Lymphoid organs of teleost fish. III. Splenic lymphoid tissue of Rutilus rutilus and Gobio gobio

Lympho-Hematopoietic Microenvironments and Fish Immune System

In the last 50 years information on the fish immune system has increased importantly, particularly that on species of marked commercial interest (i.e., salmonids, cods, catfish, sea breams), that occupy a key position in the vertebrate phylogenetical tree (i.e., Agnatha, Chondrichtyes, lungfish) or represent consolidated experimental models, such as zebrafish or medaka. However, most obtained information was based on genetic sequence analysis with little or no information on the cellular basis of the immune responses. Although jawed fish contain a thymus and lympho-hematopoietic organs equivalents to mammalian bone marrow, few studies have accounted for the presumptive relationships between the organization of these cell microenvironments and the known immune capabilities of the fish immune system. In the current review, we analyze this topic providing information on: (1) The origins of T and B lymphopoiesis in Agnatha and jawed fish; (2) the remarkable organization of the thymus of teleost fish; (3) the occurrence of numerous, apparently unrelated organs housing lympho-hematopoietic progenitors and, presumably, B lymphopoiesis; (4) the existence of fish immunological memory in the absence of germinal centers.

Histology of some major immune system organs in Lake Van fish Alburnus tarichi (Güldenstädt, 1814) (Cyprinidae)

The teleostean immune system is variable between and within taxa in terms of morphology. Accordingly, in the current study, the histology of some immune organs (thymus, kidney and spleen) of Lake Van fish (Alburnus tarichi) was investigated. For this purpose, the tissues from eight mature fish were fixed and embedded in paraffin wax, and then the sections were stained with haematoxylin and eosin, Mallory's triple, periodic acid-Schiff and Alcian blue (pH of 2.5) for histologic examinations. In the thymus, no clear discrimination of the cortex and medulla was observed. Lymphoid cells within a reticulo-epithelial network were the predominant cell type in the thymus. The other components in the thymus were macrophages and myoid, mast-like, plasma-like, cystic cells, Hassall's corpuscles, and single or multiple epithelial cystic structures. In the kidney, the head kidney, a major haematopoietic site, consisted of lymphoid and non-lymphoid zones within a reticular network. Cells exhibiting mitotic figures were also detected in the haematopoietic tissue of the head kidney. Haematopoietic tissue was also found in the trunk kidney dispersed amongst the excretory components. The spleen was composed of red and white pulp. The red pulp comprised abundant erythrocytes, whilst the white pulp contained leucocytes with a reticular network. Ellipsoids were also determined in the white pulp. Melanomacrophage centres were found in all of the examined lymphoid tissues of the fish. These findings, which were reported, herein, for the first time will provide reference knowledge for future studies of this anadromous fish.

Ontogeny of the immune system in Acipenserid juveniles

Sturgeon aquaculture has increased considerably worldwide but little is known about their immunological development and competence in early life stages. Culture of larvae is one of the most critical stages in intensive sturgeon farming, often associated with high mortality rates. The objective of this study was to characterize the developmental morphology (light and transmission electron microscopy, LM and TEM) of the meningeal myeloid tissue, spleen and thymus in Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) from hatching until 5 months old (2895°C·day (dd)). The spleen was first visible on 541 dd larvae LM sections and the other two immune organs in 768 dd samples (approximately 400 and 600 dd after onset of feeding). Generally, younger fish had significantly higher percentages of undifferentiated cells (meningeal myeloid tissue and spleen) and effective adaptive immune competence would not be expected in these fish on the onset of feeding, but further functional immune assessment is needed.

Expression of a glucocorticoid receptor (DlGR1) in several tissues of the teleost fish Dicentrarchus labrax

Since glucocorticoids have a role in maintaining the homeostatic status in fish, in the present paper mRNA expression (in situ hybridization) and tissue immunohistochemical localization of a glucocorticoid receptor (DlGR1) in several Dicentrarchus labrax organs are reported. Riboprobe and specific antibodies were prepared by using the DlGR1 that has been previously cloned and sequenced from peritoneal cavity leukocytes. Both mRNA and receptor were identified in head kidney, spleen, gills, intestine, heart and liver tissues. The functional roles of DlGR1 localization are discussed.

