Morphology and histology of the swimbladder and infrastructure of respiratory epithelium in the air-breathing catfish, Pangasius sutchi (Pangasiidae)

The swimbladder of Pangasius sutchi is made up of fibrosa, collagenic fibre walls and mucosa; its walls extend into the lumen to form dense respiratory alveoli, with the inner surface covered by a highly vascularized respiratory epithelium. The thin epithelial cells have the structural characteristics and function of type I and type II cells of lung alveoli in higher mammals. These cells and the endothelial cells compose the barrier through which gases must pass in the exchange between blood and air. The study shows that the swimbladder of P. sutchi is an important accessory respiratory organ.     

Pereopodal disk: a new type of extrabranchial ion-transporting organ in an estuarine amphipod, Melita setiflagella (crustacea)

A discoid organ, 'pereopodal disk (PD)', was found on the medial surface of the basipodite of each pereopod, except the third and the fourth, in an estuarine amphipod, Melita setiflagella. The silver methods showed that PD is an extrabranchial ion-permeable area of the body surface. The ultrastructural study revealed that PD is covered by a thin and soft cuticle layer suggesting high permeability to gases and ions, and is composed of a thick, transporting-type epithelium. This epithelium is characterized by deep basal infolding systems (BIS) of cell membranes exceeding two-thirds of the epithelial thickness and complicated interdigitations between adjacent epithelial cells, both associated with many mitochondria. Apical infolding systems (AIS) are shallow and not accompanied by any mitochondria. These characteristics resemble those of the sternal epithelia and form a striking contrast in the polarity of the infoldings to the gill epithelia, which are characterized by a well-developed AIS and sparse BIS. The results suggest that this unique organ may be involved in the active transport of electrolytes to maintain constant osmotic pressures of the body fluids under widely fluctuating salinities of the estuarine environments.     

Ultrastructural features of chloride cells in the gill epithelium of the Atlantic salmon, Salmo salar, and their modifications during smoltification

To elucidate the ultrastructural modifications of the gill epithelium during smoltification, gills of the Atlantic salmon (Salmo salar) were examined by electron microscopy at three stages of this process, which were defined as follows: "parrs" were freshwater fish that had not yet started their transformation; "freshwater smolts" were freshwater fish that were ready to enter seawater; and "seawater smolts" were smolts that had been transferred from fresh water and maintained for 4 days in seawater (35%). In the gill epithelium of parrs, there were two types of chloride cells. The large chloride cells contained deeply stained mitochondria and numerous apical, irregular, dense, membrane-bound bodies that formed 77% of the chloride cell population and were distinguished easily from small chloride cells that have distinctly paler mitochondria and no dense bodies in their apical cytoplasm. In freshwater smolts, the large chloride cells formed 95% of the chloride-cell population. In contrast to the small chloride cells that were not modified, they almost doubled in size. Their tubular system developed extensively to form a tight network with regular meshes significantly smaller than those observed in parr chloride cells. Forty percent of the large chloride cells were associated with a new type of cell, the accessory cell, to which they were bound by shallow apical junctions. Half of these accessory cells were not seen to be in contact with the external medium. In seawater smolts, 80% of the large chloride cells were associated with accessory cells. Most accessory cells reached the external medium and sent numerous cytoplasmic interdigitations within the apical portion of the adjacent chloride cells. As a result, a section through the apical portion of the chloride cells and their associated accessory cells revealed a mosaic of interlocked cell processes bound together by an extended, shallow apical junction. It was concluded that the Atlantic salmon develops in fresh water most of the ultrastructural modifications of the gill epithelium which in most euryhaline fish are triggered by exposure to seawater. The effective transfer into seawater would act only as a final stimulus to achieve some adequacy between the freshwater smolt and its new environment.     

