ber den Feinbau der Schwimmblase von Anguilla vulgaris L.: Licht- und elektronenmikroskopische Untersuchungen

The rete mirabile: a possible control site for swimbladder function

In a recent study, a large number of transport proteins was detected in the transcriptome and proteome of saline perfused rete mirabile tissue of the European eel. In this study, the data set was reanalyzed for the presence of receptor proteins and proteins involved in intracellular signaling pathways. A large number of expressed receptor proteins and proteins involved in intracellular signal transduction was detected. Several G-protein-coupled receptor signal pathways were significantly enriched in their expression level, in particular receptors and signaling pathways involved in the control of blood flow. The enriched signaling pathways also include pathways involved in trafficking of crucial transport proteins like, monocarboxylate transporters, V-ATPase, and aquaporin. The data, therefore, suggest that the rete mirabile has the capacity to control swimbladder function by regulating blood flow and by modifying countercurrent multiplication.

Oxygen consumption and acid secretion in isolated gas gland cells of the European eel Anguilla anguilla

Swimbladder gas gland cells are known to produce lactic acid required for the acidification of swimbladder blood and decreasing the oxygen carrying capacity of swimbladder blood, i.e., the onset of the Root effect. Gas gland cells have also been shown to metabolize glucose via the pentose phosphate shunt, but the role of the pentose phosphate shunt for acid secretion has not yet been evaluated. Similarly, aerobic metabolism of gas gland cells has been largely neglected so far. In the present study, we therefore simultaneously assessed the role of glycolysis and of the pentose phosphate shunt for acid secretion and recorded oxygen consumption of isolated swimbladder gas gland cells of the European eel. Presence of glucose was essential for acid secretion, and at glucose concentrations of about 1.5 mmol l-1 acid secretion of gas gland cells reached a maximum, indicating that glucose concentrations in swimbladder blood should not be limiting acid production and secretion under physiological conditions. The data revealed that most of the acid was produced in the glycolytic pathway, but a significant fraction was also contributed by the pentose phosphate shunt. Addition of glucose to gas gland cells incubated in a glucose-free medium resulted in a reduction of oxygen uptake. Inhibition of mitochondrial respiration significantly reduced oxygen consumption, but a fraction of mitochondria-independent respiration remained in presence of rotenone and antimycin A. In the presence of glucose, application of either iodo-acetate inhibiting glycolysis or 6-AN inhibiting the pentose phosphate shunt did not significantly affect oxygen uptake, indicating an independent regulation of oxidative phosphorylation and of acid production. Inhibition of the muscarinic acetylcholine receptor caused a slight elevation in acid secretion, while forskolin caused a concentration-dependent reduction in acid secretion, indicating muscarinic and c-AMP-dependent control of acid secretion in gas gland cells.

Using the swimbladder as a respiratory organ and/or a buoyancy structure-Benefits and consequences

A swimbladder is a special organ present in several orders of Actinopterygians. As a gas-filled cavity it contributes to a reduction in overall density, but on descend from the water surface its contribution as a buoyancy device is very limited because the swimbladder is compressed by increasing hydrostatic pressure. It serves, however, as a very efficient organ for aerial gas exchange. To avoid the loss of oxygen to hypoxic water at the gills many air-breathing fish show a reduced gill surface area. This, in turn, also reduces surface area available for other functions, so that breathing air is connected to a number of physiological adjustments with respect to ion homeostasis, acid-base regulation and nitrogen excretion. Using the swimbladder as a buoyancy structure resulted in the loss of its function as an air-breathing organ and required the development of a gas secreting mechanism. This was achieved via the Root effect and a countercurrent arrangement of the blood supply to the swimbladder. In addition, a detachable air space with separated blood supply was necessary to allow the resorption of gas from the swimbladder. Gas secretion as well as gas resorption are slow phenomena, so that rapid changes in depth cannot instantaneously be compensated by appropriate volume changes. As gas-filled cavities the respiratory swimbladder and the buoyancy device require surfactant. Due to high oxygen partial pressures inside the bladder air-exposed tissues need an effective reactive oxygen species defense system, which is particularly important for a swimbladder at depth.