Cytomorphological alterations of the thymus, spleen, head-kidney, and liver in cardinal fish (Apogonidae, Teleostei) as bioindicators of stress

Morphological and cytological alterations at the light microscope (LM) and transmission electron microscope (TEM) levels were observed in the thymus, spleen, head-kidney, and liver of cardinal fishes (Apogonidae, Teleostei) from the Gulf of Aqaba, Red Sea, sampled from a strongly polluted site at the northern end of the gulf, and compared to similar samples from a clean, reference site. At the polluted site, the most prominent change was the formation of numerous deposits of cells rich in phagosomes with lipofucin, melanin granules, and phagocytosed debris, including a high increase in number and dimensions of Hassall's corpuscles and melano-macrophage centers. The number of Hassall's corpuscles was 20 (+/-8.0)/mm(2) and of melano-macrophage centers 18 (+/-4.0)/mm(2) at the polluted site, and 7.0 (+/-4.0)/m(2) vs. 5.0 (+/-2.0)/mm(2) respectively at the reference site. In numerous instances the head kidney's melano-macrophage centers in fishes from the polluted site were encapsulated by reticulocytes, a phenomenon recognized as a marker of neoplasmosis and possible malignancy. In the spleens of fishes from the polluted site, numerous deposits of cell debris, peroxisomes, and enlarged lysosomes were also observed. The livers (hepatopancreas) of fishes from polluted waters demonstrated very strong hyperlipogeny. Many of their hepatocytes were laden with lipid vesicles, fragmented endoplasmic reticulula, and aberrant mitochondria. Although the observed alterations in the glands and liver do not indicate any immediate threat to the life of the fish, they can become crucial with respect to energy turnover and fecundity trajectories. This study strongly suggests the use of cytological alterations in vital organs, such as were observed, as pathological biomarkers to environmental stress.

Immune-complex trapping in the splenic ellipsoids of rainbow trout (Oncorhynchus mykiss)

Rainbow trout (Oncorhynchus mykiss), immunised with horseradish peroxidase, were given horseradish peroxidase intravenously, and the trapping of antigen in the spleen was followed 1, 24, and 48 h after injection. After 1 h, the localisation of horseradish peroxidase indicated that the antigen had been extensively trapped in the walls of the splenic ellipsoids. The colocalization of horseradish peroxidase with rainbow trout immunoglobulin M and complement factor 3 was shown with a double immunofluorescence technique and suggested that horseradish peroxidase was trapped in the form of immune complexes. After 24 and 48 h, very little horseradish peroxidase was detected in the ellipsoids, and horseradish peroxidase was mainly found in association with large cells with prominent cytoplasmic extensions. In non-immunized fish given horseradish peroxidase intravenously, antigen was not detected in ellipsoids. Thus, the observed difference between immunised and non-immunized trout suggests a specific role for the splenic ellipsoids in rapid immune-complex trapping and invites speculation on its significance in a secondary immune response.

Fish as biomarkers in immunotoxicology

This paper presents an overview on the state of the art in the development and application of biomarkers for immunotoxicology in fish. There are several reasons for developing this field: many fish diseases are related to environmental quality, various environmental pollutants have immunotoxic potential and many fish diseases have an immunological component. As in immunotoxicology in general, this aspect, in fish, has received ample attention in the recent past. Much benefit has been obtained from progress in related fields of science, such as fish immunology and rodent immunotoxicology. Meanwhile there is a broad spectrum of potential biomarkers for immunotoxicology in fish, from which macrophage parameters seem to be most widely used. The application of others, such as lymphoid cell parameters is still limited, probably due to practical problems such as lack of experience with conduct, validation and interpretation. Specific problems include the paucity of background data in the case of epidemiological field studies and the important role of other (non-chemical) stress factors in the immune response, and hence the lack of specificity of potential biomarkers.