Brush-like cells within bronchial epithelia of chicken lung (Gallus gallus)

The secondary and primary (mesobronchus) bronchi of chicken lung are lined by a typical respiratory epithelium: pseudostratified columnar ciliated with goblet cells. Up to date, four constituting epithelial cell types have been identified: ciliated, mucosecretory, basal and endocrine cells. In this study a putative new epithelial cell type, the brush-like cell, is described. The avian brush-like cells have only been found in the bronchial epithelia but never in the gas-exchange areas. They are scattered among the other epithelial cells, mainly ciliated cells, and their number is extremely low. The characteristic morphological feature of these cells is an apical protruding cytoplasm with microvilli. This cell type is similar to that found in the lung of some mammalian and non-mammalian species. The functional role of these cells is not yet clear; they could carry out absorptive processes.     

Ultrastructural features of mitochondria-rich cells in stenohaline freshwater and seawater fishes

In order to elucidate the functional significance of accessory cells in freshwater fishes, such as the rainbow trout, which displays a poor adaptability to seawater life, a search for such cells was performed in two stenohaline freshwater fishes: the loach and the gudgeon. Accessory cells were never encountered in these species; but, in contrast, two types of chloride cells were observed consistently that strikingly resembled the alpha- and beta-cells previously described in the guppy, a freshwater-adapted euryhaline fish. The alpha-cell, a pale and elongated chloride cell, was located at the base of the secondary lamellae in close contact with the arterioarterial pillar capillary. Darker, ovoid chloride cells resembling the beta-cell were found exclusively in the interlamellar region of the primary epithelium facing the central venous sinous. The latter cells frequently formed multicellular complexes linked together by deep, narrow, apical junctions. In another experiment, a stenohaline seawater fish, the turbot, was adapted to diluted 5% saltwater and to fresh water. In seawater, the gill epithelium contained only one type of chloride cell, always associated with accessory cells. Due to numerous cytoplasmic interdigitations between the accessory cells and the apical portion of the chloride cell, there was a noticeable increase in the length of the shallow apical junction, sealing off the intercellular space between the two cell types. In 5% saltwater, there was a decrease in the number of these interdigitations and a concomitant decrease in the length of the shallow apical junction. In fresh water, chloride cells were partially or completely separated from the outside medium by modified accessory cells. It is thus concluded that accessory cells are found exclusively in fish living in seawater or preadapted to seawater and that they probably are involved in the formation and modulation of paracellular pathways for ionic excretion. In contrast, the respective roles of the two types of chloride cells observed in freshwater fishes are still to be determined.     

The fine structure of the intra-uterine epithelium during late gestation in the blue shark,Prionace glauca

The fine structure of the intra-uterine epithelium of the pregnant blue shark,Prionace glauca, was examined. The intra-uterine epithelium was bilaminar and the underlying epithelial cell was extremely reduced in cytoplasm. Two cytological characteristics were shown in the outer epithelial cell; open inter-cellular spaces closed in the apical portion by a junction complex and, numerous mitochondria distributed in the basal and lateral portions of the cytoplasm. Secretive characteristics were not recognized in the outer epithelial cell, although few regions composed only of mucous cells were seen. The flattened endothelium of the capillary lay closely beneath the epithelium. These structures are thought to facilitate the water-solute transport and gaseous exchange. It suggests that the intra-uterine epithelium is involved in the osmoregulation of the uterine fluid and the exchange of respiratory gases between mother and fetus.     

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.     

Immuno- and lectin histochemistry of epithelial subtypes and their changes in a radiation-induced lung fibrosis model of the mini pig

Cell types of lung epithelia of mini pigs have been studied using a panel of monoclonal and polyclonal antibodies against cytokeratins (CKs) and vimentin and three lectins before and after radiation-induced fibrosis. In normal tissues, CK18 specific antibodies reacted above all with type II alveolar epithelial cells, while CK7 and pan CK-specific antibodies stained the whole alveolar epithelium. In bronchial epithelial cells, CKs 7, 8, 18 and focally CKs 4 and 13 as well as vimentin were found. Cell specificity of the CK pattern was confirmed by double label immunofluorescence using type II cell-specific Maclura pomifera (MPA) lectin, type I cell specific Lycopersicon esculentum (LEA) lectin and capillary endothelium-binding Dolichos biflorus (DBA) lectin. In experimental pulmonary fibrosis, enhanced coexpression of CK and vimentin was observed in bronchial epithelium. Subtypes of alveolar epithelial cells were no longer easily distinguishable. CK18 was found to be expressed in the entire alveolar epithelium. The gradual loss of the normal alveolar epithelial marker, as seen by the binding of MPA to type I-like cells, of LEA to type II-like cells and the partial loss of MPA-binding to type II cells, was paralleled by the appearance of CK4, typical for squamous epithelia, and the occurrence of DBA-binding in epithelial cells. Implications of these results for general concepts of intermediate filament protein expression and lectin binding in the fibrotic process are discussed.     

