Studies on the morphology and ultrastructure of the Malpighian tubules of Halobiotus crispae Kristensen, 1982 (Eutardigrada)

The excretory and osmoregulatory system of Halobiotus crispae consists of two lateral and one smaller dorsal Malpighian tubules, which empty into the digestive tract in the transition zone of the midgut and rectum. The tubules are identical at the ultrastructural level, and consist of an initial segment with three large cells, a thin transitional distal part lacking a nucleus, and a proximal part with 9–12 nuclei. The initial segment possesses deep basal infoldings and interdigitating, finger-shaped processes of the plasma membrane, large mitochondria and giant nuclei. The distal part is a short section which supports the initial segment. Cellular offshoots from the succeeding proximal part constitute the distal part. The distal and proximal parts contain intercellular canals with concretions of variable size. The exit of the proximal part into the digestive tract is characterized by the presence of microvilli. Correlated with the different stages in the cyclomorphosis of H. crispae , we observed size variation of the Malpighian tubules; thus, pseudosimplex stages have the largest tubules. We present suggestions concerning the physiology of the tubules and compare the Malpighian tubules of Tardigrada with the Malpighian papillae of Protura.     

INTESTINAL TRANSPORT OF ANTIBODIES IN THE NEWBORN RAT

Evidence has been reported that the proximal small intestine of the neonatal rat selectively transports antibodies into the circulation. This study describes the morphology of the absorptive epithelial cells in this region of the intestine and their transport of several immunoglobulin tracers: ferritin-conjugated immunoglobulins (IgG-Ft) and antiperoxidase antibodies. Cells exposed to rat IgG-Ft bound the tracer on the membrane of tubular invaginations of the apical cell surface. Tubular and coated vesicles within the cell also contained the tracer, as did the intercellular spaces. Uptake of tracer was highly selective and occurred only with rat or cow IgG-Ft; when cells were exposed to chicken IgG-Ft, ferritin-conjugated bovine serum albumin, or free ferritin, tracer did not enter the cell or appear in the intercellular spaces. Experiments with rat and chicken antiperoxidase showed a similar selective uptake and transport of only the homologous antibody. When cells from the distal small intestine were exposed to the tracers, all tracers were absorbed nonselectively but none were released from the cells. Cells from the proximal small intestine of the 22-day-old rat failed to absorb even rat IgG-Ft. A model is presented for selective antibody transport in proximal cells of the neonatal rat in which antibodies are selectively absorbed at the apical cell surface by pinocytosis within tubular vesicles. The antibodies are then transferred to the intercellular space within coated vesicles. Distal cells function only to digest proteins nonselectively.     

Ultrastructural localization of calcium in matrix vesicles and preodontoblasts of developing rat molar tooth germs during initial dentinogenesis

We investigated the ultrastructural localization of calcium in progenitor predentine and preodontoblasts of developing rat molar tooth germs using the potassium pyroantimonate technique. At the precalcification stage, antimonate reaction product was sparsely, randomly distributed in the preodontoblasts and in the progenitor predentine but no significant reaction could be noticed associated with matrix vesicles. At the matrix vesicle calcification stage, large amounts of antimonate reaction product tended to be localized in the region adjacent to the distal, outer surface membrane of preodontoblasts in which moderate antimonate reaction activity could be observed in mitochondria. Strong antimonate reaction was detected preferentially on the outer surface membrane of some matrix vesicles at this stage. At the subsequent collagen calcification stage, definite antimonate reaction was no longer seen within mitochondria of the late preodontoblasts, instead precipitate was mainly distributed in Golgi area, secretory granules and lateral intercellular spaces. It is suggested that although matrix vesicles contain few calcium capable of reacting to antimonate immediately after their biogenesis, subsequently, large amounts of calcium are accumulated associated with the outer surface membrane of matrix vesicles in the extracellular matrix.     