A mechanical approach to understanding the impact of the nematode Anguillicoloides crassus on the European eel swimbladder

One of the most detrimental factors in the drastic decline of the critically endangered European eel (Anguilla anguilla) was the inadvertent introduction of the invasive nematode Anguillicoloides crassus Infection primarily affects the swimbladder, a gas-filled organ that enables the eel to control its depth in the water. A reduction in swimbladder function may be fatal for eel undergoing their spawning migration to the Sargasso Sea, a journey of over 5000 km. Although the physiological damage caused by this invasive parasite is well studied through the use of quantifiable gross pathological indices, providing a good measure of the swimbladder health status, they cannot separate the role of mechanical and morphological damage. Our study examined the appropriateness of three commonly used indices as a measure of mechanical damage by performing uniaxial tensile tests on swimbladder specimens obtained from an infected eel population. When the test results were compared with the gross pathological indices it was found that thickness correlated most strongly with mechanical damage, both confirming and, more importantly, explaining the counterintuitive findings of earlier work. In a damaged swimbladder, the immune response leads to a trade-off; increasing wall thickness raises the pressure required for organ rupture but decreases strength. The results indicate that for moderate infection the mechanical integrity of the swimbladder can be maintained. For severe infection, however, a reduction in mechanical integrity may reach a tipping point, thereby affecting the successful completion of their oceanic migration.

Anguillicola crassus Infection Significantly Affects the Silvering Related Modifications in Steady State mRNA Levels in Gas Gland Tissue of the European Eel

Using Illumina sequencing, transcriptional changes occurring during silvering in swimbladder tissue of the European eel have been analyzed by comparison of yellow and silver eel tissue samples. Functional annotation analysis based on GO terms revealed significant expression changes in a number of genes related to the extracellular matrix, important for the control of gas permeability of the swimbladder, and to reactive oxygen species (ROS) defense, important to cope with ROS generated under hyperbaric oxygen partial pressures. Focusing on swimbladder tissue metabolism, levels of several mRNA species encoding glucose transport proteins were several-fold higher in silver eels, while enzymes of the glycolytic pathway were not affected. The significantly higher steady state level of a transcript encoding for membrane bound carbonic anhydrase, however, suggested that CO2 production in the pentose phosphate shunt and diffusion of CO2 was of particular importance in silver eel swimbladder. In addition, the mRNA level of a large number of genes related to immune response and to sexual maturation was significantly modified in the silver eel swimbladder. The modification of several processes related to protein metabolism and transport, cell cycle, and apoptosis suggested that these changes in swimbladder metabolism and permeability were achieved by increasing cell turn-over. The impact of an infection of the swimbladder with the nematode Anguillicola crassus has been assessed by comparing these expression changes with expression changes observed between uninfected yellow eel swimbladder tissue and infected silver eel swimbladder tissue. In contrast to uninfected silver eel swimbladder tissue, in infected tissue the mRNA level of several glycolytic enzymes was significantly elevated, and with respect to extracellular matrix, several mucin genes were many-fold higher in their mRNA level. Modification of many immune related genes and of the functional categories "response to DNA damage stimulus" and "cellular response to stress" illustrated the damaging effect of the nematode infection. This study has identified a range of cellular processes in the swimbladder of silver eels that appear to be altered by nematode infection. These altered cellular processes could contribute to detrimental changes in swimbladder function that, in turn, may lead to impairment of spawning migration.

Swimbladder function and the spawning migration of the European eel Anguilla anguilla

