Rodlet Cells Structure References Rodlet Cell Workshop
2001 Workshop Attendees
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Entire List of Abstracts

Abstracts of Papers Presented at the 1st International Workshop on Rodlet Cells

University of Ferrara
June 14, 15, 16, 2001

Phylogenetic Differences In Tissue Distribution Of Rodlet Cells
Richard L. Leino
School of Medicine, University of Minnesota, Duluth, MN, USA, 55812

Rodlet cells (RCs) are found in most species of teleosts that have been examined for them. However, their numbers and tissue distribution vary widely. Increases in numbers of RCs seem to be related to their recruitment to a given tissue, probably in response to a parasite infection or to tissue damage. Their tissue distribution is phylogenetic. The present study of over 40 species of fresh- and saltwater teleosts (and an examination of the literature) reveals three major categories of RC distribution: 1) epithelial only, e.g., RCs in epithelia of gill, nasal cavity, skin, stomach, intestine, gall bladder, bile and pancreatic ducts, or kidney; 2) epithelial and mesothelial, e.g., RCs also associated with mesothelium of body cavities; and 3) epithelial, mesothelial, and endothelial, e.g., RCs also associated with the endothelium lining the heart, bulbus arteriosus, or blood vessels. Examples of teleosts in each of these categories are given in the following table:





Yellow perch, Perca flavescens

Walleye, Stizostedion vitreum

Darter, Etheostoma nigrum


Smallmouth bass, Micropterus dolomieu

Rock bass, Ambloplites rupestris

Bluegill,  Lepomis macrochirus


Fathead minnow, Pimephales promelas

Golden shiner, Notemigonus crysoleucas

Redfin shiner, Notropsis umbratitis


Burbot, Lota lota

Cod, Gadus morhua (1)


Mud minnow, Umbra limi


White Sucker, Catastomus commersoni



Swordtail, Xiphopherus helleri

(1) Morrison & Odense, J. Fish Res. Bd. Can. 35:101-116 (1978)

The data in the table demonstrate that closely related species are in the same RC distribution category, and that, as illustrated in category 3, this grouping may include related families (Cyprinidae, Catastomidae, Poeciliidae). The reason for the different distribution of RCs in individual species is not understood. If RCs secrete an antibiotic substance, perhaps their secretion into a wide spectrum of tissue compartments has been selected forin ancestral species with a particular life style, for instance, one that exposed them to large numbers and varieties of parasites. Conversely, perhaps other host defenses such as leucocytes, complement, and antibody production take the place of RC functions in certain tissues of many species, particularly in the cardiovascular system.

New Ultrastructural Observations On The Distribution Of The Rodlet Cores In Trout.
Edith Bielek
Histological & Embryological Institute, Univ. of Vienna, Schwarzspanierstr. 17, A-1090 Vienna, Austria

During the last years, the controversial interpretation of the rodlet cell (parasitic stage or secretory cell) seemed to be decided in favour of an endogenous cell representing a regular part of the unspecific de­fence system. Evidence accumulated indicating statistical correlations with different stressors and para­sites. The specific products, the rodlets (i.e. rodlet sac and central core) are supposed to contain anti­biotic secretions and are expelled and probably dissolved at the surface of differing epithelia. To eluci­date this process further, the distribution of the discharged rodlets was ultrastructurally investigated in intestine, kidney and gills in trout (Oncorhynchus mykiss, Salmo trutta L.). In the epithelium of intestine and kidney tubules, released rodlet sacs and isolated cores displaying a paracrystalline structure and apparently broken into shorter segments were found at the epithelial surface between the microvillous border. Thin parts of cores were also detected in epithelial cells either apically or in the middle part adjacent to rodlet cells showing signs of vesicular degeneration. The released cores were surrounded by an undulating membrane or associated with short tubular elements (0: 25 - 30 nm). In the gill epithelium, cores and tubules were seen not only at the surface and in apical parts but additionally in apoptotic nuclei. In literature the core has been usually described as amorphous. A substructure ("striations") was mentioned only by few authors. The observed tubular elements do not seem to be identical with larger tubules (straight and long or bent) noted in developing rodlet sacs and interpreted e.g. as species - specific characteristics. The distribution of the cores described here is unusual and points - contrary to the concept mentioned above - rather to a noxious nature of the rodlets.