Light- and electron microscopical studies on the prolonged toxicity of trichloroethylene on rainbow trout (Oncorhynchus mykiss)

Trichloroethylene, an organochlorine compound used as solvent in numerous industrial processes, was studied with regard to its effects on rainbow trout Oncorhynchus mykiss. Fish were exposed to trichloroethylene via the surrounding water in sublethal concentrations (0.5; 2.5 mg/l) for a period of 21 or 28 days. Three different commercial products of trichloroethylene, all highly purified but varying in quality and amount of stabilizing agents were used. Subchronic exposure resulted in forced apoptosis as well as adaptive and degenerative changes at the subcellular and cellular level in gills, liver, spleen, head- and trunk kidney at the lower concentration. At the higher concentration, necrotic lesions mainly in liver, spleen and head-kidney were found. Trichloroethylene showed particular affinity to the haematopoietic tissue with proliferation and elevated phagocytic activity of reticulo-endothelial cells. Moreover, an increase in phagocytosis of red blood cells which showed abnormalities in ultrastructure was striking. The histopathologic changes after exposure to three different commercial products of trichloroethylene varying in content of stabilizing agents did not show distinct variations.

Renal haemopoietic tissue of the mudskipper Periopthalmus koelreuteri (Pallas)

The renal haemopoietic tissue of the mudskipper Periophthalmus koelreuteri was examined by light and electron microscopy. Leukopoietic zone surrounding melanomacrophage center and erythropoietic zone were differentiated. The ultrastructural features of the cellular components of the haemopoietic compartments are similar to those described in other fishes. Despite the presence of lymphoid cells, this tissue is mainly myeloid, and active granulopoiesis and erythropoiesis occur, as in the red bone marrow of higher vertebrates. © 1992 Wiley-Liss, Inc.

An ultrastructural study of the spleen of the ranid frog Rana perezi

The spleen of Rana perezi is encapsulated by connective tissue and shows by light microscopy two areas with no obvious border: the white pulp and the red pulp. The white pulp-lymphoid clusters are scattered throughout the organ and contain lymphocytes, reticular cells, and some plasma cells. The red pulp displays two different portions. The predominant region consists of reticular cells, lymphocytes, a variety of other leucocytes, and cells undergoing division. This area possibly performs a haemopoietic function. The smaller portion of the red pulp is characterized by reticular-phagocytic cells and may be haemocaretic in its function. Macrophages and pigmented cells occur in both white and red pulp. The organization of the spleen of R. perezi can be considered as a transitional or intermediate state between the primitive condition seen in certain fishes and amphibians and the more complex organ of ammiotes.

Autonomic innervation of the spleen of the coho salmon, Oncorhynchus kisutch. a histochemical demonstration and preliminary assessment of its immunoregulatory role

Regulation of immunity by the nervous system, now a well-established phenomenon in mammals, is effected in part through the autonomic innervation of lymphoid tissues. Noradrenergic fibers specifically target lymphocyte-rich areas in mammalian lymphoid tissues, and their ablation, or the administration of adrenergic agents, can significantly alter immune responses. This study demonstrates that the spleen of the coho salmon is also richly innervated by adrenergic neurons. While this innervation enters the spleen and remains largely associated with the splenic vasculature, fibers can also be observed entering the parenchyma. Although the coho spleen does not possess a well-developed white pulp, aggregations of leukocytes are found adjacent to the major blood vessels in close proximity to the vascular nervous tissue and parenchymal fibers. Chemical sympathectomy with 6-hydroxydopamine results in a significant enhancement of the splenic antibody-secreting cell response to trinitrophenylated sheep red blood cells. These results suggest that sympathectomy is removing a constraint, in the form of inhibitory catecholamines, on the immune response. The potential benefits from a teleost model of neural-immune interactions are discussed.