The spawning migration of the European eel is an extensive journey over 5000 to 7000 km from the European coast to the Sargasso Sea. Eels do not feed during this journey and on-board fuels must be sufficient to support the journey of 3.5 to 6 month, as well as sexual maturation and the spawning activity. Swimming of eels appears to be quite energy efficient compared to other fish species, and elevated hydrostatic pressure has been shown to even reduce the costs of transport. Recent studies revealed, however, that during traveling eels perform extensive diurnal migrations and swim at a depth of about 100-300 m at night time, but go down to 600-1000 m at day time. At a depth of 200 m eels are exposed to a hydrostatic pressure of 21 atmospheres (2.13 MPa), while at 800 m hydrostatic pressure increases to 81 atmospheres (8.21 MPa). Accordingly, without any compensation at a depth of 800 m swimbladder volume will be reduced to about 25% of the volume established with neutral buoyancy at 200 m. Consequently, these diurnal changes in depth must be taken into consideration for a calculation of the energy requirements of the spawning migration. Without compensation a compression of the swimbladder will result in a status of negative buoyancy, which makes swimming more costly. Trying to keep the status of neutral buoyancy during descent by gas secretion into the swimbladder in turn requires metabolic activity to enhance swimbladder perfusion and for acid production of the gas gland cells to stimulate gas secretion. During ascent gas is passively removed from the swimbladder in the resorbing section and in the blood transported to the gills, where it is lost into the water. Accordingly, the swimbladder appears to be a crucial organ for the spawning migration. It can be assumed that an impairment of swimbladder function for example due to an infection with the nematode Anguillicola crassus significantly threatens the success of the spawning migration.

Detection without deflection? A hypothesis for direct sensing of sound pressure by hair cells

It is widely thought that organisms detect sound by sensing the deflection of hair-like projections, the stereocilia, at the apex of hair cells. In the case of mammals, the standard interpretation is that hair cells in the cochlea respond to deflection of stereocilia induced by motion generated by a hydrodynamic travelling wave. But in the light of persistent anomalies, an alternative hypothesis seems to have some merit: that sensing cells (in particular the outer hair cells) may, at least at low intensities, be reacting to a different stimulus - the rapid pressure wave that sweeps through the cochlear fluids at the speed of sound in water. This would explain why fast responses are sometimes seen before the peak of the travelling wave. Yet how could cells directly sense fluid pressure? Here, a model is constructed of the outer hair cell as a pressure vessel able to sense pressure variations across its cuticular pore, and this 'fontanelle' model, based on the sensing action of the basal body at this compliant spot, could explain the observed anomalies. Moreover, the fontanelle model can be applied to a wide range of other organisms, suggesting that direct pressure detection is a general mode of sensing complementary to stereociliar displacement.

pH regulation and swimbladder function in fish

Gas gland cells of the European eel (Anguilla anguilla) are specialized for the production and secretion of acidic metabolites. Although typically exposed to high oxygen partial pressures, they convert glucose mainly into lactate, but also produce CO2 in the pentose phosphate shunt. Only a very small fraction of glucose is oxidized via aerobic metabolism. Although the buffer capacity of gas gland cells appears to be high, even at low extracellular pH values intracellular pH is always kept about 0.2-0.3 pH-units more acidic. Thus, under all physiological conditions proton concentration within gas gland cells is higher than in the extracellular fluid, facilitating proton extrusion. Diffusion of CO2, Na+/H+-exchange, sodium-dependent anion exchange and a V-ATPase represent the pathways available for proton secretion. While under resting conditions the sodium-dependent pathways and diffusion of CO2 appear to be the dominating mechanisms for acid secretion, at low intracellular pH the contribution of Na+/H+-exchange and of V-ATPase appear to increase, while sodium-dependent anion exchange becomes less important. The mechanisms regulating the activity of these acid-secreting pathways and of the metabolism responsible for the production of protons are largely unknown.

The Origin and Evolution of the Surfactant System in Fish: Insights into the Evolution of Lungs and Swim Bladders