Rodlet Cell Forms In The Hepatopancreas Of The Nile Tilapia, Oreochromis niloticus
Thomas Caceci (1), Hany A. El-Habback (2), and Stephen A. Smith (1)
(1) Aquatic Medicine Laboratory, Department of Biomedical Sciences & Pathobiology
Virginia-Maryland Regional College of Veterinary Medicine
Virginia Polytechnic Institute & State University
Blacksburg, Virginia USA 2406 1-0442
(2) Department of Cytology & Histology, Faculty of Veterinary Medicine, Cairo University, GIza, Egypt

The rodlet cells of the hepatopancreas of Orechromis niloticus fall into three forms: an immature precursor cell, a mature dormant cell, and a mature, active secretory cell. The immature form migrates from the lumen of a blood vessel to the intercellular space, then passes across the epithelial layer covering the pancreas, where it become located between pancreatic acini. The immature form has the same structure as the mature form but lacks the fibrillar cytoplasm and intra-cytoplasmic rodlets. Its nucleus is spherical to elongated and is centrally situated; the cytoplasm contains free ribosomes, some RER, and many dense mitochondria. Matured dormant cells are spherical to elongate in shape and are usually associated with the epithelial covering of the pancreas. The mature dormant cell's nucleus is irregular in shape and eccentrically located, and the cytoplasm contains the characteristic rodlets as well as low-density mitochondria and many vacuoles. In cross section the rodlets are round with a low-density outer cortex and a dot-like inner core. The periphery of the mature cell's cytoplasm is condensed into a fibrous band. In the secretory form, the rodlets become more elongated, are polarized towards the apex of the cell, and show increased electron density in their cores. Mitochondria accumulate at the apex of the cell. At the time of release of the cellular contents the apex attenuates and fuses with the overlying epithelial cells, creating an opening through which the entire contents of the cell are discharged into the intercellular compartment. After discharge the remnant, consisting mainly of the fibrous capsule, is removed by macrophage activity.

Alteration Of Rodlet Cells Of The Fish Leuciscus cephalus As Response To Herbicide-Contaminated Water: Laboratory Exposures
B.S. Dezfuli (1), L. Giari (1), E. Simoni (1), D. Palazzi (2), P. Boldrini (3), M. Manera (4)
(1) Department of Biology, University of Ferrara, Italy
(2) Environmental Protection Agency, Ferrara
(3) Centre of Electron Microscopy, University of Ferrara
(4) Faculty of Veterinary Medicine, University of Teramo

Propanil (3, 4'-Dichloropropionanilide), the active ingredient in the herbicide is probably the most extensively used herbicide for rice production in the world. Propanil and its metabolites are transported within characteristic ditch ecosystems in the production landscape of Province of Ferrara. Thus, the effect of Propanil on occurrence and structure of rodlet cells in different organs of chub, Leuciscus cephalus was investigated.

Thirty-six chub were obtained from a local supplier, total length of fish ranged from 105 to 137 mm, were divided in 6 groups: 1 group kept in a tank as control; 1 group in a tank with no treatment and 4 other groups were exposed to increasing concentration of herbicide (3.16 mg/L; 6.31 mg/I; 12.6 mg/I; 25.1 mg/L) for 48 h. Within the precise interval of time (t0, t6, t24, t48) from each tank, fish were scarified and heart and tissues of gills, liver and kidney were fixed and then processed for light and electron microscopy.

Distribution and structure of the rodlet cells of treated fish were affected by Propanil. With regard to biometrics data, the bulb of the heart apparently exerts as storage site for these cells. Both herbicide and handling stress induced a negative exponential depletion of the number of RCs in bulb; this depletion is positively correlated with stress intensity.

Rodlet Cells In The Gall Bladder Of The Black Mollie
Terry C. Hrubec and Thomas Caceci
Department of Biomedical Sciences & Pathobiology
Virginia-Maryland Regional College of Veterinary Medicine
Virginia Polytechnic Institute & State University
Blacksburg, Virginia USA 2406 1-0442