Phagocytosis in the spleen of the sunfish Lepomis spp

A histological investigation of the filtering function of the spleen of the sunfish Lepomis spp. was conducted by light, scanning, and transmission electron microscopy. The parenchyma of the organ is predominantly red pulp, a system of splenic cords and sinuses. The white pulp consists of loose lymphoid tissue which forms a cuff around the pulp arteries. Filtering of particulate matter from the blood occurs in the red pulp by phagocytes of the pulp cords and ellipsoids (periarterial macrophage sheaths). The ellipsoids are pale-staining cuffs of macrophages and reticular cells in a framework of reticular fibres surrounding the arterial capillaries. Destruction of effete blood cells (especially erythrocytes) is confined to the pigment nodules; particulate matter is not taken up by the nodules. These yellow-brown bodies are dispersed throughout the red pulp and are bounded by a reticular capsule. They contain masses of phagocytes and have the appearance of a morula. They are associated with blood vessels and are surrounded by sinusoids. Prussian Blue stain shows the presence of haemosiderin within their phagocytes. The phagocytes of the pigment nodules are filled with inclusions such as residual bodies, siderosomes, and fragments of erythrocytes. The early filtering of particulate matter by the phagocytes of the pulp cords and ellipsoids may allow for a more efficient phagocytosis of erythrocytes by the pigment nodules, followed by storage and reutilization of iron-containing compounds uncontaminated by other phagocytosed material.

Fine structure of the thymus in the adult cling fish Sicyases sanguineus (Pisces, Gobiesocidae)

The structure of the thumus in adult specimens of a marine teleost, the cling fish Sicyases sanguineus, has been studied by light and transmission electron microscopy. Most cling fishes have an outer thymus located beneath the opercular epithelium. A few of them, however, have a large inner thymus besides a poorly developed outer thymus. In the well-developed outer thymus of cling fish there are three different zones: outer cortex, inner cortex, and medulla. The inner cortex is similar to the cortical region of the thumus in other vertebrates, whereas the outer cortex is a specialized lympho-epithelial zone containing cystic cells (also present in medullary region) and true Hassall's corpuscles. In accordance with the development of the thymic parenchyma, the medullary or basal region may appear either like a true thymic medulla or like a subcapsular region. In the inner thymus, a subcapsular or peripheral "medullary" region and a central area (inverted cortex) show structural features like those of the medullary (basal) and deep cortical regions of the outer thymus, respectively. In addition to the above regions, sometimes there is a lymphomyeloid perithymic infiltration that often extends along connective tissue septa into the perivascular spaces of the gland. Reticuloepithelial, mesenchymal, and unidentified types of stromal cells within the thymus are described. Some erythrocytes, granulocytes, and monocytoid cells are found, but no plasma cells nor erythropoietic foci are evident. The probable significance of these findings is discussed.

Non-lymphoid cells of the anuran spleen: an ultrastructural study in the natterjack, Bufo calamita

The present study is concerned with the ultrastructure of the spleen in the natterjack, Bufo calamita (Anura, Bufonidae), with special emphasis on the structure and function of the non-lymphoid elements occurring in the red and white pulp. The organ consists of two clearly distinguishable areas, the white and the red pulp, separated by a prominent marginal zone. Thus, the pattern of lymphocytic arrangement in the spleen of Bufo calamita corresponds to a follicular model and is similar to that reported in the primitive anurans of the Pipidae family, such as Xenopus laevis. The white pulp presents a reticular network consisting of two different cell types and free cells, such as lymphocytes, plasma cells, and macrophages. The red pulp is formed by cell cords, where reticular cells and fibers, macrophages and lymphocytes occur, and blood sinuses which sometimes contain developing erythroid elements. Colloidal carbon particles injected via the lymph sac are trapped exclusively by free macrophages in the red pulp which then move through the marginal zone to the white pulp. Giant, ramified, non-phagocytic cells appear in both white and red pulp. They have been functionally related with the trapping of antigen-antibody complexes on their surface, and a possible dendritic significance is discussed on the basis of their morphologic characteristics.