Several times throughout their radiation fish have evolved either lungs or swim bladders as gas-holding structures. Lungs and swim bladders have different ontogenetic origins and can be used either for buoyancy or as an accessory respiratory organ. Therefore, the presence of air-filled bladders or lungs in different groups of fishes is an example of convergent evolution. We propose that air breathing could not occur without the presence of a surfactant system and suggest that this system may have originated in epithelial cells lining the pharynx. Here we present new data on the surfactant system in swim bladders of three teleost fish (the air-breathing pirarucu Arapaima gigas and tarpon Megalops cyprinoides and the non-air-breathing New Zealand snapper Pagrus auratus). We determined the presence of surfactant using biochemical, biophysical, and morphological analyses and determined homology using immunohistochemical analysis of the surfactant proteins (SPs). We relate the presence and structure of the surfactant system to those previously described in the swim bladders of another teleost, the goldfish, and those of the air-breathing organs of the other members of the Osteichthyes, the more primitive air-breathing Actinopterygii and the Sarcopterygii. Snapper and tarpon swim bladders are lined with squamous and cuboidal epithelial cells, respectively, containing membrane-bound lamellar bodies. Phosphatidylcholine dominates the phospholipid (PL) profile of lavage material from all fish analyzed to date. The presence of the characteristic surfactant lipids in pirarucu and tarpon, lamellar bodies in tarpon and snapper, SP-B in tarpon and pirarucu lavage, and SPs (A, B, and D) in swim bladder tissue of the tarpon provide strong evidence that the surfactant system of teleosts is homologous with that of other fish and of tetrapods. This study is the first demonstration of the presence of SP-D in the air-breathing organs of nonmammalian species and SP-B in actinopterygian fishes. The extremely high cholesterol/disaturated PL and cholesterol/PL ratios of surfactant extracted from tarpon and pirarucu bladders and the poor surface activity of tarpon surfactant are characteristics of the surfactant system in other fishes. Despite the paraphyletic phylogeny of the Osteichthyes, their surfactant is uniform in composition and may represent the vertebrate protosurfactant.

Swimbladder gas gland cells cultured on permeable supports regain their characteristic polarity

A cell culture system has been developed in which swimbladder gas gland cells from the European eel (Anguilla anguilla) were cultured on a permeable support. Cells seeded on Anodisc 13 (Whatman) or Costar Transwell 13 mm membranes form a confluent cell layer within the first 2 or 3 days of culture but, on the basis of measurements of transepithelial resistance, it is a ‘leaky’ cell layer. In a superfusion system, the apical and basal sides of the cells were superfused asymmetrically, with saline on the apical side and a glucose-containing cell culture medium on the basal side. Under these conditions, the cells continuously produced lactic acid, and approximately 60–70 % of this lactate was released at the basal side. To mimic the in vivo situation, the saline solution supplied to the apical side was replaced by humidified air in an additional series of experiments. Cells cultured in an air/liquid system produced even more lactate, and this lactate was only released to the basal side; there was no leakage of fluid to the apical side. After 4 or 5 days in the superfusion system, the cells were fixed for histological examination. The cells were columnar, similar to gas gland cells in vivo, and showed a clear polarity, with some small microvilli at the apical membrane and extensive membrane foldings at lateral and basal membranes. Immunohistochemical localization of Na+/K+-ATPase revealed that this ATPase was present mainly in the lateral membranes; it was never found in the apical membranes. Cells cultured in the air/liquid system showed a similar structure and polarity.

The comparative biology of pulmonary surfactant: past, present and future

Richard E. Pattle contributed enormously to the biology of the pulmonary surfactant system. However, Pattle can also be regarded as the founding father of comparative and evolutionary research of the surfactant system. He contributed eight seminal papers of the 167 publications we have located on this topic. In particular, Pattle produced a synthesis interpreting the evolution of the surfactant system that formed the foundation for the area. Prepared 25 years ago this synthesis spawned the three great discoveries in the comparative biology of the surfactant system: (1) that the surfactant system has been highly conserved throughout the enormous radiation of the air breathing vertebrates; (2) that temperature is the major selective condition that influences surfactant composition; (3) that acting as an anti-adhesive is one primitive and ubiquitous function of vertebrate surfactant. Here we review the literature and history of the comparative and evolutionary biology of the surfactant system and highlight the areas of comparative physiology that will contribute to our understanding of the surfactant system in the future. In our view the surfactant system is a neatly packaged system, located in a single cell and highly conserved, yet spectacularly complex. The surfactant system is one of the best systems we know to examine evolutionary processes in physiology as well as gain important insights into gas transfer by complex organisms.