Secretion of collagen types I and II by epithelial and endothelial cells in the developing chick cornea demonstrated by in situ hybridization and immunohistochemistry

Cells involved in the synthesis of collagen types I and II in the cornea of developing chick embryos have been studied by using in situ hybridization and immunohistochemistry. Corneas processed for in situ hybridization with the type I and II collagen probes demonstrated specific mRNAs in the epithelium of embryos at stage 18 with an increase at stages between 26 and 31, and then gradual decrease to the background level in the next several days. In the endothelium, a small amount of specific mRNA was recognized through these stages. In the stroma, only sections hybridized with the type I probe demonstrated mRNA in fibroblasts. Immunostaining demonstrated specific collagen types in the stroma at sites which were closely associated with cells containing specific mRNAs. Both collagens type I and II were present beneath the epithelium as narrow bands at stage 18; as the thicker primary stroma at stages 20 and 26; and as subepithelial, subendothelial and stromal staining at stage 31. Thereafter, type I collagen was increased in the stroma but it was also noted in the subepithelial and, to a lesser degree, subendothelial regions, whereas type II collagen was gradually confined to the subendothelial matrix. Electron microscopic examination of sections from 5-day-old (stage-27) embryo corneas using antibodies against the carboxyl propeptides of type I and II procollagens revealed the presence of these procollagens within the cisternae of the endoplasmic reticulum and Golgi vesicles in both epithelial and endothelial cells. In the epithelial cells both the periderm and basal cells contained these procollagens within the cytoplasmic organelles. These results indicate that not only the epithelial cells, but also the endothelial cells secrete collagen types I and II during the formation of the primary corneal stroma and for several days after invasion of fibroblasts.     

Allogeneic Human Mesenchymal Stem Cells Restore Epithelial Protein Permeability in Cultured Human Alveolar Type II Cells by Secretion of Angiopoietin-1

Acute lung injury is characterized by injury to the lung epithelium that leads to impaired resolution of pulmonary edema and also facilitates accumulation of protein-rich edema fluid and inflammatory cells in the distal airspaces of the lung. Recent in vivo and in vitro studies suggest that mesenchymal stem cells (MSC) may have therapeutic value for the treatment of acute lung injury. Here we tested the ability of human allogeneic mesenchymal stem cells to restore epithelial permeability to protein across primary cultures of polarized human alveolar epithelial type II cells after an inflammatory insult. Alveolar epithelial type II cells were grown on a Transwell plate with an air-liquid interface and injured by cytomix, a combination of IL-1β, TNFα, and IFNγ. Protein permeability measured by ¹³¹I-labeled albumin flux was increased by 5-fold over 24 h after cytokine-induced injury. Co-culture of human MSC restored type II cell epithelial permeability to protein to control levels. Using siRNA knockdown of potential paracrine soluble factors, we found that angiopoietin-1 secretion was responsible for this beneficial effect in part by preventing actin stress fiber formation and claudin 18 disorganization through suppression of NFκB activity. This study provides novel evidence for a beneficial effect of MSC on alveolar epithelial permeability to protein.     

Fine structure of the corpuscles of stannius in the toadfish.