Rodlet cells have been reported in over 114 fish species. Morphologically they demonstrate minor species differences, mainly in rodlet shape. We observed large numbers of them in the gall bladder of the black mollie (Poecilia spp.). While rodlet cells are frequently associated with inflammation, we found them to be a consistent feature of all our specimens, all clinically normal and displaying no signs of inflammation. They were present throughout the bladder and particularly numerous in the bile ducts, always contained within the thickness of the epithelium and not extending below the basement membrane. They had a heavy, fibrous capsule and abundant cytoplasm, with numerous vesicles of smooth endoplasmic reticulum, and a prominent Golgi apparatus. Each contained round to oval rods with an electron-dense core and a less-dense but distinct amorphous outer "halo". The rods stained with eosin but not PAS, with no suggestion of crystallinity. Comparing the mollie bladder to that of the goldfish, we were struck by the absence of rodlet cells in the latter. Our observation of a Golgi apparatus and abundant smooth endoplasmic reticulum implies a secretory function. We suggest that the rodlet cell is an endogenous cell whose definitive appearance is masked In the absence of inflammation. Their abundance in the gall bladder of the mollie may be due to conditions in situ that trigger migration into the epithelium and/or final differentiation to the mature form. Their paucity in the goldfish may be due to difference of pH, bile concentration, or some other factor.

Formation And Release Of The Secretory Product In Rodlet Cells
Richard L. Leino
School of Medicine, University of Minnesota, Duluth, MN, USA, 55812

Immature rodlet cells, in which secretory granule formation is occurring, are found deep in the epithelia of organs such as gill and intestine of many fish species. These cells have a euchromatic nucleus with a prominent nucleolus indicative of active synthesis of m-RNA and ribosomes. Electron microscopy reveals considerable development of the organelles involved in protein synthesis, segregation, and concentration, i.e., the rough endoplasmic reticulum and the Golgi apparatus, along with many mitochondria and several incipient secretory granules. Coated (presumably copII) vesicles appear to bud off of the transition elements of the RER, lose their coat, and then fuse to form larger vesicles that compose the ER-Golgi intermediate compartment (ERGIC). About 3-7 cisternae usually form the cis to trans elements of the Golgi apparatus proper. A mixture of uncoated and coated vesicles bud from the trans-Golgi network. By analogy to other cell-types, the coated vesicles are of two kinds: 1) copI, involved in recycling of Golgi resident proteins, and 2) clathrin, involved in production of endosomes. The more numerous uncoated trans-Golgi vesicles appear to carry cargo to the developing secretory granules where most of the concentration of the secretory product occurs. While the production of secretory granules in rodlet cells is typical of that described for numerous secretory cells and for granulocytes, the release of the secretory granules is unique. Rodlet cells at times appear to explosively discharge all of their membrane-bounded granules through an opening in the cell apex. Granule release into the extracellular environment is accomplished by a sudden contraction of the actin-myosin rich cell cortex. Some evidence suggests that this secretory process delivers a concentrated dose of an antibiotic substance to combat protozoan or metazoan parasites. Moreover, coordinated secretion from large numbers of rodlet cells could lead to high concentrations of antibiotic substances in localized regions such as body cavities, kidney tubules, and interlamellar spaces of gills, without producing tissue damaging inflammation.

Seasonal Increases of Rodlet Cells and Other Cell-types in Percid Gills: Association with Parasitic Infections
Richard L. Leino
School of Medicine, University of Minnesota, Duluth, MN, USA, 55812

A wide variety of cell-types involved in innate defense against parasites are associated with epithelia of aquatic organisms. In epithelia of percid gills, this defense system principally appears to comprise three cell-types: mucous cells, rodlet cells, and eosinophilic granular cells. In previous observations of many species from clean freshwater lakes in northern latitudes we noted a near absence of gill parasites in overwintering fish. In late spring and early summer, increased incidences of gill parasitism occurred, but the infestations were rapidly dampened, and numbers of parasites remained at low densities thereafter. In the present study, three species of percids, walleye, Stizostedion vitreum (from Island Lake, MN), yellow perch, Perca flavescens (from Little Rock Lake, WI), and ruffe, Gymnocephalus cernuus (from the Duluth-Superior harbor on western Lake Superior) were examined quantitatively for gill cell types and for parasites. Monthly samples were taken beginning shortly after ice out (April, May) to shortly before ice over (November). The goal of the study was to determine if a relationship exists between changes in numbers of gill "defense" cells and 1) incipient spring parasite activity and 2) subsequent parasite levels. Histological examination of the gills revealed that in all three species: 1) in April-May the filament epithelium is thin and consists mostly of pavement cells and chloride cells; few rodlet, mucous, and granular cells are present; 2) during May-June dramatic increases in rodlet, mucous, and granular cells occur and are accompanied by, or result in, a thickened stratified filament epithelium. High numbers of these three cell-types persist through September, but their densities begin to decline in October November when water temperature drops sharply; 3) Parasitic (primarily protozoan) infections appearing in May-June did not reach high levels throughout the study period. It is suggested that the combined secretions of rodlet and mucous cells serve to prevent or dampen parasitic infections at gill epithelial surfaces. Eosinophilic granular cells, which lie deeper in the gill epithelium, may form a second line of defense, helping to prevent endoparasites from becoming established in the epithelium and deeper tissues.