Expression of two vacuolar-type ATPase B subunit isoforms in swimbladder gas gland cells of the European eel: nucleotide sequences and deduced amino acid sequences

The poly(A)(+) RNA of swimbladder gas gland cells of the European eel Anguilla anguilla was isolated and used for cDNA synthesis. Using a pair of degenerate PCR primers directed towards the evolutionary highly conserved central part of the B subunit of vacuolar type H(+)-ATPase (V-ATPase) a fragment of 388 bp was amplified. By sequencing the cloned PCR products two different amplicons with a sequence identity of about 86% were obtained. BLASTN searches revealed a high degree of similarity of both to V-ATPase B subunits of other species. The sequences were completed by performing rapid amplification of cDNA ends PCR, subsequent cloning, and sequencing of the obtained products. The expression of two different isoforms of the V-ATPase B subunit is already demonstrated for Homo sapiens and Bos taurus. This is the first report that attributes the same phenomenon to a non-mammalian species, A. anguilla. The first isoform found in eel (vatB2) shows the highest degree of amino acid sequence homology with the human brain isoform (98.2%), the second one (vatB1) with the B subunit sequence of rainbow trout (Oncorhynchus mykiss) gill and kidney (98, 6%). The alignment of the deduced amino acid sequences of vatB1 and vatB2 shows that the highest sequence variation between these two isoforms is found at the amino-terminus, where vatB1 is nine amino acids shorter than vatB2, while at the carboxy-terminus it is two amino acids longer than vatB2. This has also been reported for the human and bovine kidney isoforms when compared with the brain isoforms. Northern blot analysis using specific hybridization probes revealed the expression of two mRNA's with lengths of about 2.9 kb and 3.5 kb for vatB1 and vatB2, respectively. For mammals, it is well known that V-ATPases containing the kidney isoforms of the B subunit are responsible for the extrusion of protons across the plasma membranes of several cell types. The fact that eel vatB1 seems to share structural features with the kidney isoforms in mammals supports the hypothesis that in gas gland cells a V-ATPase contributes to the acidification of the blood in the swimbladder.

Functional morphology of the gas-gland cells of the air-bladder of Oreochromis alcalicus grahami (teleostei: cichlidae): an ultrastructural study on a fish adapted to a severe, highly alkaline environment

Oreochromis alcalicus grahami is a small cichlid fish that lives in shallow peripheral lagoons of Lake Magadi, Kenya. The internal surface of the air-bladder is highly vascularized, illustrating possible utilization as an accessory respiratory organ. The wall of the bladder consists of five distinct tissue layers. From the outer to the inner surfaces are: a squamous, undifferentiated epithelial cell; a collagen-elastic tissue space; a smooth muscle tissue band; an inner connective tissue space; and columnar gas-gland cells projecting into the lumen. The cell membrane over the perikarya of the gas-gland cells was sporadically broken. The disruptions were ascribed to possible physical damage by discharging gas-bubbles. Juxtaluminally, the gas-gland cells attached across tight junctions. The cells have highly euchromatic nuclei and conspicuously large intracytoplasmic secretory bodies. On the blood capillary facing (basal) aspect, the cell membrane of the gas-gland cells is highly amplified. The cells insert onto a smooth muscle tissue band. The morphological features and the topographical arrangement of the gas-gland cells in O. a. grahami are indicative of an operative exchange of materials between them and the underlying tissue components especially the blood capillaries. For a fish that subsists in hot, highly saline and alkaline water heavily invested by avian predators and where the partial pressure of oxygen diurnally shifts from virtual anoxia to hyperoxia, development of a versatile air-bladder for efficient buoyancy control conforms to the functional demands placed on it by a unique environment. Illustratively, instead of the gas-gland morphology in O. a. grahami resembling that in the freshwater fishes, the group from which the fish has evolved, it compares more closely to that of the marine fish. This similarity suggests amazing parallel evolutionary adaptation to biophysically corresponding aquatic milieus.

