations were found to not penetrate any endothelial cells. A computer-aided reconstruction revealed that the caveolae derived from the tubular invaginations were in close apposition to the surface-connected canaliculi. The reaction product of Concanavalin A conjugated to horseradish peroxidase was present on the outer leaflet of the membranes of the coated pits and coated vesicles and also in the contents of the endosomes, but it was absent from any caveolae. Based on our observations, the functional significance of the tubular invaginations in sinus endothelial cells is discussed. Accepted: 13 September 1999     

The ecdysial gland of the spider crab,Libinia emarginata (L). I. Ultrastructure of the gland in the male

The ecdysial glands of mature male Libinia emarginata are pale, yellowish organs composed of lobes of epithelial cells having oval nuclei which are often eccentric and which have one or two nucleoli containing amorphous granular material and coarse strands. The plasma membrane bordering the basal lamina consists of invaginations containing microtubules which may serve to increase the surface area for metabolic exchange. Masses of smooth endoplasmic reticulum and associated vesicles are scattered throughout the cytoplasm. Two or more vacuoles may coalesce. Larger vesicles lie close to the cell surface. Numerous mitochondria with tubular cristae surround the nucleus and frequently are associated with SER. A few Golgi complexes consisting of flattened sacs, cisternae or vesicles, lipid droplets and free ribosomes were seen. Adjacent plasma membranes may be in close apposition or separated by a space filled with vesicles, granules, or blood or supporting cells. This type of ultrastructure is associated with steroid-secreting cells.     

Characean charasome-complex and plasmalemma vesicle development

Summary We report on an unusual phenomenon which occurs in some characean algae as a normal plasma membrane activity and also in association with charasome formation. The phenomenon of formation of coated invaginations of the plasma membrane was observed in twoChara and 6Nitella species. These invaginations are coated on their cytoplasmic surface, are 50–60 nm in diameter and rarely exceed 60 nm in length. They are abundant in the young cells ofChara andNitella and also occur in mature cells, but at a lower frequency.N. translucent is an exception in that coated invaginations were few in the young cells and absent in mature cells. Coated vesicles (50–60 nm diameter) were closely associated with these invaginations. Our observations suggest the vesicles may be derived from the invaginations by endocytosis.A close relationship was noted between the development of charasomes (plasmalemma modifications) and coated invaginations. Numerous coated invaginations are seen along the membranes of young charasomes; these invaginations appear to be associated with growth of the charasomes. Coated vesicles were not associated with the coated invaginations of the charasome membrane. The tubular network of cytoplasm and wall space seen in the mature charasome may be formed by fusion of coated invaginations of the developing charasomes, leaving cytoplasmic strands between the fused portions. Coated invaginations were not present along charasomes of the mature cells.     

Unusual features of intercellular junctions in the larvacean Oikopleura dioica

Summary In the pelagic larvacean Oikopleura dioica, the epithelium lining the alimentary tract consists of ciliated and unciliated cell types. The ciliated cells also exhibit an apical border of long microvilli. Between the microvilli, the cellular membrane often projects deeply down into the cytoplasm; the membranes of these invaginations and those of apicolateral interdigitations may be associated with one another by tight junctions. Some of these junctions may be autocellular. The tight junctions are seen by freeze-fracture to be very simple in construction, composed of a single row of intramembranous particles, which may be fused into a P-face ridge. There is a dense cytoplasmic fuzz associated with these tight junctions which may extend into adjoining zonula adhaerens-like regions. The invaginations of the apical membranes are, in addition, associated by gap junctions which may also be autocellular. More conventional homocellular and heterocellular tight and gap junctions occur along the lateral borders of ciliated cells and between ciliated and unciliated cells. These gap junctions possess a reduced intercellular cleft and typical P-face connexons arranged in macular plaques, with complementary E-face pits. Both cell types exhibit extensive stacks of basal and lateral interdigitations. The tight junctions found here are unusual in that they are associated with a dense cytoplasmic fuzz which is normally more characteristic of zonulae adhaerentes.     