Ultrastructural Observations On Rodlet Cells In The Head Kidney Of The Platyfish, Xiphophorus maculatus (Teleostei: Poeciliidae)

Hugh Potter (1), Charles R. Kramer (2)
(1) Biology Department, Union County College, Cranford, New Jersey, USA
(2) Department of Biology, College of Staten Island (CUNY), Staten Island, New York, USA

The enigmatic rodlet cell (RC) has been observed in tissues of marine and freshwater teleosts. The origin and function of this cell remain unclear. In this present study, we describe the association of the rodlet cell with the head kidney tissue of the platyfish, Xiphophorus maculatus.

In previous studies, we have observed rodlet cells in a variety of tissues of the platyfish and in hybrids of the platyfish and swordtail, Xiphophorus helleri. Rodlet cells were commonly seen in the hemopoietic compartments if the head kidney, associated with gill epithelia and within ovarian tissue. Rodlet cells were also found in melanotic tumors of the hybrids.

More recently, we have found rodlet cells within mesothelial membranes lining body spaces and musculature of the platyfish. In our most recent work, RCs could be observed within the walls of the ovarian ducts of this species. Intrigued by these observations, we began our current study of the RC-head kidney association in X. maculatus. A series of developmental stages of the rodlet cell could be discerned within the kidney's hemopoietic tissue. These included early precursor cells through degraded terminal stages. An isolated observation revealed a single mature RC within a kidney tubule epithelial cell.

Tight junctions and desmosomes were observed between mature RCs and endothelial cells. Desmosomal junctions were occasionally seen between maturing RCs and neighboring cells within the intertubular areas. We never observed junctional complexes between adjacent RCs at any stage of maturity.

A discussion will relate our observations to the theories proposing either an endogenous or exogenous origin of the rodlet cell.

Presence Of Rodlet Cells And Other Inflammatory Cells In Sparidae Infected By Myxosporidia
Francesco Quaglio (1), Maria Lourdes Delgado (2), Maria Letizia Fioravanti (2), Monica Caffara (2), Birgit Oberkofler (1)
(1) Centro Interdipartimentale suite Tecnologie e I'Igiene degli Allevamenti Intensivi delle PiccoleSpecie, Università di Bologna.
(2) Dipartimento di Sanità Pubblica Veterinaria e Patologia Animate, Facoltâ di Medicina Veterinaria, Università di Bologna.

Histopathological survey was carried out in myxosporeans infected sparids cultured in different systems in Italy. Two hundred-forty sea bream (Sparus aurata) and 28 sharpsnout sea bream (Diplodus puntazzo) were examined. Myxidium leei were detected in 8 sea bream (Sparus aurata) and 21 sharpsnout sea bream (Diplodus puntazzo); Polysporoplasma spans in 2 sea bream; Henneguya sp. in 4 sea bream; Polysporoplasma spans associated with Henneguya sp. in 1 sharpsnout sea bream. Histological sections: were prepared from gill, lever, gall bladder, gut, pancreas, kidney, spleen, heart and brain and stained with Haematoxylin and Eosin (H&E), Giemsa, Periodic Acid-Schiff (PAS), Mallory's Trichrome and Ziehl Neelsen Methods. Histopathological lesions observed in the gut infected by Myxidium leei were: severe enteritis, detachment of epithelium, haemorrhages and inflammation of the subepithelial connective tissue with massive presence of eosinophilic granule cells (EGGs), particularly in sharpsnout sea bream. Polysporoplasma spans was found in the trunk kidney determining serious damages only in sharpsnout sea bream. Glomerular capillaries were invaded by the parasite. Spores were also detected in the lumen of renal tubuli, in the interstitial tissue and in cysts replete with cellular debris. Inflammatory responses consisted mainly of melanomacrophages near spores degenerated and EGCs. The presence of rodlet cells inside the cysts and in the nephric tubules were distinct after staining with Mallory's Trichrome. Henneguya spores discovered in sharpsnout sea bream were evident free in the kidney interstitial tissue and damaged in melanomacrophages centres.