Localization of carbonic anhydrase in swimbladder of European eel (Anguilla anguilla) and perch (Perca fluviatilis)

The distribution of carbonic anhydrase in swimbladder tissue and especially in gas gland cells of the European eel (Anguilla anguilla) and the perch (Perca fluviatilis) was analysed using histochemical staining according to Hansson (1967), with modifications proposed by Riddersträle (1991). While in the European eel, gas gland cells are distributed as a single layered epithelium over the whole secretory part of the swimbladder, the gas gland of the perch consists of a compact, richly vascularized 'multilayered' epithelium, in which gas gland cells have contact with the swimbladder lumen via small channels. In spite of these differences in organization, membranes of gas gland cells near blood vessels are richly folded in both species. A strong histochemical staining for carbonic anhydrase was observed in these membrane foldings. With prolonged incubation times a positive reaction was also observed in the cytoplasm of gas gland cells. In addition, the vascular endothelium and the erythrocytes showed a positive histochemical reaction. No staining, however, was visible in apical membranes towards the lumen of the swimbladder. In the perch, swimbladder epithelial cells outside the gas gland showed no positive staining of carbonic anhydrase. The results thus indicate that carbonic anhydrase activity is especially concentrated in membranes facing blood vessels. This suggests that a rapid equilibrium of the CO2/HCO3- reaction in the intracellular as well as in the extracellular space is essential for swimbladder function.

Physiology and biochemistry of the pseudobranch: an unanswered question?

The structure and function of the pseudobranch has long interested scientists, but its overall role has remained a mystery. Previous studies have attributed respiratory, endocrine, osmoregulatory and sensory roles to the pseudobranch, and the present review concentrates on new findings. Perfusion experiments on the pseudobranch of the rainbow trout (Oncorhynchus mykiss) using both erythrocyte suspensions and Ringer solution have shown that this organ is able to generate values for the respiratory quotient (RQ) greater than 1.0. The release of carbon dioxide into the perfusate was found to be largely independent of flow between perfusion rates of 120-190 microl/min and could be inhibited by acetazolamide (10(-5) M), indicating a role for carbonic anhydrase. Noradrenaline (10(-5) M) had no effect on oxygen consumption or carbon dioxide release of the pseudobranch. The rate of carbon dioxide release was also dependent on the pH of the pre-pseudobranch perfusate, carbon dioxide release being reduced at lower perfusate pH values. Based on the glucose balance of the isolated saline-perfused rainbow trout pseudobranch and on the enzyme profiles for the rainbow trout, cod, swordfish and deep-water grenadier pseudobranch, it is suggested that the pentose phosphate shunt might be a source of carbon dioxide, yielding the high RQ values found for this organ. Most evidence now available indicates that the pseudobranch is integrally linked with the choroid rete and the supply of oxygen to the retina of the fish eye.

pH-dependent proton secretion in cultured swim bladder gas gland cells

The pH dependence of acid production and of acid release has been analyzed in cultured gas gland cells of the European eel using a cytosensor microphysiometer. Total acid release of gas gland cells showed an optimum at pH 7.4-7.6, with only a minor reduction at acidic (pH 7.0) as well as at alkaline pH (pH 8.0). The acid production was largely dependent on the availability of extracellular glucose and was almost completely abolished if glucose was replaced by succinate, alanine, or even pyruvate. Phloretin, an inhibitor of glucose uptake, significantly reduced acid release of gas gland cells with a Ki of approximately 1 x 10(-5) M, irrespective of pH. Although the glucose dependence of acid production was not modified by pH, acid release became increasingly sodium dependent with decreasing pH, but at low pH significantly higher sodium concentrations were necessary to achieve maximal rate of proton secretion. This sodium-dependent proton secretion could only in part be inhibited by application of 5-(N-methyl-N-isobutyl)-amiloride. Removal of extracellular potassium caused a slow reduction in the rate of acid secretion. A similar reduction was observed in the presence of ouabain, a specific inhibitor of Na(+)-K(+)-adenosinetriphosphatase, and both effects significantly increased with decreasing pH. The results demonstrate a significant pH dependence of the mechanisms of acid release in swim bladder gas gland cells and indicate that sodium-dependent pathways become especially important at low pH.