Structural heterogeneity of the basement membrane in the rat proximal tubule

Summary The ultrastructure of the basement membrane of the rat proximal tubule was observed by transmission electron microscopy after the use of a cold dehydration technique. The basement membrane of the P1 segment is thick and possesses several structural specializations that are rare in other basement membranes; these include intraepithelial ridges, dense bars, and basement membrane vesicles. The intraepithelial ridges are found in the intercellular spaces between interdigitating processes of the proximal tubule cells. The ridges and the interdigitating processes run circumferentially around the tubule. The dense bars are frequently found in the intraepithelial ridges. They are especially prominent on the concave side of the tubular bends and to a lesser extent near sites where intracellular actin filaments anchor onto the basal cell membranes. The basement membrane vesicles are bounded by unit membranes; they are variable in both their electron density and their size. They are usually found in association with dense bars, and the grade of their accumulation is positively correlated with the development of the dense bars. These three specializations have no topographical relationship with the interstitial structures, such as fibrobalasts and collagen fibrils. The specializations are best developed on the concave side of tubular bends where the circumferential stresses caused by the intraluminal hydraulic pressure are presumably the largest; we therefore propose that they are an adaptation to, or a manifestation of, the increased wall stress in the proximal tubule.     

Ultrastructural markers of renal tubular transport in rats under physiological conditions

Mitochondria of the proximal and distal tubules which are in different configurational states of epithelial cells and their surface--volume relationship of intercellular spaces and basal infolded channels were evaluated in rats. The evaluation was performed with stereological methods. The studies were carried out on 5 rats under physiological conditions using electron microscopy. Mitochondria within the proximal and distal tubules were found to occur in transitional states close to the orthodox state. However, mitochondria within the proximal tubules were in a higher energy state, closer to the orthodox state when compared with those within the distal tubules. Surface--volume parameters of intercellular spaces and basal infolded channels were unexpectedly higher than the relation to active ion transport as well as indiscernible permeability of the distal tubular basement membrane.     

FINE STRUCTURE OF THE OCTOPUS RETINA

The fine structure of the visual and the supporting cells and of the blood capillaries in the octopus retina is described. Lamellated structures contained in the proximal segment of the visual cell consist of compact arrays of dense membranes each of which is quintuple-layered and divides at its margins into two thinner sheets or membranes which are connected directly with the agranular or granular endoplasmic reticulum. Proximal to the deeper extremities of the rhabdomeres, the lateral plasma membranes of two adjoining visual cells contact each other forming a quintuple-layered compound membrane, which results in occlusion of the intercellular space. The central layer of the compound membrane is of high density, so that the membrane, as a whole, appears to be a single thick layer at low magnifications. The supporting cells are connected with the neighboring visual cells by two types of junctions. Long slender processes extend from the supporting cells to the surface of the retina through narrow spaces among the distal segments of the visual cells. The capillary endothelial cells are characterized by luminal surfaces irregularly contoured and by lateral surfaces elaborately interdigitated. The functional significance of the close contact between adjoining visual cells is discussed.     

The pelvic kidney of male Ambystoma maculatum (Amphibia,urodela, ambystomatidae) with special reference to the sexual collecting ducts

This study details the gross and microscopic anatomy of the pelvic kidney in male Ambystoma maculatum. The nephron of male Ambystoma maculatum is divided into six distinct regions leading sequentially away from a renal corpuscle: (1) neck segment, which communicates with the coelomic cavity via a ventrally positioned pleuroperitoneal funnel, (2) proximal tubule, (3) intermediate segment, (4) distal tubule, (5) collecting tubule, and (6) collecting duct. The proximal tubule is divided into a vacuolated proximal region and a distal lysosomic region. The basal plasma membrane is modified into intertwining microvillus lamellae. The epithelium of the distal tubule varies little along its length and is demarcated by columns of mitochondria with their long axes oriented perpendicular to the basal lamina. The distal tubule possesses highly interdigitating microvillus lamellae from the lateral membranes and pronounced foot processes of the basal membrane that are not intertwined, but perpendicular to the basal lamina. The collecting tubule is lined by an epithelium with dark and light cells. Light cells are similar to those observed in the distal tuble except with less mitochondria and microvillus lamellae of the lateral and basal plasma membrane. Dark cells possess dark euchromatic nuclei and are filled with numerous small mitochondria. The epithelium of the neck segment, pleuroperitoneal funnel, and intermediate segment is composed entirely of ciliated cells with cilia protruding from only the central portion of the apical plasma membrane. The collecting duct is lined by a highly secretory epithelium that produces numerous membrane bound granules that stain positively for neutral carbohydrates and proteins. Apically positioned ciliated cells are intercalated between secretory cells. The collecting ducts anastomose caudally and unite with the Wolffian duct via a common collecting duct. The Wolffian duct is secretory, but not to the extent of the collecting duct, synthesizes neutral carbohydrates and proteins, and is also lined by apical ciliated cells intercalated between secretory cells. Although functional aspects associated with the morphological variation along the length of the proximal portions of the nephron have been investigated, the role of a highly secretory collecting duct has not. Historical data that implicated secretory activity concordant with mating activity, and similarity of structure and chemistry to sexual segments of the kidneys in other vertebrates, lead us to believe that the collecting duct functions as a secondary sexual organ in Ambystoma maculatum. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