The micro-anatomy of the corpuscles of Stannius of the toadfish, Opsanus tau, an aglomerular marine teleost, has been studied by light and electron microscopy. The corpuscles are composed of extensively anastomosed cords of epithelial cells which maintain intimate contact with blood capillaries. Most of the epithelial cells contain acidophilic granules which also show a positive reaction with the periodic acid-Schiff technique and aldehyde fuchsin. On the basis of fine structural criteria, three cell types can be recognized. The granular cells contain abundant quantities of granular endoplasmic reticulum, ribosomes, Golgi apparatus with prosecretory granules, coated vesicles, polymorphic mitochondria with lamellar cristae, filaments, microtubules, a cilium, a variety of lysosome-like dense bodies, glycogen particles, lipid droplets, secretory granules and intranuclear lipid-like inclusions. One variety of agranular cell (type I) is characterized by the total absence of secretory granules, but it contains large amounts of granular endoplasmic reticulum and ribosomes, conspicuous profiles of Golgi apparatus, coated vesicles and sometimes an abundance of glycogen. Another variety of agranular cell (type II) has poorly developed cytoplasmic organelles. The perivascular space between the capillary and parenchyma contains connective tissue cells and abundant nerve fibers. The different types of epithelial cells observed in the corpuscles of Stannius of this fish may represent functional stages of the secretory cycle in a single cell type.     

Accessory cells in the gill epithelium of the freshwater rainbow trout Salmo gairdneri

Two types of mitochondria-rich cells were identified in the gill epithelium of the freshwater-adapted rainbow trout, Salmo gairdneri, after selective impregnation of their tubular system with reduced osmium. A first type consisted of large cells with a poorly developed and loosely anastomosed tubular system; thus, that resembled the chloride cells commonly encountered in the gill epithelium of freshwater-adapted euryhaline fishes. A second type comprised smaller cells with an extensively developed and tightly anastomosed tubular system. These never reached the basal lamina of the gill epithelium and were adjacent to chloride cells, to which they were linked by shallow apical junctions (100-200 nm); thus, they resembled accessory cells, which are currently found in the gill epithelium of seawater-adapted fishes but are usually lacking in freshwater living fishes. Transfer of the freshwater-adapted trout into seawater induced the proliferation of the tubular system in the chloride cells and the formation of lateral plasma membrane interdigitations between accessory cells and the apical portion of the chloride cells. The length of the apical junction sealing off this extended intercellular space was reduced to 20-50 nm. The tubular system of the accessory cells was not modified. The extension of the tubular system in the chloride cells of the seawater-adapted fishes indicated that, as in most euryhaline fishes, these cells have a role in the adaptation of the rainbow trout to seawater. In contrast, the function of the presumptive accessory cells in freshwater trout remains to be established.     

Ultrastructure of the endolymphatic sac in the mouse.

The ultrastructure of the endolymphatic sac (ES) in the mouse was examined by light and electron microscopy. This organ was divided into three parts: proximal, intermediate and distal. In the proximal portion of the ES, the epithelium consisted of thin squamous cells. The epithelial cells had acquired basolateral processes, numerous small vesicles and well-developed Golgi apparatus. In the intermediate portion, the epithelium consisted of columnar or cuboidal cells. The epithelial cells could be classified into two types: type I and type II. The type I cells had abundant microvilli, pinocytotic vesicles, vacuoles, multivesicular bodies, lysosomes and mitochondria. The type II cells had fewer numbers of these organelles. A few free-floating cells could be observed in the lumen of this intermediate portion, most of which were macrophages. In the distal portion, the epithelium consisted of squamous or cuboidal cells. The epithelial cells had a few cytoplasmic organelles. In the ultrastructural study, each portion of the mouse ES was found to have a very distinct morphological feature. It was suggested that each of these three portions has a different function.     