Evidence of a tubular system for transendothelial transport in arterial capillaries of the rete mirabile

The arterial endothelial cells of the rete capillaries of the eel were examined by transmission electron microscopy on thin sections, on freeze-fracture replicas, by scanning electron microscopy, after cytochemical osmium impregnation and perfusion with peroxidase. The study revealed the existence of membrane-bound tubules and vesicles that open at both the luminal and abluminal poles of the cell and at the level of the intercellular space. The tubules are straight or present successive dilations and constrictions. They branch in various directions and intrude deeply into the cell cytoplasm, forming a complex tubular network within the cell. Immunocytochemical techniques were applied on immersion-fixed tissues and on perfusion of the capillaries with albumin and insulin. These demonstrated that the tubular-vesicular system is involved in the transport of circulating proteins. Furthermore, protein A-gold immunocytochemistry has revealed the association of actin with the membranes of this system. On the basis of these results, we suggest that the transendothelial transport of serum proteins takes place by a transcytotic process through a membrane-bound tubular-vesicular system and is equivalent to the large pore system presumed from functional studies.

SEM of capillary pericytes prepared by ultrasonic microdissection: evidence for the existence of a pericapillary syncytium

Retia mirabile of the eel swimbladder were exsanguinated, perfusion-fixed and subjected to prolonged osmication. They were then microdissected by ultrasonication which delaminated the capillary bed along planes which revealed the surfaces of arterial and venous capillaries. This procedure resulted in cleaned capillary surfaces largely free of connective tissue elements and basement membrane material. The arterial capillary segments were heavily invested with pericytes characterized by plump cell bodies containing nuclei and an extensive system of processes encircling the capillary wall. These processes exhibited a hierarchical organization consisting of primary, secondary, and tertiary elements arising roughly at right angles to each other. Primary and secondary processes exhibited frequent anastomoses and resulted in cytoplasmic continuity between adjacent cell bodies. Processes were also observed to form connections between pericytes on adjacent capillaries. These observations are evidence for the existence of a pericapillary syncytium in which cell bodies may be connected in series and in parallel throughout the arterial capillary bed. This syncytial organization would provide for a coordinated and global contractile response of pericytes to vasoactive hormones and other effectors. It may also provide for synchrony of nuclear division during developmental spread of pericytes along capillary surfaces.

Ultrastructure of capillaries in the red body (rete mirabile) of the eel swim bladder

The retea mirabilia are paired capillary organs located on the dorsal surface of the swimbladder of the common eel. They consist of bundles of closely apposed capillary segments which function in countercurrent exchange of gases and other solutes and concentrate oxygen in the swim bladder. Ultrastructural features of the afferent arterial capillaries and efferent venous capillaries were studied by scanning EM of corrosion casts and critical-point dried retes and transmission EM of thin sections and freeze-fractured retes. A loose association between endothelial cells in the venous capillaries is indicated by penetration of casting material into interendothelial clefts and the appearance of clefts bounded by cytoplasmic flaps in exposed critical-point dried specimens. In thin sections, open gaps between venous endothelial cells are bounded by cytoplasmic processes. Sections through arterial capillaries exhibit tight occluding junctions joining endothelial cells together and these can be seen in freeze-fracture replicas to extend without interruption along the length of the arterial capillaries. These studies indicate the absence of open or hydraulically conductive pathways across the arterial capillary walls and that they probably constitute a rate-limiting barrier in countercurrent exchange of solutes.

Cytochemistry of the gas-exchange area in vertebrate lungs

Considerable progress has been made in the localization of chemical substances within the gas-exchange zones of vertebrate lungs since cytochemical techniques suitable for use with the electron microscope have been developed. The light microscope, an instrument with an effective resolution limit of about 0.2 micron, is ill-suited for studying regions such as these where small tissue elements are arranged in a complex manner. A wide range of acid hydrolases have been detected in the vacuoles and dense bodies of alveolar macrophages by means of cytochemical techniques. The enzymes demonstrated in this way include acid phosphatase, aryl sulphatase, cathepsin D, beta-glucuronidase, acetyl glucosaminidase, nonspecific esterase, dipeptidyl peptidase II and dipeptidyl peptidase IV. Such enzymes are, of course, to be expected in the lysosomes of cells which have a primary phagocytic role. Nevertheless, it must be confessed that very little is yet known about the actual mechanism of phagocytosis or of the fate of the digested material. It is fortunate, however, that some of the tools which are likely to be of value in research on these aspects of macrophage function are currently being developed. Of particular interest in this connection are the immunocytochemical techniques which permit the localization of surface-associated antigens and intracellular contractile proteins. It must be emphasized that phagocytosis is not the only function of macrophages in the gas-exchange zone of the lung. These cells are thought to be involved in the presentation of exogenous antigenic material to the reactive cells of the lymphoid system. Recent research has also indicated that mammalian alveolar macrophages synthesize a diverse range of substances. Furthermore, the elastases associated with pulmonary macrophages are now thought to be involved in the pathogenesis of emphysema. All of the above-mentioned activities are of great biological and clinical significance and, consequently, merit the cytochemists' attention in future. The epithelial lining of the greater part of the pulmonary gas-exchange area is composed of type I pneumonocytes. In terms of ultrastructure, these are very specialized cells; their extensive and highly-attenuated cytoplasmic processes form the outer layer of the air-blood barrier. No special carrier systems have been identified within type I pneumonocytes and this is in keeping with the claims that oxygen is transferred across the alveolar tissue barrier by a process of simple diffusion. Type II pneumonocytes, in contrast, have considerable metabolic activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural and morphometric study of the lung of the European salamander, Salamandra salamandra L