The structure of epidermal feet during their development

Epidermal cells in Calpodes and other insects form basal processes or feet that at first extend axially and later shorten at the same time as the larval segment shortens to the pupal shape. The feet grow into spaces at the surfaces of other cells to make a basal interlacing meshwork of cellular extensions that are combined mechanically by their desmosomal attachments to cell bodies above and to the basal lamina below. Microtubules and microfilaments are linked to these junctions by a reticular fibrous matrix. Gap junctions on the feet may couple cells that are several cell bodies removed from one another. The meshwork is also a sieve separating the hemolymph from the spaces between cells to form an intercellular compartment. Entry to the intercellular compartment is through the sieve made by the negatively charged basolateral cell surfaces that can prevent the entry of positively charged molecules such as cationic ferritin. As the cells become columnar, coincident with the metamorphic change in segment shape, the feet shorten and pack more densely together. At this time the basal lamina buckles axially as if responding to contraction of the feet. Segment shape change involves cell rearrangement and relative cell movement, necessitating the transient loss of plasma membrane plaque attachments to the cuticle apically and the loss of junctions laterally. Gap junctions involute in characteristic vacuoles. The metamorphic reduction in cell surface area coincides with the loss of basolateral membrane in smooth tubes and vesicles and the turnover of the apical surface in multivesicular bodies. New apical plasma membrane plaques and new lateral and basal junctions stabilize the cells in their pupal positions.     

Ultrastructure of the crayfish antennal gland revealed by scanning and transmission electron microscopy combined with ultrasonic microdissection

The antennal gland of the crayfish Pacifasticus leniusculus was studied using standard techniques for scanning electron microscopy as well as newer procedures for ultrasonic microdissection. To clarify relationships in the nephron tubule, transmission electron microscopy was employed. The coelomosac contains elongated cells (podocytes) displaying microvilli and extensive apical blebbing. A smooth basal lamina lines the blood space that furnishes hemolymph to the coelomosac. The labyrinth consists of tall columnar cells displaying apical microvilli, numerous blebs that seem to represent an expansion of apical plasma membrane, and lateral interdigitations. The nephron tubule consists of two distinctly different areas: a proximal region of flattened cells with extensive intercellular fusions, and a distal segment of separate, dome-shaped cells. Despite many similarities between the crayfish kidney and the vertebrate nephron, there are striking differences. The amount of surface blebbing that occurs in the coelomosac and labyrinth far exceeds that of the vertebrate nephron and may reflect its importance in the function of the crayfish kidney. The cells of the coelomosac are taller than are the vertebrate podocytes and possess less obvious arms and pedicels. In addition, the proximal segment of the nephron tubule is notable for its intercellular fusions, which are not present in the vertebrate nephron. Although the function of the intercellular fusions is unknown, they may play a role in cellular communication or the redistribution of fluids or electrolytes between cells.     

A correlated scanning and transmission electron microscopic study of maturation ameloblasts in developing molar teeth of rats

Summary Maturation ameloblasts of developing molar teeth of the rat were studied by both scanning and transmission electron microscopy. After fixation, teeth were frozen and split. One face of the fractured tooth was used for SEM, the other for TEM.It was found that in some regions proximal junctional complexes separate the interameloblast space from the intercellular space of the papillary layer. Thereby an intercellular ameloblastic compartment is delineated which in some specimens contains a substance interpreted to be colloidal. Elsewhere the proximal junctions of ameloblasts are not present and free communication between the extracellular spaces is evident. The apical pole of ameloblasts varies in structure. Over some areas there is a distinct distal border zone with membranous infoldings which in some regions resembles a striated or ruffled border, but in other regions the membranes show whorl configurations. The distal border zone also contains granules with flocculent material. Elsewhere the ameloblasts display no distal border zone and the cells show a smooth membrane (except for pinocytotic vesicles and hemidesmosomes) facing the enamel surface. The lateral surface of ameloblasts exhibits a variety of surface configurations similar to but not as pronounced as those reported previously in rat incisor maturation ameloblasts.The authors wish to thank Pauletta Sanders and Helen Ruane for technical assistance. This project was supported in part by USPHS NIH Grant DE04059-03 and by the Medical Research Council of Great Britain     