A mechanism for rapid transport of colloidal particles by flounder renal epithelium

Large quantities of colloidal particles were rapidly transported around the junctional complex into the lateral intercellular spaces by flounder renal epithelial cells. Large invaginations containing particles developed in the apical cytoplasm of cells when tracer particles were injected into the tubular lumens. Some membranebounded profiles containing particles appeared close to the lateral intercellular spaces. Particles were then found in the lateral intercellular spaces, between the basal plasmalemma and the basement membrane, and within the basement membrane. It is suggested that this transport might operate in situ and provide a morphological mechanism to explain a type of protein transport noted in the renal tubules of another flounder species by Maack and Kinter ('67). It is interesting to consider that perhaps a similar mechanism for the transport of intact proteins might also operate in mammalian nephrons as well.     

Membrane differentiations of an absorptive epithelium covering embryonic trophotaeniae in a goodeid teleost

Summary The trophotaenial absorptive cells (TACs) in goodeid embryos facilitate nutrient absorption during prolonged periods of intraovarian gestation. In a study of membrane differentiations associated with solute and ligand transfer in the trophotaeniae of Xenotoca eiseni, embryos were incubated in vivo with cationized ferritin (CF) prior to freeze-cleaving. This exposure to high concentrations of an adsorptive ligand was meant to induce swelling of the endosomal compartment. Macromolecular trafficking in TACs occurs via an apical endocytic complex consisting of plasma membrane invaginations, a large population of small vesicles, uniformly thick apical tubules, and endosomes. Freeze-fracture replicas showed that the microvillar plasma membrane P-face of TACs was studded with intramembrane particles (IMPs) at a fairly high density, whereas that of the cell surface proper contained a distinctly lower density and the tubulovesicular endocytic pits contained almost no IMPs. The majority of small vesicles and apical tubules in a near surface position displayed P-fracture faces with only a few odd IMPs, indicating that membrane, shuttling between the apical plasma membrane and intracellular sorting organelles, obviously does not carry along many large-sized integral membrane proteins. The distended endosomal compartment had many P-face-associated particles primarily clustered into patches. Specializations of the lateral plasma membrane included 4–8 tight junctional strands, relatively large complements of gap junction proteins, and numerous plaques of desmosomal membrane particles. A system of lamellar cisternae underlay the lateral cell surface that was in continuity with the intraepithelial space by numerous tubular canals, giving rise to an intracellular amplification of the basolateral plasma membrane. Their outward openings appeared as tiny pits on the cytoplasmic faces of freeze-cleaved cell membrane. The density of IMPs on the P-faces of the surface plasma membrane was apparently lower than that on its invaginated lamellar complex. Hence, it is concluded that the mobility of integral membrane proteins in the plane of the membrane may be hampered in movement across the surface pores.Supported by the Deutsche Forschungsgemeinschaft (Schi 268/1-1)     

Ultrastructure of the colon of Locusta migratoria

The ultrastructure of the colon of Locusta migratoria is described. The colon is lined by a thick cuticle that, for the most part, adheres to the underlying epithelium. The cuboid epithelial cells are characterized by moderate invaginations of the apical and, to a lesser extent, basal plasma membranes; the lateral plasma membranes are relatively flat. The bulk of the mitochondria are located in the apical region of the cell and are not particularly associated with any of the plasma membranes. The basal region of the cells contains much rough endoplasmic reticulum, glycogenlike granules, and a predominance of spherical, electron-dense bodies of various sizes. Where muscle fibers make contact with the epithelium, the cells are much reduced; the cytoplasm is usually less electron-dense, and, typically, the nucleus has a thick layer of granular material associated with the inner nuclear membrane. The apical and basal plasma membranes of the reduced epithelial cells contain numerous hemidesmosomes. The apical hemidesmosomes occur in pairs around an extracellular space that contains electron-opaque material. The latter forms tonofibrillae that extend into the endocuticle. Bundles of microtubules are associated with the hemidesmosomes. The tubules traverse the cell from the apical to the basal region. The possible significance of these findings is discussed.     