Ultrastructural and morphometric investigations were performed on the lung of the European salamander, Salamandra salamandra L. Folds of first and second order are covered with a ciliated epithelium containing goblet cells. The respiratory surface of the lung is lined by a single type of cell which, in amphibians, combines features of type I and type II alveolar cells of the mammalian lung. In the salamander the respiratory and ciliated epithelial cells as well as goblet cells possess electron dense and lucent vesicles in their cytoplasm as well as lamellar bodies. A small amount of surfactant, composed most probably of phospholipids and mucopolysaccharides, was observed covering the entire inner surface of the lung. Morphometric methods were used to determine the dimensions of the perinuclear region of pneumocytes, the thickness of the air-blood barrier and lung wall, and also the diameter of capillaries. The thickness of the respiratory air-blood barrier was found to be considerably higher than that of the corresponding barrier in mammals.

The capillary permeability of the rete mirabile of the eel, Anguilla vulgaris L

Direct measurements of transcapillary exchange during steady-state conditions have been performed in the rete mirabile of the eel. The capillaries were found to be more than 30 times as permeable to THO (PTHO = 33.2 times 10(-5) cm sec-1), ethanol and antipyrine than to K+. The mutual relationship of the permeabilities for K+, Na+, urea and sucrose were similar to that between the corresponding free diffusion coefficients in water. The permeability characteristics did not change when the perfusate contained metabolic inhibitors. Additional of 1 mM/l of albumin lead to a significant transcapillary osmotic flow. NaCl, urea and sucrose, however, did not cause osmotic flow. Based on the quantitative values for permeability and volume flow the capillary pores should be about 1300 A and have a frequency of 1.3 times 10(6) per cm2 of the capillary surface, occupying an area of about 0.1% of the total area. The results obtained in the present investigation raise questions as to the validity of the Pappenheimer single-pore theory for transcapillary transport as well as to the fine structure of the capillary pores. As an alternative to Pappenheimer's theory a multi-pathway model is proposed in order to explain transcapillary transport.

Qualitative and quantitative studies of the capillary structure in the rete mirabile of the eel, Anguilla vulgaris L

The capillaries of the counter-current capillary organ, rete mirabile, have been studied by light microscopy and electron microscopy. The vasculature of this rete was found to have a cross-sectional area of about 5.25 mm2, and it consisted of 34,000 efferent (=arterial) and 22,000 afferent (=venous) capillaries. The total surface area was the same for the two types of capillaries. The capillary endothelial cells showed numerous pinocytotic vesicles, scattered giant vacuoles and cytoplasmic extrusions (microvilli) at the luminal surface. The majority of the intercellular junctions appeared to have patent gaps with a width of 110-120 A. The arterial capillaries appeared in many ways to be morphologically similar to skeletal muscle capillaries of mammals, whereas the fenestrated venous capillaries resembled those in the intestinal mucosa of mammals. Measurements of the ionic composition of the rete tissue indicated that the endothelial cells contained much less K+ than other cells, the rete containing approximately equimolar amounts of K+ and Na+. The functional significance of the structural and chemical observations are discussed.