Morphometric analysis of coated pits and vesicles in the proximal and distal caput epididymidis

A morphometric analysis of coated and uncoated structures found in the apical portion of principal cells from both the proximal and distal caput epididymidis has been carried out. Almost all endocytic, coated vesicles are found within 1 micron of the luminal surface of principal cells and the volume fraction of these and of uncoated vesicles is much greater in the proximal caput epididymidis. A serial section analysis indicated that many coated "vesicles" are tangentially sectioned coated pits and that a complex network of interconnected vesicular and tubular structures exists in the apical cytoplasm. Efferent duct ligation has no effect on the number of size of large coated and uncoated vesicles in either the proximal or distal caput epididymidis, indicating that substances delivered to principal cells from the lumen are not required to maintain the endocytic machinery. However, this treatment does result in a considerable increase in the number of large coated vesicles associated with the basal surface of principal cells from the proximal but not the distal caput epididymidis. The volume fraction of small, presumably exocytic, coated vesicles is significantly greater in the apical cytoplasm of cells from the distal caput epididymidis in control animals. Efferent duct ligation results in a significant increase in the volume fraction of these vesicles in the proximal but not distal caput epididymidis. These results show that there are marked differences in structure among principal cells from these two regions of the epididymis and that this may reflect differences in control and function.     

FINE STRUCTURE AND MORPHOGENIC MOVEMENTS IN THE GASTRULA OF THE TREEFROG, HYLA REGILLA

The blastoporal groove of the early gastrula of the treefrog, Hyla regilla, was examined with the electron microscope. The innermost extension of the groove is lined with invaginating flask- and wedge-shaped cells of entoderm and mesoderm. The distal surfaces of these cells bear microvilli which are underlain with an electron-opaque layer composed of fine granular material and fibrils. The dense layer and masses of vesicles proximal to it fill the necks of the cells. In flask cells bordering the forming archenteron the vesicles are replaced by large vacuoles surrounded by layers of membranes. The cells lining the groove are tightly joined at their distal ends in the region of the dense layer. Proximally, the cell bodies are separated by wide intercellular spaces. The cell body, which is migrating toward the interior of the gastrula, contains the nucleus plus other organalles and inclusions common to amphibian gastrular cells. A dense layer of granular material, vesicles, and membranes lies beneath the surface of the cell body and extends into pseudopodium-like processes and surface undulations which cross the intercellular spaces. A special mesodermal cell observed in the dorsal lining of the groove is smaller and denser than the surrounding presumptive chordamesodermal cells. A long finger of cytoplasm, filled with a dense layer, vesicles and membranes, extends from its distal surface along the edge of the groove, ending in a tight interlocking with another mesodermal cell. Some correlations between fine structure and the mechanics of gastrulation are discussed, and a theory of invagination is proposed, based on contraction and expansion of the dense layer and the tight junctions at distal cell surfaces.     

Cuprolinic Blue visualization of cytosolic and membrane-associated glycosaminoglycans in the rat junctional epithelium and gingival epithelia