The ultrastructure of the posterior midgut caecum of Pachygrapsus crassipes (Decapoda, Brachyura) adapted to low salinity

The effects of salinity adaptation and of composition and tonicity of fixatives upon the ultrastructure of the posterior midgut caecum (PMC) of Pachygrapsus crassipes have been studied. The PMC epithelium consists of a single layer of columnar cells with a microvillous border. The apical cytoplasm contains numerous mitochondria, lysosomes, and much smooth endoplasmic reticulum. Rough endoplasmic reticulum and Golgi apparatus are situated in the perinuclear cytoplasm. This epithelium resembles other transporting epithelia in that the basal cytoplasm has an extensive system of branched tubules formed from invaginations of the lateral and basal plasma membrane. Numerous mitochondria are associated with the basal tubular system. To determine the possible contribution of the PMC to the osmoregulatory ability of Pachygrapsus, the ultrastructure of the PMC from animals adapted to 40, 50, 100 and 150% sea water was investigated. Enlargement of basal tubules and intercellular spaces at low salinity, suggestive of fluid-transport activity, was found to be an artifact of fixation. The most consistent response when animals were acclimated to dilute salinities was that some basal mitochondria assume a more complex shape, usually appearing as rings in cross sections of the caecum. A hypothesis concerning the functional significance of these mitochondria is proposed.     

Lamellar to tubular conformational changes in the endoplasmic reticulum of the retinal pigment epithelium of the newt,Notophthalmus viridescens

Summary The retinal pigment epithelium (RPE) of the newt (Notophthalmus viridescens) was examined ultrastructurally under both in-vivo and in-vitro conditions. Five distinct conformations of smooth endoplasmic reticulum (SER), two lamellar and three tubular, were observed. The two lamellar conformations included myeloid bodies, which have previously been described (Yorke and Dickson 1984), and fenestrated SER. The latter appeared as layers of flattened or curved cisternae which were penetrated by fenestrations. Fenestrated SER became indistinguishable from the highly branched and convoluted random-tubular SER through the formation of an intermediate configuration (tubular sheets). The remaining tubular SER conformations appeared to arise from random-tubular SER through a progressive reduction in branching and a straightening of individual tubules. Fascicular SER was represented by the hexagonal organization of straight, unbranched tubules into bundles (fascicles). Spiral SER consisted of a similar hexagonal arrangement, but the unbranched tubules spiralled about one another. Neighbouring tubules in areas of spiral SER were also joined together by pairs of electrondense bars. Although lamellar (especially myeloid bodies) and random-tubular configurations of the SER were common features in vivo, fascicular and spiral SER were primarily conformations encountered in vitro. Conditions favouring bilayer lipid phases also appear to facilitate the formation of both myeloid bodies and fascicular SER. These conditions included increased duration of incubation, low (<20° C) incubation temperatures, and Ca²⁺-free incubations with EGTA. Random-tubular SER was most prevalent in media supplemented with fetal calf serum and also after warmer (30° C) incubation temperatures. We speculate that the different conformations of SER observed in the newt RPE may be due, in part, to lipid phase transitions within the membranes of this organelle. However, the specific formation of fascicular and spiral SER may also involve some additional factor, possibly a protein.Supported by grant # MT-5039 from the Medical Research Council of Canada to DHD     