Henneguya sp. was found in cysts on the bulbus arteriosus of sea bream. Foci of infection were observed throughout the musculature and elastic fibres of the bulbus. EGGs and rodlet cells were abundant in the wall.

Distribution Of Rodlet Cells In Different Teleosts: A Comparison With Mast Cells/Eosinophilic Granule Cells
Ola B. Reite
Department of Morphology, Genetics and Aquatic Biology, Norwegian School of
Veterinary Science, P.O. Box 8146 Dep., 0033 Oslo, Norway

During studies on the distribution of mast cells/eosinophilic granule cells (MCs/EGCs) in species representing 5-12 genera from each of the teleostean families Salmonidae, Cyprinidae, Gadidae, Labridae and Pleuronectidae, a characteristic distribution pattern common to species of the same genus and great similarities also between the different genera of a family were found. Furthermore, the studies showed that persistent inflammatory reactions, e.g. those caused by helminths in tissues of the viscera, induced local recruitment of MCs/EGCs, except in gadids, where this cell type was not found in any tissue. In all families, rodlet cells were recruited when cestodes or trematodes affected epithelial tissues.

Fish of the salmonid, gadid and pleuronectid families showed great individual variations in the distribution of rodlet cells. The cells were absent in some individuals of a species and present in different organs in other individuals. Their occurrence was common in salmonids caught in their natural environment, whereas those in aquaculture, kept under controlled conditions with respect to water quality, showed extremely few rodlet cells.

In the other two of the studied families, the labrids and the cyprinids, the picture was different. All species of labrids showed a fairly high number of rodlet cells in the mesentery, where larval specimens of helminths were common, whereas such cells were only occasionally present in the epithelia of gills and intestine. The cyprinids were infected with trematodes, and rodlet cells were consistently present under the endothelium of branchial blood vessels and in the bulbus arteriosus. In individuals with eggs and miracidiae of blood flukes in tissues of the bulbus arteriosus, massive aggregations of rodlet cells were present at levels not encountered in any other tissue of fishes examined. With respect to other epithelial tissues of cyprinids, rodlet cells were found in some individuals but were absent in others. MCs/EGCs were consistently present in large numbers in tissues of the intestine of cyprinids and labrids. They were located throughout the lamina propria and were also quite common in the intestinal epithelium. In this context, it is noteworthy that cyprinids and labrids are stomachless fish, lacking the disinfectant function of hydrochloric acid. The results of these studies indicate that rodlet cells, like MCs/EGCs, have a role in the defence mechanisms of teleosts.

Teleosts have come to inhabit more diverse environments than any other group of vertebrates, which suggests the presence of special adaptations with respect to tissue distribution of cells involved in host defence. It has previously been suggested that evolution has created a large "standing force" of MCs/BGCs in particular tissues of fish consistently exposed to pathogens, whereas an efficient "mobilization force" has been an advantage for fish in other more pathogen-free environments. Since present evidence points to a role for the rodlet cells as participants in defence functions, e.g. in combating helminths, the suggestion made for MCs/EGCs, explaining differences between teleostean families with respect to their distribution pattern, may also apply to the rodlet cells.

Expression of Tumour Necrosis Factor (TNF) in rodlet cells from pathological fish tissue: immunohistochemical study
A. Sfacteria, G. Costantino, F. Marino, G. Mazzullo, and G. De Vico
Department of Pathology, Parasitic and Infectious Diseases, and Food Inspection - Unit of General and Anatomic Pathology, Via S. Cecilia 30, 98123, Messina, Italy

The so-called "rodlet cells" (RCs) are enigmatic cells usually associated with the epithelium of various organs of both freshwater and marine teleost fish. At date, the proper biological role of these elements is still to be clarified. It has been suggested that they could represent an inflammatory cell type, but this suggestion should be definitively demonstrated. In this order, sections from pathological tissues of both Liza aurata and Leuciscus cephalus where the presence of RCs and other inflammatory cells has been previously assessed in routine histopathology, have been also tested by Tumour Necrosis Factor (TNF)­immunohistochemistry. TNF, in fact, is one of the major pro-inflammatory cytokine secreted by many inflammatory cells in mammals, being involved in leukocyte traffic and natural immunity. In the examined tissues RCs were frequently associated to endothelial cells in blood vessels, and always showed a positive and strong TNF-immunoreaction. These findings strongly support the inflammatory role of RCs in teleost fish.