Summary The gingiva of rat molars was studied at the light microscope level using glutaraldehyde as fixative, Cuprolinic Blue for visualizing polyanionic glycosaminoglycans and the autometallographic technique for enhancing the copper signal of the cationic dye. The polyanions were located inside the epithelial cells in the junctional epithelium, whereas a network located along either the plasma membrane or the intercellular spaces, or both, of the gingival oral epithelium and sulcular oral epithelium was evident with autometallography. In these cases, positive staining was limited to the basal and spinous layers, the granular and keratinized layers being unstained. With the transmission electron microscope, electron-dense aggregates were seen in the gingival lamina propria, in the basement membrane and along the plasma membrane of the keratinocytes of the basal and spinous layers of the gingival and sulcular oral epithelia. In the junctional epithelium, Cuprolinic Blue-positive granules, 25 nm in diameter, were seen in the cytoplasm. Together with some vesicles containing electron-dense material, they may account for the staining process noted after autometallography. When the ultra-thin sections were digested with bovine testicular hyaluronidase, the staining was abolished. This indicates that glycosaminoglycans were primarily responsible for the staining pattern visualized with these methods. In the junctional epithelium, the cytosolic location of the 25 nm granules reflects either transcellular transfer between the plasma membrane and the nucleus or accumulation of glycosaminoglycans in this group of keratinocytes. The glycoconjugates located inside vesicles or vacuoles are related to endocytosis and lysosomal degradation. Interstitial glycosaminoglycans seen in the two types of oral epithelium may play a role in the diffusion of water and nutriments.     

Ultrastructure of the crayfish kidney--coelomosac, labyrinth, nephridial canal

Regions of the crayfish kidney were examined by electron microscopy. Coelsmosac cells are loosely bound together by desmosome-like spot junctions, and connected to the basal lamina via characteristic pedicels. The cytoplasm contains numerous vesicles and vacuoles of various sizes and is often crowded with large, lysosome-like granules or dense bodies. The morphology suggests a filtration mechanism with reabsorption of materials such as protein from the filtrate and secretion of other substances into the lumen. The labyrinth is composed of cuboidal to columnar cells which possess a brush border, long and narrow intercellular spaces, basal plasmalemmal invaginations and typical cytoplasmic components. Two sub-regions are distinguishable. The morphology of labyrinth I suggests that these cells move fluid isotonically across the epithelium. Labyrinth II, in addition to isotonic transport, appears to be more active in the endocytic uptake and intracellular digestion of large molecules such as protein. The nephridial canal consists of cells which lack a brush border, but display extensive basal invaginations associated with elongated mitochondria. A proximal and distal region are cytologically distinguishable. Proximally, the cells are small and filled with mitochondria throughout. Scattered within the cytoplasm are vesicles, vacuoles, diffuse glycogen, free ribosomes, dense bodies and some rough endoplasmic reticulum. Distally, the cells are less compact, larger, and cuboidal to columnar in shape. The cytoplasm is similar to that of the proximal cells, but the basal invaginations are even larger and more extensive. The morphology of cells in both regions of the nephridial canal is highly suggestive of active solute reabsorption, probably occurring against an osmotic gradient.     

The fine structure of the nephronic tubule of the mudskipper Periophthalmus koelreuteri (Pallas)

Ultrastructural examination of the head kidney of Periophthalmus koelreuteri (Pallas) (Teleostei, Gobiidae) revealed that the nephronic tubule cells are bound by tight junctions and desmosomes with little intercellular space. The first proximal segment (PI) consists of low columnar cells with well developed brush borders, indented nuclei, and numerous apical endocytic vesicles and lysosomes. A second cell type possessing clusters of apical cilia and lacking brush border and lysosomes is occasionally found between PI cells. The second proximal segment (PII) is formed of high columnar cells with brush border, regular spherical nuclei and numerous mitochondria located between well developed infoldings of the basal membrane. Single ciliary structures protrude into the lumen from PI and PII cells. The distal segment is lined by low columnar epithelium with few microvilli, regular spherical nuclei, numerous scattered mitochondria, and microbodies. The collecting tubule cells are cuboidal with few euchromatic nuclei, some mitochondria, and secondary lysosomes.     

Internal structure of the postcapillary high-endothelial venules of rodent lymph nodes and Peyer's patches and the transendothelial lymphocyte passage