HISTOCHEMICAL STAINING AND CHARACTERIZATION OF GLYCOPROTEINS IN ACID-SECRETING CELLS OF FROG STOMACH

Glycoproteins were histochemically localized in oxyntic cells of the frog stomach by staining with periodic acid-silver methenamine. Reduction of silver was most intense on (a) the outer aspect of the apical plasmalemma, (b) within the tubular smooth membrane system characteristic of oxyntic cells, and (c) within cisternae and vesicles of the Golgi complex. Other membrane components such as those from the mitochondria, nucleus, junctional complex, lateral and basal cell membranes showed little or no stainability. Gastric mucosal homogenates were fractionated by centrifugation for further morphological and chemical analysis. The staining reaction of the microsomal fraction (40,000 g x 60 min) was similar to that of the tubular membranous components of intact oxyntic cells. Carbohydrate analyses showed that all cell fractions are extremely low in acidic sugars, uronic and sialic acids, while neutral sugars and hexosamines are relatively abundant. The microsomal fraction contains the largest proportion of carbohydrates, ca. 9% of the fat-free dry weight. Another distinguishing feature is that glucosamine is the only detectable hexosamine in the microsomal fraction. These histochemical and chemical data indicate that neutral glycoproteins are associated with membranous components which have been implicated in the process of HCl secretion by oxyntic cells. The staining pattern within the cells supports the hypothesis of interrelationships between the Golgi membranes, tubular smooth membranes, and apical surface membrane.     

Molluscan visceral muscle fine structure. General structure and sarcolemmal organization in the smooth muscle of the intestinal wall of Buccinum undatum L.

Fine structure of intestinal muscle in the gastropod Buccinum undatum is described. Myofibrillar organization is typical of non-pseudostriated molluscan muscles. The dense body system is poorly developed but there are extensive attachment plaques. The sarcolemma is elaborately modified. Deep infoldings of the membrane give the cells an irregular outline. Such infoldings enclose extracellular matrix and are associated with attachment plaques. Arising from these and from the general sarcolemma are numerous tubular membranous invaginations ending blindly at varying depth in the sarcoplasm. These structures have a helical coat of particles on the cytoplasmic face. Associated with both types of invagination are subsarcolemmal vesicles. The possibility that the tubular invaginations are analogues of vertebrate smooth muscle caveolae or striated muscle T-tubules and that the vesicles are the corresponding sarcoplasmic reticulum is discussed. The occurrence of such structures in molluscan muscle and elsewhere is reviewed.     

Fine structure of the hemocytes and nephrocytes of Argas (Persicargas) arboreus (Ixodoidea: Argasidae)

The fine structure of the hemocytes and nephrocytes in Argas (Persicargas) arboreus is described and compared with that of similar cells in other tick species and insects. The hemocytes are of three types: prohemocytes, with a relatively undifferentiated cytoplasm lacking granular inclusions and probably serving as progenitors of the other hemolymph cell types; plasmatocytes, containing abundant mitochondria, cisternae of rough endoplasmic reticulum (RER), and free ribosomes, as well as some small granular inclusions; granulocytes, the predominant cell type in the hemolymph, containing numerous granules of variable electron density and maturity, and pseudopodia-like processes on the cell surface. Plasmatocytes and granulocytes are phagocytic and possibly also have other functions in the tick body. Cells with intermediate features appear to be in a stage of transition from plasmatocyte to granulocyte. Nephrocytes contain vacuoles enclosing fibrillar material, some electrondense granules, and moderate amounts of the active organelles—mitochondria, RER, and ribosomes. The nephrocyte is surrounded by a basal lamina and its plasma membrane infolds to form many deep invaginations coated by a fine fibrillar material. Openings to these invaginations are closed by membranous diaphragms. Coated tubular elements connect the surface invaginations with large coated vesicles, which appear to be specialized for internalization of proteins from the hemolymph. The dense granules may represent an advanced stage of condensation of ingested protein and thus may be lysosomal residual bodies, or they may develop by accumulation of secretory products.     