Ultrastructure of the lung of the rattlesnake,Crotalus viridis oreganus

Scanning and transmission electron microscopic observations were made on the rattlesnake lung, which has the form of a cigar-shaped bag enclosing a large axial air chamber. The lungs were fixed by tracheal instillation of fixative to preserve the structural features of inflated lungs. An open tracheal groove along the ventral aspect of the lung is the only structural “airway” present. The wall of the lung has two histologically distinct regions: anteriorly, a respiratory portion, where up to three generations of septa subdivide the wall into cup-shaped gas-exchange chambers, termed faveoli; and posteriorly, a simple, thin-walled saccular portion. The epithelium lining the internal surface of the lung is composed of several cell types: (1) ciliated cells; (2) type I pneumonocytes; (3) type II pneumonocytes, secretory cells characterized by the presence of lamellar bodies; and (4) serous epithelial cells, secretory cells characterized by the presence of homogeneous, densely staining secretory granules. However, the distinctiveness of the secretory cell types in the snake lung is blurred because intermediate-appearing cells have both the lamellar body and homogenous type of secretory granule. The nonepithelial components of the pulmonary wall and septa consist of blood vessels and lymphatics, smooth muscle cells and fibroblasts, embedded in a matrix of extracellular connective tissue fibers. Tubular myelin figures were observed in the faveolar lining layer.     

A reexamination of the epithelial sensory cells of amphioxus (Branchiostoma)

The rostral epithelium of a newly metamorphosed juvenile of Branchiostoma floridae was examined at the EM level to confirm previous reports on its sensory cells. The majority of the sensory cells are of three types: two type I variants, with simple collars of unbranched microvilli surrounding their cilia, and one kind of type II cell, with an extended collar of repeatedly branched microvilli. The two type I variants differ in the structure and arrangement of the microvilli, basal body and rootlet, and the length of the cilium. Both variants are probably primary sensory cells (i.e. each has its own axon), but the data supporting this conclusion are much better for one variant than for the other. Type II cells are secondary sensory cells, with synaptic terminals borne on short extensions of the cell body. The presence of degenerating type II cells suggests that they may be subject to a regular process of loss and renewal. The results do not resolve the evolutionary issue of how amphioxus sensory cells relate to the epithelial sensory and receptor cells of vertebrates. Being primary, the type I cells resemble the supposed ancestral type more closely than do type II cells. Type II cells may be chemosensory, however, and should not be ruled out a priori as possible homologues of either primary or secondary chemosensory cells in vertebrates.     

Characterization of epithelial domains in the nasal passages of chick embryos: spatial and temporal mapping of a range of extracellular matrix and cell surface molecules during development of the nasal placode

The formation of the nasal passages involves complex morphogenesis and their lining develops a spatially ordered pattern of differentiation, with distinct domains of olfactory and respiratory epithelium. Using antibodies to the neural cell adhesion molecule (N-CAM), keratan sulphate and heparan sulphate proteoglycan (HSPG) and a panel of lectins (agglutinins of Canavalia ensiformis (ConA), Dolichos biflorus (DBA), peanut (PNA), Ricinis communis (RCA1), soybean (SBA), Ulex europaeus (UEA1), and wheatgerm (WGA], we have documented cell surface characteristics of each epithelial domain. Binding of antibodies to N-CAM and to keratan sulphate, and the lectins ConA, PNA, RCA1, SBA and WGA marks the olfactory epithelial domain only. The restriction of N-CAM to the sensory region of the epithelium has also been reported in the developing ear. This striking similarity is consistent with the idea that N-CAM may be involved in the division of functionally and histologically distinct cell groups within an epithelium. We traced the olfactory-specific cell markers during development to gain insights into the origin of the epithelial lining of the nasal passages. All reagents bind at early stages to the thickened nasal placode and surrounding head ectoderm and then become progressively restricted to the olfactory domain. The expression of these characteristics appears to be modulated during development rather than being cell autonomous. The distribution of keratan sulphate was compared with collagen type II in relation to the specification of the chondrocranium. Keratan sulphate and collagen type II are only colocalized at the epithelial-mesenchymal interface during early nasal development. At later stages, only collagen type II is expressed at the interface throughout the nasal passages, whereas keratan sulphate is absent beneath the respiratory epithelium.     