Scanning electron microscopy (SEM) shows that the postcapillary high-endothelial venules of lymph nodes and Peyer's patches consist of two segments each with a different surface relief: a proximal segment with a cobblestone surface pattern and a distal segment of interlacing cytoplasmic plates. Both segments have deep adluminal crevices in which lymphocytes are lodged. The internal structural configuration of this endothelium has been examined by transmission electron microscopy (TEM) of serial sections of lymph nodes and Peyer's patches of mice, rats, and guinea pigs. The serial sections revealed that the endothelial cell bodies and their cytoplasmic extensions were disposed in a direction generally lateral to the luminal surface and intruded into the intercellular spaces of similarly disposed neighboring endothelial cells, resulting in a complex interlacing cellular pattern. Lymphocytes penetrated the endothelial cell body and secondarily followed an intracellular pathway through which they entered the extravascular compartment. At the exposed surfaces of the adluminal venule wall, recirculating lymphocytes were seen in SEM images to enter the endothelium by penetrating the endothelial cell body. The mode of migration of lymphocytes lodged in the endothelial crevices could be determined by SEM and has been examined by TEM of serial sections. At these locations as at the exposed surfaces, lymphocytes also entered the venule by penetrating the endothelial cell body. At both sites this transcellular pathway was followed by lymphocyte entry into the intercellular spaces from which they migrated into the extravascular compartment.     

Fine structure of the rectal pads in the desert locust Schistocerca gregaria with reference to the mechanism of water uptake

The rectal pads of Schistocerca gregaria are composed of three different cell types: epithelial, secondary and junctional cells. The rectal pads are interconnected by simple rectal cells and both are lined internally by a articular intima. The epithelial cells exhibit extensive infoldings of the apical plasma membranes that are closely associated with mitochondria. Their lateral plasma membranes are highly folded around large mitochondria and enclose intercellular channels and spaces. They are united by belt and spot desmosomes, septate junctions, gap junctions and scalariform junctions, but terminate in a basal syncytium without contacting the basal plasma membranes. The apical and basal cytoplasm contain coated vesicles, dense tubular elements, multivesicular bodies and lysosomes, suggesting receptor-mediated endocytosis of small peptide molecules into the epithelial cells. The apical membrane infoldings of the secondary cells are also associated with large mitochondria. Their basal plasma membranes are covered by connective cell processes and connected with them by spot desmosomes which may be involved in solute recycling. The presence of neurosecretory-like axons near the secondary cells suggests that they exert local control on the function of these cells. The ultrastructural details are examined in relation to their role in solute and water transport.     

Ultrastructural immunolocalization of apolipoprotein B within human jejunal absorptive cells

Apolipoprotein B (apoB) was localized by electron microscopy within absorptive cells of human jejunal biopsy specimens taken fasting and after micellar fat infusion. Nakane's double antibody immunoperoxidase technique was used to label apoB near open cut surfaces of 60-Micrometers fixed tissue slices sectioned by a Ralph knife in a Vibratome. In fasting tissue, apoB label was found within structurally intact peri-mitochondrial rough endoplasmic reticulum (RER) and within Golgi cisternae of absorptive cells covering the tips of jejunal villi. After fat infusion, apoB label was found adjacent to very low density lipoproteins (VLDL) and chylomicrons within apical smooth endoplasmic reticulum (SER). Less label was seen within RER than in fasting absorptive cells, and RER-SER connections containing apoB label were occasionally seen. Expanded Golgi vesicles and cisternae contained VLDL, chylomicrons, and apoB label. Vesicles containing chylomicrons and apoB label were occasionally visualized bordering the lateral plasma membrane in a configuration suggesting exocytosis. Specific apoB label was regularly seen within intercellular spaces and capillaries, but the in vivo significance of this Localization was problematical. These observations suggest that apoB is synthesized in RER, transfers to SER where it is incorporated into new VLDL and chylomicrons, and moves to Golgi cisternae and vesicles to be prepared for exocytosis through the plasma membrane.     

Plasma membrane specializations in the cells of the kidney distal segment of the lamprey, Lampetra japonica (von Martens)

The unique and highly specialized structural features of the plasma membrane in the cells of the kidney distal segment of the lamprey, Lampetra japonica, were studied by electron microscopy. The cells of the distal segment are largely filled by a continuous network of cytoplasmic tubules which are derived from the basolateral plasma membrane. Thin sections and freeze-fracture replicas of the membrane of the cytoplasmic tubules show spirally wound parallel rows of particles. The rows are approximately 17 nm apart and are wound at a pitch of approximately 45 degrees with respect to the major axis of the tubules. Another type of membrane specialization was found in the freeze-cleaved surface of the basolateral plasma membrane. It consists of large square aggregations of membrane particles containing 100-400 cuboidal subunits. The distribution of these particles in this cell, as well as in other systems in which they have been noted, suggests a polarization of membrane activity.