Vinculin is a component of an extensive network of myofibril-sarcolemma attachment regions in cardiac muscle fibers

Immunofluorescent staining of bovine and avian cardiac tissue with affinity-purified antibody to chicken gizzard vinculin reveals two new sites of vinculin reactivity. First, vinculin is organized at the sarcolemma in a striking array of rib-like bands, or costameres. The costameres encircle the cardiac muscle cell perpendicular to the long axis of the fiber and overlie the I bands of the immediately subjacent sarcomeres. The second new site of vinculin reactivity is found in bovine cardiocytes at tubular invaginations of the plasma membrane. The frequency and location of these invaginations correspond to the known frequency and distribution of the transverse tubular system in bovine atrial, ventricular, and Purkinje fibers. We do not detect tubular invaginations that stain with antivinculin in avian cardiocytes and, in fact, a transverse tubular system has not been found in avian cardiac fibers. Apparent lateral Z-line attachments to the sarcolemma and its invaginations have been observed in cardiac muscle by electron microscopy in the same regions where we find vinculin. On the basis of these previous ultrastructural findings and our published evidence for a physical connection between costameres and the underlying myofibrils in skeletal muscle, we interpret the immunofluorescence data of this study to mean that, in cardiac muscle, vinculin is a component of an extensive system of lateral attachment of myofibrils to the plasma membrane and its invaginations.     

Quantitative immunogold localization of Na, K-ATPase along rat nephron.

Ultrastructural localization of Na, K-ATPase alpha-subunit along rat nephron segments was investigated quantitatively by immunogold electron microscopy on LR-White ultrathin sections using affinity-purified antibody against alpha-subunit of the enzyme. Ultrathin sections were incubated with the antibody at a saturation level and the number of gold particles bound per micron of the plasma membrane (particle density) of the tubular epithelial cells from the proximal tubule to the collecting duct was determined. In all the tubular epithelial cells, gold particles were located exclusively on the basolateral surface, and no significant binding of gold particles to the apical surface was observed. Distribution of gold particles on the basolateral membranes was quite heterogeneous; lateral membranes and infolded basal membranes were highly labeled, whereas the basal membranes which are in direct contact with the basal lamina were scarcely labeled. The average particle density on the basal surface was highest in the distal straight tubule cells (11.4 units), very high in the distal convoluted tubule cells (9.8 units), intermediate in the proximal tubule cells (3.3 units), in the connecting tubule cells (4.3 units), and in the principal cells of the collecting duct (5.6-3.8 units), low in the thin limb of Henle's loop (1.0 unit), and at the control level in the intercalated cells in the connecting and collecting duct. The relative number of gold particles/mm nephron segment and the relative number of gold particles in the various nephron segments were calculated using quantitative morphological data. The estimated distribution profile of the former was in good agreement with the Na, K-ATPase activity profile in rat nephron, which was determined biochemically with a microenzymatic method.     

Electron tomographic characterization of a vacuolar reticulum and of six vesicle types that occupy different cytoplasmic domains in the apex of tip-growing <Emphasis Type="Italic">Chara</Emphasis> rhizoids

We provide a 3D ultrastructural analysis of the membrane systems involved in tip growth of rhizoids of the green alga Chara. Electron tomography of cells preserved by high-pressure freeze fixation has enabled us to distinguish six different types of vesicles in the apical cytoplasm where the tip growth machinery is accommodated. The vesicle types are: dark and light secretory vesicles, plasma membrane-associated clathrin-coated vesicles (PM-CCVs), Spitzenkoerper-associated clathrin-coated vesicles (Sp-CCVs) and coated vesicles (Sp-CVs), and microvesicles. Each of these vesicle types exhibits a distinct distribution pattern, which provides insights into their possible function for tip growth. The PM-CCVs are confined to the cytoplasm adjacent to the apical plasma membrane. Within this space they are arranged in clusters often surrounding tubular plasma membrane invaginations from which CCVs bud. This suggests that endocytosis and membrane recycling are locally confined to specialized apical endocytosis sites. In contrast, exocytosis of secretory vesicles occurs over the entire membrane area of the apical dome. The Sp-CCVs and the Sp-CVs are associated with the aggregate of endoplasmic reticulum membranes in the center of the growth-organizing Spitzenkoerper complex. Here, Sp-CCVs are seen to bud from undefined tubular membranes. The subapical region of rhizoids contains a vacuolar reticulum that extends along the longitudinal cell axis and consists of large, vesicle-like segments interconnected by thin tubular domains. The tubular domains are encompassed by thin filamentous structures resembling dynamin spirals which could drive peristaltic movements of the vacuolar reticulum similar to those observed in fungal hyphae. The vacuolar reticulum appears to serve as a lytic compartment into which multivesicular bodies deliver their internal vesicles for molecular recycling and degradation. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