Heterocellular cultures of pulmonary alveolar epithelial cells grown on laminin-5 supplemented matrix

The pulmonary alveolar epithelium consists of alveolar type I (AT1) and alveolar type II (AT2) cells. Interactions between these two cell types are necessary for alveolar homeostasis and remodeling. These interactions have been difficult to study in vitro because current cell culture models of the alveolar epithelium do not provide a heterocellular population of AT1 and AT2 cells for an extended period of time in culture. In this study, a new method for obtaining heterocellular cultures of AT1- and AT2-like alveolar epithelial cells maintained for 7 d on a rat tail collagen-fibronectin matrix supplemented with laminin-5 is described. These cultures contain cells that appear by their morphology to be either AT1 cells (larger flattened cells without lamellar bodies) or AT2 cells (smaller cuboidal cells with lamellar bodies). AT1-like cells stain for the type I cell marker aquaporin-5, whereas AT2-like cells stain for the type II cell markers surfactant protein C or prosurfactant protein C. AT1/AT2 cell ratios, cell morphology, and cell phenotype-specific staining patterns seen in 7-d-old heterocellular cultures are similar to those seen in alveoli in situ. This culture system, in which a mixed population of phenotypically distinct alveolar epithelial cells are maintained, may facilitate in vitro studies that are more representative of AT1-AT2 cell interactions that occur in vivo.     

Ultrastructure of the Malpighian Tubule Cells in the Mosquito Larvae, Anopheles sinesis

Ultrastructure of epithelial cells constituting the Malpighian tubule of Anopheles sinesis last instar larvae was observed with electron microscope. Malpighian tubule consists of four long and narrow tubule structures with principal cells in typical absorptive cells and regenerative cells forming the simple epithelium. Apical plasma membrane of the principal cell is differentiated into microvilli with one mitochondrion in each microvilli. Basal plasma membrane had extreme infolding to form a canaliculi and a well developed mitochondria was attached in the infoldings. And, rER, ribosomes, and vacuoles were well developed inside the cells. However, there were two main cell types depending on the differentiation of cell organelles. Type 1 cell was cubic, forming the distal portion of Malpighian tubule. The length of microvilli was approximately 4 μm and the basal infoldings were introjected to the depth of 2 μm inside the cell. On the other hand, Type II cell that formed the main proxinal portion was a low squamous type cells with shorter 2 μm of microvilli and the basal infoldings were introjected to the depths of 4 μm inside the cell. As for vacuoles scattered inside the cells, they were regularly observed in both Type I and II and the Type II cells had better developed cellular organelles. Although regenerative cells were extremely small, their cellular organelles were developed and their overall electron density was high that they appeared darker than the principal cells.     

Choanocyte-like cells in the digestive system of the starfish Marthasterias glacialis (Echinodermata)

Two types of choanocyte-like cells have been found in the digestive tract of the starfish. Type I choanocytes are in the lining epithelium of all organs of the digestive system. These are narrow, columnar cells strongly anchored basally and expanded apically into a protuberance projecting into the lumen. A prominent flagellum surrounded by microvilli projects from the center of this protuberance. Apical cytoplasm contains numerous mitochondria, secondary lysosomes, and multivesicular bodies. A distinctive characteristic of these cells is a filament bundle that traverses the length of the cell from its region of attachment on the rootlet of the flagellar basal body to its terminus on the basal plasma membrane. Between the attenuated basal ends of type I cells are the nerve fibers of an intraepithelial nerve plexus. Thickness of the plexus is correlated with the quantity of type I cells in the epithelium. Type II choanocytes are in the cuboidal coelomic epithelium that forms the outer layer of digestive tract organs. These cells are smaller than those of type I, and they have an apical collar surmounted by a ring of 13 microvilli. Within the collar is a cup-shaped depression with a central flagellum. Coated vesicles, secondary lysosomes, and phagocytic infoldings are observed in and near the collar cytoplasm. Filament bundles similar to those in type I choanocytes are also observed in coelomic epithelial cells that are sufficiently tall. Injection of peroxidase into the stomach and ferritin into the coelom results in phagocytic uptake of these macromolecules by type I and type II choanocytes, respectively.