In dry pear (Pyrus communis L.) pollen,membranes assume a tightly packed multilamellate aspect that disappears rapidly upon hydration

Summary Ultrastructural details of dry (7% moisture content) and hydratedPyrus communis L. pollen are revealed following freezesubstitution preparation for electron microscopy. Dry pollen is characterized by tightly packed, multilamellate membranous profiles found in association with plasma membrane, vesicles, ER, dictyosomes and some double-membrane bound organelles. Dry pollen also shows unit-membrane bound, densely osmiophilic bodies often with tightly packed multilamellations contained within and, at times, in their bounding membranes. These features are not evident in hydrated pollen. Results suggest that multilamellate membranes form as the plasma membrane, vesicles, ER, and double-membrane bound organelles undergo dehydration, and that upon hydration they rapidly resume normal unilamellate structure.Abbreviations DOB densely osmiophilic body - IMP intramembrane particles - MO multilamellate organelle     

An ultrastructural analysis of the ecdysoneless ((l(3)ecd 1ts) ring gland during the third larval instar of Drosophila melanogaster

Summary In the late third larval instar of Drosophila melanogaster, the prothoracic gland, an endocrine portion of the ring gland, synthesizes ecdysteroids at an accelerated rate. The resultant ecdysteroid titer peak initiates the events associated with metamorphosis. The normal prothoracic gland displays several ultrastructural features at this developmental stage that reflect increased steroidogenic activity, including extensive infoldings of the plasma membrane (membrane invaginations) and an increase in both the concentration of smooth endoplasmic reticulum (SER) (or transitional ER) and elongated mitochondria. By contrast, the prothoracic glands of larvae homozygous for a conditional larval lethal mutation, l(3)ecd ^1ts, not only fail to produce ecdysteroids at normal levels at the restrictive temperature (29° C), but also acquire abnormal morphological features that reflect the disruptive effects of the mutation. These abnormalities include an accumulation of lipid droplets presumed to contain sterol precursors of ecdysteroids, a disappearance of SER and a drastic reduction of membrane invaginations in the peripheral area of the cell. These morphological defects are observed in prothoracic glands dissected from larvae transferred from 18° C to 29° C approximately 24 h before observation and also within 4 h of an in vitro transfer to 29° C following dissection from wandering third instar larvae reared at 18° C. No ultrastructural abnormalities were noted in the corpus allatum portion of mutant ring glands. These observations further indicate the direct involvement of the ecd gene product in ecdysteroid synthesis and suggest a role for the gene in the proper transport of precursors to the site where they can be utilized in ecdysteroid biosynthesis.     

Freeze fracture studies on the annelid septate junction

Summary Freeze-fractured preparations of septate junctions between epidermal cells of annelids (Lumbricus terrestris and Tubifex spec.) have been investigated. In Lumbricus the protoplasmic face (PF) of the plasma membrane is characterized by variously arranged rows of particles. Apically the rows take an undulating course and often are separated by wide distances. In the basal part of the junction the rows run closely together and more or less in parallel. The diameter of the particles measures 80–120 Å, the distance between two particles (centre to centre) is 150–250 Å. Additionally striking rows of large particles (long diameter 150–200 Å). Are to be observed mainly near the basal part of the junction. In Tubifex both faces of the plasma membrane could be studied in detail. The protoplasmic face (PF) contains rows of distinct individual particles (mean diameter 100–150 Å, centre to centre distance approx. 250 Å) whereas the particles of the extracellular face (EF, mean diameter 200–250 Å) usually form continuous strands in which the individual particles seem to fuse. The density of arrangement of the strands varies considerably. Additionally ladder-shaped membrane structures have been observed in plasma membranes of this species.     

Freeze-fracture electron microscopic studies of age-related plasma membrane changes in Sporothrix schenckii

Characteristics of the plasma membrane of Sporothrix scheckii cells as revealed by freeze-fracture techniques have been classified into eight types (Y1, Y2a, Y2b, Y3a, Y3b, Y4a, Y4b, and Y5) in yeastlike cells grown under the following two conditions: brain heart infusion agar medium at 27 degrees C, and brain heart infusion agar medium at 37 degrees C. Type Y1 cells are yeastlike cells having smooth plasma membranes without any invagination. Typical characteristics of the other types are as follows: type Y2a, smooth plasma membranes with few trenchlike invaginations; type Y2b, wavy plasma membranes with few oval or irregularly formed invaginations; type Y3a, plasma membranes with many randomly distributed trenchlike invaginations; type Y3b, plasma membranes with many cocoonlike or irregularly formed invaginations; type Y4a, plasma membranes with longer trenchlike invaginations; type Y4b, plasma membranes with irregularly formed, enlarged invaginations; and type Y5, smooth or wavy plasma membranes with aggregations of intramembranous particles and with many vacuoles between cell walls and plasma membranes or in the cytoplasm in some cells. By counting the proportion of each type of yeastlike cell under the two conditions and with different cultivation periods, it appears that plasma membrane types change as aging progresses in the following order: type Y1, Y2a, Y3a, Y4a, and Y5 in conidia and type Y1, Y2b, Y3b, Y4b, and Y5 in yeastlike vegetative cells. These observations provide us with an important advantage when studying the effects of antifungal agents on the plasma membrane of Sporothrix scheckii, as it is important to know the natural course of changes in membrane structure during aging.     

On the fine structure of the aglomerular renal tubule in Lophius piscatorius

Summary The fine structure of the secretory tubules in the kidney of the aglomerular goose-fish (Lophius piscatorius) is described. The cells have a pyramidal shape, are joined together by multiple desmosomes, and share as main characteristics: abundant and deep inflections of the basal and lateral cell membranes; coated luminal plasma membranes forming multiple microvilli or a genuine brush border; moderate numbers of comparatively small mitochondria, usually unassociated with the basal and lateral plasma membrane specializations; numerous multivesicular bodies occuring in the apical cytoplasm; abundant large lysosome-like bodies in the intermediate regions of the cytoplasm; and comparatively poor development of endoplasmic reticulum and Golgi apparatus.The observations suggest that the cells perform both absorptive and secretory functions and are metabolically unusually active in autolytic and heterolytic work. Comparisons with other aglomerular species indicate that the ability for active secretory function is not necessarily dependent on a close association between plasma membrane and mitochondria; however, this ability does appear to require a markedly increased basal and/or lateral cell surface created by multiple invaginations of the plasma membrane. The abundance of desmosomes and associated structures appears to represent a unique structural specialization of the goosefish tubule, and indicates that the cells must be firmly anchored to one another to supply a rigid and mechanically continuous lining of the tubule. The multivesicular bodies probably represent endocytic vacuoles which fuse with apical vesicles and invaginate their outer membrane to form the internal vesicles; they appear to transform to ambilysosomes via a function as heterophagosomes and — later — combined hetero- and autophagosomes.Supported by grants from Karolinska Institutet, Fonden til Videnskabens Fremme and Konsul Johannes Fogh-Nielsen og fru Ella Fogh-Nielsens Légat. Part of the study was performed at the Zoological Station at Naples, Italy. The assistance of Mrs. Britt-Marie Karlsson is gratefully acknowledged.